Some raster data sources, such as simple scanned maps, lack proper spatial referencing, and we have to georeference them before we can use them in a GIS. In QGIS, we can georeference rasters using the Georeferencer GDAL plugin, which can be accessed by going to Raster | Georeferencer. (Enable it by going to Plugins | Manage and Install Plugins if you cannot find it in the Raster menu).

The Georeferencer plugin covers the following use cases:

After loading a raster into Georeferencer by going to File | Open raster or using the Open raster toolbar button, we are asked to specify the CRS of the ground control points that we are planning to add. Next, we can start adding ground control points by going to Edit | Add point. We can use the pan and zoom tools to navigate, and we can place GCPs by clicking on the map. We are then prompted to insert the coordinates of the new point or pick them from the reference map in the main QGIS window. The placed GCPs are displayed as red circles in both Georeferencer and the QGIS window, as you can see in the following screenshot:

Georeferencer shows a screenshot of the OCM Landscape map © Thunderforest, Data © OpenStreetMap contributors (http://www.opencyclemap.org/?zoom=4&lat=62.50806&lon=-145.01953&layers=0B000)

After placing the GCPs, we can define the transformation algorithm by going to Settings | Transformation Settings. Which algorithm you choose depends on your input data and the level of geometric distortion you want to allow. The most commonly used algorithms are polynomial 1 to 3. A first-order polynomial transformation allows scaling, translation, and rotation only.

A second-order polynomial transformation can handle some curvature, and a third-order polynomial transformation consequently allows for even higher degrees of distortion. The thin-plate spline algorithm can handle local deformations in the map and is therefore very useful while working with very low-quality map scans. Projective transformation offers rotation and translation of coordinates. The linear option, on the other hand, is only used to create world files, and as mentioned earlier, this does not actually transform the raster.

The resampling method depends on your input data and the result you want to achieve. Cubic resampling creates smooth results, but if you don't want to change the raster values, choose the nearest neighbor method.

Before we can start the georeferencing process, we have to specify the output filename and target CRS. Make sure that the Load in QGIS when done option is active and activate the Use 0 for transparency when needed option to avoid black borders around the output image. Then, we can close the Transformation Settings dialog and go to File | Start Georeferencing. The georeferenced raster will automatically be loaded into the main map window of QGIS. In the following screenshot, you can see the result of applying projective transformation using the five specified GCPs:

Some raster data sources, such as simple scanned maps, lack proper spatial referencing, and we have to georeference them before we can use them in a GIS. In QGIS, we can georeference rasters using the Georeferencer GDAL plugin, which can be accessed by going to Raster | Georeferencer. (Enable it by going to Plugins | Manage and Install Plugins if you cannot find it in the Raster menu).

The Georeferencer plugin covers the following use cases:

After loading a raster into Georeferencer by going to File | Open raster or using the Open raster toolbar button, we are asked to specify the CRS of the ground control points that we are planning to add. Next, we can start adding ground control points by going to Edit | Add point. We can use the pan and zoom tools to navigate, and we can place GCPs by clicking on the map. We are then prompted to insert the coordinates of the new point or pick them from the reference map in the main QGIS window. The placed GCPs are displayed as red circles in both Georeferencer and the QGIS window, as you can see in the following screenshot:

Georeferencer shows a screenshot of the OCM Landscape map © Thunderforest, Data © OpenStreetMap contributors (http://www.opencyclemap.org/?zoom=4&lat=62.50806&lon=-145.01953&layers=0B000)

After placing the GCPs, we can define the transformation algorithm by going to Settings | Transformation Settings. Which algorithm you choose depends on your input data and the level of geometric distortion you want to allow. The most commonly used algorithms are polynomial 1 to 3. A first-order polynomial transformation allows scaling, translation, and rotation only.

A second-order polynomial transformation can handle some curvature, and a third-order polynomial transformation consequently allows for even higher degrees of distortion. The thin-plate spline algorithm can handle local deformations in the map and is therefore very useful while working with very low-quality map scans. Projective transformation offers rotation and translation of coordinates. The linear option, on the other hand, is only used to create world files, and as mentioned earlier, this does not actually transform the raster.

The resampling method depends on your input data and the result you want to achieve. Cubic resampling creates smooth results, but if you don't want to change the raster values, choose the nearest neighbor method.

Before we can start the georeferencing process, we have to specify the output filename and target CRS. Make sure that the Load in QGIS when done option is active and activate the Use 0 for transparency when needed option to avoid black borders around the output image. Then, we can close the Transformation Settings dialog and go to File | Start Georeferencing. The georeferenced raster will automatically be loaded into the main map window of QGIS. In the following screenshot, you can see the result of applying projective transformation using the five specified GCPs:

Let's get started with a point layer! Load airport.shp from your sample data. In the top-left corner of the Style dialog, below the drop-down list, we find the symbol preview. Below this, there is a list of symbol layers that shows us the different layers the symbol consists of. On the right-hand side, we find options for the symbol size and size units, color and transparency, as well as rotation. Finally, the bottom-right area contains a preview area with saved symbols.

Point layers are, by default, displayed using a simple circle symbol. We want to use a symbol of an airplane instead. To change the symbol, select the Simple marker entry in the symbol layers list on the left-hand side of the dialog. Notice how the right-hand side of the dialog changes. We can now see the options available for simple markers: Colors, Size, Rotation, Form, and so on. However, we are not looking for circles, stars, or square symbols—we want an airplane. That's why we need to change the Symbol layer type option from Simple marker to SVG marker. Many of the options are still similar, but at the bottom, we now find a selection of SVG images that we can choose from. Scroll through the list and pick the airplane symbol, as shown in the following screenshot:

Before we move on to styling lines, let's take a look at the other symbol layer types for points, which include the following:

Simple marker layers can have different geometric forms, sizes, outlines, and angles (orientation), as shown in the following screenshot, where we create a red square without an outline (using the No Pen option):

Font marker layers are useful for adding letters or other symbols from fonts that are installed on your computer. This screenshot, for example, shows how to add the yin-and-yang character from the Wingdings font:

Ellipse marker layers make it possible to draw different ellipses, rectangles, crosses, and triangles, where both the width and height can be controlled separately. This symbol layer type is especially useful when combined with data-defined overrides, which we will discuss later. The following screenshot shows how to create an ellipse that is 5 millimeters long, 2 millimeters high, and rotated by 45 degrees:

In this exercise, we create a river style for the majriver.shp file in our sample data. The goal is to create a line style with two colors: a fill color for the center of the line and an outline color. This technique is very useful because it can also be used to create road styles.

To create such a style, we combine two simple lines. The default symbol is one simple line. Click on the green + symbol located below the symbol layers list in the bottom-left corner to add another simple line. The lower line will be our outline and the upper one will be the fill. Select the upper simple line and change the color to blue and the width to 0.3 millimeters. Next, select the lower simple line and change its color to gray and width to 0.6 millimeters, slightly wider than the other line. Check the preview and click on Apply to test how the style looks when applied to the river layer.

You will notice that the style doesn't look perfect yet. This is because each line feature is drawn separately, one after the other, and this leads to a rather disconnected appearance. Luckily, this is easy to fix; we only need to enable the so-called symbol levels. To do this, select the Line entry in the symbol layers list and tick the checkbox in the Symbol Levels dialog of the Advanced section (the button in the bottom-right corner of the style dialog), as shown in the following screenshot. Click on Apply to test the results.

Before we move on to styling polygons, let's take a look at the other symbol layer types for lines, which include the following:

A common use case for Marker line symbol layers are train track symbols; they often feature repeating perpendicular lines, which are abstract representations of railway sleepers. The following screenshot shows how we can create a style like this by adding a marker line on top of two simple lines:

Another common use case for Marker line symbol layers is arrow symbols. The following screenshot shows how we can create a simple arrow by combining Simple line and Marker line. The key to creating an arrow symbol is to specify that Marker placement should be last vertex only. Then we only need to pick a suitable arrow head marker and the arrow symbol is ready.

In this exercise, we will create a style for the alaska.shp file. The goal is to create a simple fill with a blue halo. As in the previous river style example, we will combine two symbol layers to create this style: a Simple fill layer that defines the main fill color (white) with a thin border (in gray), and an additional Simple line outline layer for the (light blue) halo. The halo should have nice rounded corners. To achieve these, change the Join style option of the Simple line symbol layer to Round. Similar to the previous example, we again enable symbol levels; to prevent this landmass style from blocking out the background map, we select the Multiply blending mode, as shown in the following screenshot:

Before we move on, let's take a look at the other symbol layer types for polygons, which include the following:

A common use case for Point pattern fill symbol layers is topographic symbols for different vegetation types, which typically consist of a Simple fill layer and Point pattern fill, as shown in this screenshot:

When we design point pattern fills, we are, of course, not restricted to simple markers. We can use any other marker type. For example, the following screenshot shows how to create a polygon fill style with a Font marker pattern that shows repeating alien faces from the Webdings font:

As an alternative to simple fills with only one color, we can create Gradient fill symbol layers. Gradients can be defined by Two colors, as shown in the following screenshot, or by a Color ramp that can consist of many different colors. Usually, gradients run from the top to the bottom, but we can change this to, for example, make the gradient run from right to left by setting Angle to 270 degrees, as shown here:

The Shapeburst fill symbol layer type, also known as a "buffered" gradient fill, is often used to style water areas with a smooth gradient that flows from the polygon border inwards. The following screenshot shows a fixed-distance shading using the Shade to a set distance option. If we select Shade whole shape instead, the gradient will be drawn all the way from the polygon border to the center.

Let's get started with a point layer! Load airport.shp from your sample data. In the top-left corner of the Style dialog, below the drop-down list, we find the symbol preview. Below this, there is a list of symbol layers that shows us the different layers the symbol consists of. On the right-hand side, we find options for the symbol size and size units, color and transparency, as well as rotation. Finally, the bottom-right area contains a preview area with saved symbols.

Point layers are, by default, displayed using a simple circle symbol. We want to use a symbol of an airplane instead. To change the symbol, select the Simple marker entry in the symbol layers list on the left-hand side of the dialog. Notice how the right-hand side of the dialog changes. We can now see the options available for simple markers: Colors, Size, Rotation, Form, and so on. However, we are not looking for circles, stars, or square symbols—we want an airplane. That's why we need to change the Symbol layer type option from Simple marker to SVG marker. Many of the options are still similar, but at the bottom, we now find a selection of SVG images that we can choose from. Scroll through the list and pick the airplane symbol, as shown in the following screenshot:

Before we move on to styling lines, let's take a look at the other symbol layer types for points, which include the following:

Simple marker layers can have different geometric forms, sizes, outlines, and angles (orientation), as shown in the following screenshot, where we create a red square without an outline (using the No Pen option):

Font marker layers are useful for adding letters or other symbols from fonts that are installed on your computer. This screenshot, for example, shows how to add the yin-and-yang character from the Wingdings font:

Ellipse marker layers make it possible to draw different ellipses, rectangles, crosses, and triangles, where both the width and height can be controlled separately. This symbol layer type is especially useful when combined with data-defined overrides, which we will discuss later. The following screenshot shows how to create an ellipse that is 5 millimeters long, 2 millimeters high, and rotated by 45 degrees:

In this exercise, we create a river style for the majriver.shp file in our sample data. The goal is to create a line style with two colors: a fill color for the center of the line and an outline color. This technique is very useful because it can also be used to create road styles.

To create such a style, we combine two simple lines. The default symbol is one simple line. Click on the green + symbol located below the symbol layers list in the bottom-left corner to add another simple line. The lower line will be our outline and the upper one will be the fill. Select the upper simple line and change the color to blue and the width to 0.3 millimeters. Next, select the lower simple line and change its color to gray and width to 0.6 millimeters, slightly wider than the other line. Check the preview and click on Apply to test how the style looks when applied to the river layer.

You will notice that the style doesn't look perfect yet. This is because each line feature is drawn separately, one after the other, and this leads to a rather disconnected appearance. Luckily, this is easy to fix; we only need to enable the so-called symbol levels. To do this, select the Line entry in the symbol layers list and tick the checkbox in the Symbol Levels dialog of the Advanced section (the button in the bottom-right corner of the style dialog), as shown in the following screenshot. Click on Apply to test the results.

Before we move on to styling polygons, let's take a look at the other symbol layer types for lines, which include the following:

A common use case for Marker line symbol layers are train track symbols; they often feature repeating perpendicular lines, which are abstract representations of railway sleepers. The following screenshot shows how we can create a style like this by adding a marker line on top of two simple lines:

Another common use case for Marker line symbol layers is arrow symbols. The following screenshot shows how we can create a simple arrow by combining Simple line and Marker line. The key to creating an arrow symbol is to specify that Marker placement should be last vertex only. Then we only need to pick a suitable arrow head marker and the arrow symbol is ready.

In this exercise, we will create a style for the alaska.shp file. The goal is to create a simple fill with a blue halo. As in the previous river style example, we will combine two symbol layers to create this style: a Simple fill layer that defines the main fill color (white) with a thin border (in gray), and an additional Simple line outline layer for the (light blue) halo. The halo should have nice rounded corners. To achieve these, change the Join style option of the Simple line symbol layer to Round. Similar to the previous example, we again enable symbol levels; to prevent this landmass style from blocking out the background map, we select the Multiply blending mode, as shown in the following screenshot:

Before we move on, let's take a look at the other symbol layer types for polygons, which include the following:

A common use case for Point pattern fill symbol layers is topographic symbols for different vegetation types, which typically consist of a Simple fill layer and Point pattern fill, as shown in this screenshot:

When we design point pattern fills, we are, of course, not restricted to simple markers. We can use any other marker type. For example, the following screenshot shows how to create a polygon fill style with a Font marker pattern that shows repeating alien faces from the Webdings font:

As an alternative to simple fills with only one color, we can create Gradient fill symbol layers. Gradients can be defined by Two colors, as shown in the following screenshot, or by a Color ramp that can consist of many different colors. Usually, gradients run from the top to the bottom, but we can change this to, for example, make the gradient run from right to left by setting Angle to 270 degrees, as shown here:

The Shapeburst fill symbol layer type, also known as a "buffered" gradient fill, is often used to style water areas with a smooth gradient that flows from the polygon border inwards. The following screenshot shows a fixed-distance shading using the Shade to a set distance option. If we select Shade whole shape instead, the gradient will be drawn all the way from the polygon border to the center.

In this exercise, we create a river style for the majriver.shp file in our sample data. The goal is to create a line style with two colors: a fill color for the center of the line and an outline color. This technique is very useful because it can also be used to create road styles.

To create such a style, we combine two simple lines. The default symbol is one simple line. Click on the green + symbol located below the symbol layers list in the bottom-left corner to add another simple line. The lower line will be our outline and the upper one will be the fill. Select the upper simple line and change the color to blue and the width to 0.3 millimeters. Next, select the lower simple line and change its color to gray and width to 0.6 millimeters, slightly wider than the other line. Check the preview and click on Apply to test how the style looks when applied to the river layer.

You will notice that the style doesn't look perfect yet. This is because each line feature is drawn separately, one after the other, and this leads to a rather disconnected appearance. Luckily, this is easy to fix; we only need to enable the so-called symbol levels. To do this, select the Line entry in the symbol layers list and tick the checkbox in the Symbol Levels dialog of the Advanced section (the button in the bottom-right corner of the style dialog), as shown in the following screenshot. Click on Apply to test the results.

Before we move on to styling polygons, let's take a look at the other symbol layer types for lines, which include the following:

A common use case for Marker line symbol layers are train track symbols; they often feature repeating perpendicular lines, which are abstract representations of railway sleepers. The following screenshot shows how we can create a style like this by adding a marker line on top of two simple lines:

Another common use case for Marker line symbol layers is arrow symbols. The following screenshot shows how we can create a simple arrow by combining Simple line and Marker line. The key to creating an arrow symbol is to specify that Marker placement should be last vertex only. Then we only need to pick a suitable arrow head marker and the arrow symbol is ready.

In this exercise, we will create a style for the alaska.shp file. The goal is to create a simple fill with a blue halo. As in the previous river style example, we will combine two symbol layers to create this style: a Simple fill layer that defines the main fill color (white) with a thin border (in gray), and an additional Simple line outline layer for the (light blue) halo. The halo should have nice rounded corners. To achieve these, change the Join style option of the Simple line symbol layer to Round. Similar to the previous example, we again enable symbol levels; to prevent this landmass style from blocking out the background map, we select the Multiply blending mode, as shown in the following screenshot:

Before we move on, let's take a look at the other symbol layer types for polygons, which include the following:

A common use case for Point pattern fill symbol layers is topographic symbols for different vegetation types, which typically consist of a Simple fill layer and Point pattern fill, as shown in this screenshot:

When we design point pattern fills, we are, of course, not restricted to simple markers. We can use any other marker type. For example, the following screenshot shows how to create a polygon fill style with a Font marker pattern that shows repeating alien faces from the Webdings font:

As an alternative to simple fills with only one color, we can create Gradient fill symbol layers. Gradients can be defined by Two colors, as shown in the following screenshot, or by a Color ramp that can consist of many different colors. Usually, gradients run from the top to the bottom, but we can change this to, for example, make the gradient run from right to left by setting Angle to 270 degrees, as shown here:

The Shapeburst fill symbol layer type, also known as a "buffered" gradient fill, is often used to style water areas with a smooth gradient that flows from the polygon border inwards. The following screenshot shows a fixed-distance shading using the Shade to a set distance option. If we select Shade whole shape instead, the gradient will be drawn all the way from the polygon border to the center.

In this exercise, we will create a style for the alaska.shp file. The goal is to create a simple fill with a blue halo. As in the previous river style example, we will combine two symbol layers to create this style: a Simple fill layer that defines the main fill color (white) with a thin border (in gray), and an additional Simple line outline layer for the (light blue) halo. The halo should have nice rounded corners. To achieve these, change the Join style option of the Simple line symbol layer to Round. Similar to the previous example, we again enable symbol levels; to prevent this landmass style from blocking out the background map, we select the Multiply blending mode, as shown in the following screenshot:

Before we move on, let's take a look at the other symbol layer types for polygons, which include the following:

A common use case for Point pattern fill symbol layers is topographic symbols for different vegetation types, which typically consist of a Simple fill layer and Point pattern fill, as shown in this screenshot:

When we design point pattern fills, we are, of course, not restricted to simple markers. We can use any other marker type. For example, the following screenshot shows how to create a polygon fill style with a Font marker pattern that shows repeating alien faces from the Webdings font:

As an alternative to simple fills with only one color, we can create Gradient fill symbol layers. Gradients can be defined by Two colors, as shown in the following screenshot, or by a Color ramp that can consist of many different colors. Usually, gradients run from the top to the bottom, but we can change this to, for example, make the gradient run from right to left by setting Angle to 270 degrees, as shown here:

The Shapeburst fill symbol layer type, also known as a "buffered" gradient fill, is often used to style water areas with a smooth gradient that flows from the polygon border inwards. The following screenshot shows a fixed-distance shading using the Shade to a set distance option. If we select Shade whole shape instead, the gradient will be drawn all the way from the polygon border to the center.

The first group of tools in the Attributes toolbar allows us to select features on the map using the mouse. The following screenshot shows the Select Feature(s) tool. We can select a single feature by clicking on it, or select multiple features by drawing a rectangle. The other tools can be used to select features by drawing different shapes (polygons, freehand areas, or circles) around the features. All features that intersect with the drawn shape are selected.

The second type of select tool is called Select by Expression, and it is also available in the Attribute toolbar. It selects features based on expressions that can contain references and functions that use feature attributes and/or geometry. The list of available functions in the center of the dialog is pretty long, but we can use the search box at the top of the list to filter it by name and find the function we are looking for faster. On the right-hand side of the window, we find the function help, which explains the functionality and how to use the function in an expression. The function list also shows the layer attribute fields, and by clicking on all unique or 10 samples, we can easily access their content. We can choose between creating a new selection or adding to or deleting from an existing selection. Additionally, we can choose to only select features from within an existing selection. Let's take a look at some example expressions that you can build on and use in your own work:

The third type of tool is called Spatial Query and allows us to select features in one layer based on their location relative to features in a second layer. These tools can be accessed by going to Vector | Research Tools | Select by location and Vector | Spatial Query | Spatial Query. Enable it in Plugin Manager if you cannot find it in the Vector menu. In general, we want to use the Spatial Query plugin as it supports a variety of spatial operations such as Crosses, Equals, Intersects, Is disjoint, Overlaps, Touches, and Contains, depending on the layer geometry type.

Let's test the Spatial Query plugin using railroads.shp and pipelines.shp from the sample data. For example, we might want to find all railroad features that cross a pipeline; therefore, we select the railroads layer, the Crosses operation, and the pipelines layer. After we've clicked on Apply, the plugin presents us with the query results. There is a list of IDs of the result features on the right-hand side of the window, as you can see in the next screenshot. Below this list, we can check the Zoom to item box, and QGIS will zoom into the feature that belongs to the selected ID. Additionally, the plugin offers buttons for direct saving of all the resulting features to a new layer:

The first group of tools in the Attributes toolbar allows us to select features on the map using the mouse. The following screenshot shows the Select Feature(s) tool. We can select a single feature by clicking on it, or select multiple features by drawing a rectangle. The other tools can be used to select features by drawing different shapes (polygons, freehand areas, or circles) around the features. All features that intersect with the drawn shape are selected.

The second type of select tool is called Select by Expression, and it is also available in the Attribute toolbar. It selects features based on expressions that can contain references and functions that use feature attributes and/or geometry. The list of available functions in the center of the dialog is pretty long, but we can use the search box at the top of the list to filter it by name and find the function we are looking for faster. On the right-hand side of the window, we find the function help, which explains the functionality and how to use the function in an expression. The function list also shows the layer attribute fields, and by clicking on all unique or 10 samples, we can easily access their content. We can choose between creating a new selection or adding to or deleting from an existing selection. Additionally, we can choose to only select features from within an existing selection. Let's take a look at some example expressions that you can build on and use in your own work:

The third type of tool is called Spatial Query and allows us to select features in one layer based on their location relative to features in a second layer. These tools can be accessed by going to Vector | Research Tools | Select by location and Vector | Spatial Query | Spatial Query. Enable it in Plugin Manager if you cannot find it in the Vector menu. In general, we want to use the Spatial Query plugin as it supports a variety of spatial operations such as Crosses, Equals, Intersects, Is disjoint, Overlaps, Touches, and Contains, depending on the layer geometry type.

Let's test the Spatial Query plugin using railroads.shp and pipelines.shp from the sample data. For example, we might want to find all railroad features that cross a pipeline; therefore, we select the railroads layer, the Crosses operation, and the pipelines layer. After we've clicked on Apply, the plugin presents us with the query results. There is a list of IDs of the result features on the right-hand side of the window, as you can see in the next screenshot. Below this list, we can check the Zoom to item box, and QGIS will zoom into the feature that belongs to the selected ID. Additionally, the plugin offers buttons for direct saving of all the resulting features to a new layer:

The second type of select tool is called Select by Expression, and it is also available in the Attribute toolbar. It selects features based on expressions that can contain references and functions that use feature attributes and/or geometry. The list of available functions in the center of the dialog is pretty long, but we can use the search box at the top of the list to filter it by name and find the function we are looking for faster. On the right-hand side of the window, we find the function help, which explains the functionality and how to use the function in an expression. The function list also shows the layer attribute fields, and by clicking on all unique or 10 samples, we can easily access their content. We can choose between creating a new selection or adding to or deleting from an existing selection. Additionally, we can choose to only select features from within an existing selection. Let's take a look at some example expressions that you can build on and use in your own work:

The third type of tool is called Spatial Query and allows us to select features in one layer based on their location relative to features in a second layer. These tools can be accessed by going to Vector | Research Tools | Select by location and Vector | Spatial Query | Spatial Query. Enable it in Plugin Manager if you cannot find it in the Vector menu. In general, we want to use the Spatial Query plugin as it supports a variety of spatial operations such as Crosses, Equals, Intersects, Is disjoint, Overlaps, Touches, and Contains, depending on the layer geometry type.

Let's test the Spatial Query plugin using railroads.shp and pipelines.shp from the sample data. For example, we might want to find all railroad features that cross a pipeline; therefore, we select the railroads layer, the Crosses operation, and the pipelines layer. After we've clicked on Apply, the plugin presents us with the query results. There is a list of IDs of the result features on the right-hand side of the window, as you can see in the next screenshot. Below this list, we can check the Zoom to item box, and QGIS will zoom into the feature that belongs to the selected ID. Additionally, the plugin offers buttons for direct saving of all the resulting features to a new layer:

The third type of tool is called Spatial Query and allows us to select features in one layer based on their location relative to features in a second layer. These tools can be accessed by going to Vector | Research Tools | Select by location and Vector | Spatial Query | Spatial Query. Enable it in Plugin Manager if you cannot find it in the Vector menu. In general, we want to use the Spatial Query plugin as it supports a variety of spatial operations such as Crosses, Equals, Intersects, Is disjoint, Overlaps, Touches, and Contains, depending on the layer geometry type.

Let's test the Spatial Query plugin using railroads.shp and pipelines.shp from the sample data. For example, we might want to find all railroad features that cross a pipeline; therefore, we select the railroads layer, the Crosses operation, and the pipelines layer. After we've clicked on Apply, the plugin presents us with the query results. There is a list of IDs of the result features on the right-hand side of the window, as you can see in the next screenshot. Below this list, we can check the Zoom to item box, and QGIS will zoom into the feature that belongs to the selected ID. Additionally, the plugin offers buttons for direct saving of all the resulting features to a new layer:

This is the basic Digitizing toolbar:

The Digitizing toolbar contains tools that we can use to create and move features and nodes as well as delete, copy, cut, and paste features, as follows:

The Advanced Digitizing toolbar offers very useful Undo and Redo functionalities as well as additional tools for more involved geometry editing, as shown in the following screenshot:

The Advanced Digitizing tools include the following:

One of the challenges of digitizing features by hand is avoiding undesired gaps or overlapping features. To make it easier to avoid these issues, QGIS offers a snapping functionality. To configure snapping, we go to Settings | Snapping options. The following screenshot shows how to enable snapping for the Current layer. Similarly, you can choose snapping modes for All layers or the Advanced mode, where you can control the settings for each layer separately. In the example shown in the following screenshot, we enable snapping To vertex. This means that digitizing tools will automatically snap to vertices/nodes of existing features in the current layer. Similarly, you can enable snapping To segment or To vertex and segment. When snapping is enabled during digitizing, you will notice bold cross-shaped markers appearing whenever you go close to a vertex or segment that can be snapped to:

This is the basic Digitizing toolbar:

The Digitizing toolbar contains tools that we can use to create and move features and nodes as well as delete, copy, cut, and paste features, as follows:

The Advanced Digitizing toolbar offers very useful Undo and Redo functionalities as well as additional tools for more involved geometry editing, as shown in the following screenshot:

The Advanced Digitizing tools include the following:

One of the challenges of digitizing features by hand is avoiding undesired gaps or overlapping features. To make it easier to avoid these issues, QGIS offers a snapping functionality. To configure snapping, we go to Settings | Snapping options. The following screenshot shows how to enable snapping for the Current layer. Similarly, you can choose snapping modes for All layers or the Advanced mode, where you can control the settings for each layer separately. In the example shown in the following screenshot, we enable snapping To vertex. This means that digitizing tools will automatically snap to vertices/nodes of existing features in the current layer. Similarly, you can enable snapping To segment or To vertex and segment. When snapping is enabled during digitizing, you will notice bold cross-shaped markers appearing whenever you go close to a vertex or segment that can be snapped to:

The Advanced Digitizing toolbar offers very useful Undo and Redo functionalities as well as additional tools for more involved geometry editing, as shown in the following screenshot:

The Advanced Digitizing tools include the following:

One of the challenges of digitizing features by hand is avoiding undesired gaps or overlapping features. To make it easier to avoid these issues, QGIS offers a snapping functionality. To configure snapping, we go to Settings | Snapping options. The following screenshot shows how to enable snapping for the Current layer. Similarly, you can choose snapping modes for All layers or the Advanced mode, where you can control the settings for each layer separately. In the example shown in the following screenshot, we enable snapping To vertex. This means that digitizing tools will automatically snap to vertices/nodes of existing features in the current layer. Similarly, you can enable snapping To segment or To vertex and segment. When snapping is enabled during digitizing, you will notice bold cross-shaped markers appearing whenever you go close to a vertex or segment that can be snapped to:

One of the challenges of digitizing features by hand is avoiding undesired gaps or overlapping features. To make it easier to avoid these issues, QGIS offers a snapping functionality. To configure snapping, we go to Settings | Snapping options. The following screenshot shows how to enable snapping for the Current layer. Similarly, you can choose snapping modes for All layers or the Advanced mode, where you can control the settings for each layer separately. In the example shown in the following screenshot, we enable snapping To vertex. This means that digitizing tools will automatically snap to vertices/nodes of existing features in the current layer. Similarly, you can enable snapping To segment or To vertex and segment. When snapping is enabled during digitizing, you will notice bold cross-shaped markers appearing whenever you go close to a vertex or segment that can be snapped to:

All three use cases are covered by the functionality available through the attribute table. We can access it by going to Layer | Open Attribute Table, using the Open Attribute Table button present in the Attributes toolbar, or in the layer name context menu.

Besides the classic attribute table view, QGIS also supports a form view, which you can see in the lower dialog of the previous image. You can switch between these two views using the buttons in the bottom-right corner of the attribute table dialog.

Another option to edit the attributes of one feature is to open the attribute form directly by clicking on the feature on the map using the Identify tool. By default, the Identify tool displays the attribute values in read mode, but we can enable the Auto open form option in the Identify Results panel, as shown here:

What you can see in the previous screenshot is the default feature attributes form that QGIS creates automatically, but we are not limited to this basic form. By going to Layer Properties | Fields section, we can configure the look and feel of the form in greater detail. The Attribute editor layout options are (in an increasing level of complexity) autogenerate, Drag and drop designer, and providing a .ui file. These options are described in detail as follows.

If we want to change the attributes of multiple or all features in a layer, editing them manually usually isn't an option. This is what the Field calculator is good for. We can access it using the Open field calculator button in the attribute table, or by pressing Ctrl + I. In the Field calculator, we can choose to update only the selected features or update all the features in the layer. Besides updating an existing field, we can also create a new field. The function list is the same one that we explored when we selected features by expression. We can use any of the functions and variables in this list to populate a new field or update an existing one. Here are some example expressions that are often used:

An alternative to the Field calculator—especially if you already know the formula you want to use—is the field calculator bar, which you can find directly in the Attribute table dialog right below the toolbar. In the next screenshot, you can see an example that calculates the area of all census areas (use the New Field button to add a Decimal number field called CENSUSAREA first). This example uses a CASE WHEN – THEN – END expression to check whether the value of TYPE_2 is Census Area:

CASE WHEN TYPE_2 = 'Census Area' THEN $area / 27878400 END

if(TYPE_2 = 'Census Area', $area / 27878400, CENSUSAREA)

if(TYPE_2 = 'Census Area', $area / 27878400, NULL)

Enter the formula and click on the Update All button to execute it:

Since it is not possible to directly change a field data type in a Shapefile or SpatiaLite attribute table, the field calculator and calculator bar are also used to create new fields with the desired properties and then populate them with the values from the original column.

All three use cases are covered by the functionality available through the attribute table. We can access it by going to Layer | Open Attribute Table, using the Open Attribute Table button present in the Attributes toolbar, or in the layer name context menu.

Besides the classic attribute table view, QGIS also supports a form view, which you can see in the lower dialog of the previous image. You can switch between these two views using the buttons in the bottom-right corner of the attribute table dialog.

Another option to edit the attributes of one feature is to open the attribute form directly by clicking on the feature on the map using the Identify tool. By default, the Identify tool displays the attribute values in read mode, but we can enable the Auto open form option in the Identify Results panel, as shown here:

What you can see in the previous screenshot is the default feature attributes form that QGIS creates automatically, but we are not limited to this basic form. By going to Layer Properties | Fields section, we can configure the look and feel of the form in greater detail. The Attribute editor layout options are (in an increasing level of complexity) autogenerate, Drag and drop designer, and providing a .ui file. These options are described in detail as follows.

If we want to change the attributes of multiple or all features in a layer, editing them manually usually isn't an option. This is what the Field calculator is good for. We can access it using the Open field calculator button in the attribute table, or by pressing Ctrl + I. In the Field calculator, we can choose to update only the selected features or update all the features in the layer. Besides updating an existing field, we can also create a new field. The function list is the same one that we explored when we selected features by expression. We can use any of the functions and variables in this list to populate a new field or update an existing one. Here are some example expressions that are often used:

An alternative to the Field calculator—especially if you already know the formula you want to use—is the field calculator bar, which you can find directly in the Attribute table dialog right below the toolbar. In the next screenshot, you can see an example that calculates the area of all census areas (use the New Field button to add a Decimal number field called CENSUSAREA first). This example uses a CASE WHEN – THEN – END expression to check whether the value of TYPE_2 is Census Area:

CASE WHEN TYPE_2 = 'Census Area' THEN $area / 27878400 END

if(TYPE_2 = 'Census Area', $area / 27878400, CENSUSAREA)

if(TYPE_2 = 'Census Area', $area / 27878400, NULL)

Enter the formula and click on the Update All button to execute it:

Since it is not possible to directly change a field data type in a Shapefile or SpatiaLite attribute table, the field calculator and calculator bar are also used to create new fields with the desired properties and then populate them with the values from the original column.

Another option to edit the attributes of one feature is to open the attribute form directly by clicking on the feature on the map using the Identify tool. By default, the Identify tool displays the attribute values in read mode, but we can enable the Auto open form option in the Identify Results panel, as shown here:

What you can see in the previous screenshot is the default feature attributes form that QGIS creates automatically, but we are not limited to this basic form. By going to Layer Properties | Fields section, we can configure the look and feel of the form in greater detail. The Attribute editor layout options are (in an increasing level of complexity) autogenerate, Drag and drop designer, and providing a .ui file. These options are described in detail as follows.

If we want to change the attributes of multiple or all features in a layer, editing them manually usually isn't an option. This is what the Field calculator is good for. We can access it using the Open field calculator button in the attribute table, or by pressing Ctrl + I. In the Field calculator, we can choose to update only the selected features or update all the features in the layer. Besides updating an existing field, we can also create a new field. The function list is the same one that we explored when we selected features by expression. We can use any of the functions and variables in this list to populate a new field or update an existing one. Here are some example expressions that are often used:

An alternative to the Field calculator—especially if you already know the formula you want to use—is the field calculator bar, which you can find directly in the Attribute table dialog right below the toolbar. In the next screenshot, you can see an example that calculates the area of all census areas (use the New Field button to add a Decimal number field called CENSUSAREA first). This example uses a CASE WHEN – THEN – END expression to check whether the value of TYPE_2 is Census Area:

CASE WHEN TYPE_2 = 'Census Area' THEN $area / 27878400 END

if(TYPE_2 = 'Census Area', $area / 27878400, CENSUSAREA)

if(TYPE_2 = 'Census Area', $area / 27878400, NULL)

Enter the formula and click on the Update All button to execute it:

Since it is not possible to directly change a field data type in a Shapefile or SpatiaLite attribute table, the field calculator and calculator bar are also used to create new fields with the desired properties and then populate them with the values from the original column.

If we want to change the attributes of multiple or all features in a layer, editing them manually usually isn't an option. This is what the Field calculator is good for. We can access it using the Open field calculator button in the attribute table, or by pressing Ctrl + I. In the Field calculator, we can choose to update only the selected features or update all the features in the layer. Besides updating an existing field, we can also create a new field. The function list is the same one that we explored when we selected features by expression. We can use any of the functions and variables in this list to populate a new field or update an existing one. Here are some example expressions that are often used:

An alternative to the Field calculator—especially if you already know the formula you want to use—is the field calculator bar, which you can find directly in the Attribute table dialog right below the toolbar. In the next screenshot, you can see an example that calculates the area of all census areas (use the New Field button to add a Decimal number field called CENSUSAREA first). This example uses a CASE WHEN – THEN – END expression to check whether the value of TYPE_2 is Census Area:

CASE WHEN TYPE_2 = 'Census Area' THEN $area / 27878400 END

if(TYPE_2 = 'Census Area', $area / 27878400, CENSUSAREA)

if(TYPE_2 = 'Census Area', $area / 27878400, NULL)

Enter the formula and click on the Update All button to execute it:

Since it is not possible to directly change a field data type in a Shapefile or SpatiaLite attribute table, the field calculator and calculator bar are also used to create new fields with the desired properties and then populate them with the values from the original column.

If we want to change the attributes of multiple or all features in a layer, editing them manually usually isn't an option. This is what the Field calculator is good for. We can access it using the Open field calculator button in the attribute table, or by pressing Ctrl + I. In the Field calculator, we can choose to update only the selected features or update all the features in the layer. Besides updating an existing field, we can also create a new field. The function list is the same one that we explored when we selected features by expression. We can use any of the functions and variables in this list to populate a new field or update an existing one. Here are some example expressions that are often used:

An alternative to the Field calculator—especially if you already know the formula you want to use—is the field calculator bar, which you can find directly in the Attribute table dialog right below the toolbar. In the next screenshot, you can see an example that calculates the area of all census areas (use the New Field button to add a Decimal number field called CENSUSAREA first). This example uses a CASE WHEN – THEN – END expression to check whether the value of TYPE_2 is Census Area:

CASE WHEN TYPE_2 = 'Census Area' THEN $area / 27878400 END

if(TYPE_2 = 'Census Area', $area / 27878400, CENSUSAREA)

if(TYPE_2 = 'Census Area', $area / 27878400, NULL)

Enter the formula and click on the Update All button to execute it:

Since it is not possible to directly change a field data type in a Shapefile or SpatiaLite attribute table, the field calculator and calculator bar are also used to create new fields with the desired properties and then populate them with the values from the original column.

If we want to change the attributes of multiple or all features in a layer, editing them manually usually isn't an option. This is what the Field calculator is good for. We can access it using the Open field calculator button in the attribute table, or by pressing Ctrl + I. In the Field calculator, we can choose to update only the selected features or update all the features in the layer. Besides updating an existing field, we can also create a new field. The function list is the same one that we explored when we selected features by expression. We can use any of the functions and variables in this list to populate a new field or update an existing one. Here are some example expressions that are often used:

An alternative to the Field calculator—especially if you already know the formula you want to use—is the field calculator bar, which you can find directly in the Attribute table dialog right below the toolbar. In the next screenshot, you can see an example that calculates the area of all census areas (use the New Field button to add a Decimal number field called CENSUSAREA first). This example uses a CASE WHEN – THEN – END expression to check whether the value of TYPE_2 is Census Area:

CASE WHEN TYPE_2 = 'Census Area' THEN $area / 27878400 END

if(TYPE_2 = 'Census Area', $area / 27878400, CENSUSAREA)

if(TYPE_2 = 'Census Area', $area / 27878400, NULL)

Enter the formula and click on the Update All button to execute it:

Since it is not possible to directly change a field data type in a Shapefile or SpatiaLite attribute table, the field calculator and calculator bar are also used to create new fields with the desired properties and then populate them with the values from the original column.

If we want to change the attributes of multiple or all features in a layer, editing them manually usually isn't an option. This is what the Field calculator is good for. We can access it using the Open field calculator button in the attribute table, or by pressing Ctrl + I. In the Field calculator, we can choose to update only the selected features or update all the features in the layer. Besides updating an existing field, we can also create a new field. The function list is the same one that we explored when we selected features by expression. We can use any of the functions and variables in this list to populate a new field or update an existing one. Here are some example expressions that are often used:

An alternative to the Field calculator—especially if you already know the formula you want to use—is the field calculator bar, which you can find directly in the Attribute table dialog right below the toolbar. In the next screenshot, you can see an example that calculates the area of all census areas (use the New Field button to add a Decimal number field called CENSUSAREA first). This example uses a CASE WHEN – THEN – END expression to check whether the value of TYPE_2 is Census Area:

CASE WHEN TYPE_2 = 'Census Area' THEN $area / 27878400 END

if(TYPE_2 = 'Census Area', $area / 27878400, CENSUSAREA)

if(TYPE_2 = 'Census Area', $area / 27878400, NULL)

Enter the formula and click on the Update All button to execute it:

Since it is not possible to directly change a field data type in a Shapefile or SpatiaLite attribute table, the field calculator and calculator bar are also used to create new fields with the desired properties and then populate them with the values from the original column.

After loading the tabular data from either the spreadsheet or text file, we can continue to join this non-spatial data to a vector layer (for instance, our airports.shp dataset, as shown in the following example). To do this, we go to the vector's Layer Properties | Joins section. Here, we can add a new join by clicking on the green plus button. All we have to do is select the tabular Join layer and Join field (of the tabular layer), which will contain values that match those in the Target field (of the vector layer). Additionally, we can—if we want to—select a subset of the fields to be joined by enabling the Choose which fields are joined option. For example, the settings shown in the following screenshot will add only the some value field. Additionally, we use a Custom field name prefix instead of using the entire join layer name, which would be the default option.

Once the join is added, we can see the extended attribute table and use the new appended attributes (as shown in the following screenshot) for styling and labeling. The way joins work in QGIS is as follows: the join layer's attributes are appended to the original layer's attribute table. The number of features in the original layer is not changed. Whenever there is a match between the join and the target field, the attribute value is filled in; otherwise, you see NULL entries.

You can save the joined layer permanently using Save as… to create the new file.

After loading the tabular data from either the spreadsheet or text file, we can continue to join this non-spatial data to a vector layer (for instance, our airports.shp dataset, as shown in the following example). To do this, we go to the vector's Layer Properties | Joins section. Here, we can add a new join by clicking on the green plus button. All we have to do is select the tabular Join layer and Join field (of the tabular layer), which will contain values that match those in the Target field (of the vector layer). Additionally, we can—if we want to—select a subset of the fields to be joined by enabling the Choose which fields are joined option. For example, the settings shown in the following screenshot will add only the some value field. Additionally, we use a Custom field name prefix instead of using the entire join layer name, which would be the default option.

Once the join is added, we can see the extended attribute table and use the new appended attributes (as shown in the following screenshot) for styling and labeling. The way joins work in QGIS is as follows: the join layer's attributes are appended to the original layer's attribute table. The number of features in the original layer is not changed. Whenever there is a match between the join and the target field, the attribute value is filled in; otherwise, you see NULL entries.

You can save the joined layer permanently using Save as… to create the new file.

Once the join is added, we can see the extended attribute table and use the new appended attributes (as shown in the following screenshot) for styling and labeling. The way joins work in QGIS is as follows: the join layer's attributes are appended to the original layer's attribute table. The number of features in the original layer is not changed. Whenever there is a match between the join and the target field, the attribute value is filled in; otherwise, you see NULL entries.

You can save the joined layer permanently using Save as… to create the new file.

In QGIS, we can use the Topology Checker plugin; it is installed by default and is accessible via the Vector menu Topology Checker entry (if you cannot find the menu entry, you might have to enable the plugin in Plugin Manager). When the plugin is activated, it adds a Topology Checker Panel to the QGIS window. This panel can be used to configure and run different topology checks and will list the detected errors.

To see the Topology Checker in action, we create a temporary scratch layer with polygon geometries and digitize some polygons, as shown in the following screenshot. Make sure you use snapping to create polygons that touch but don't overlap. These could, for example, represent a group of row houses. When the polygons are ready, we can set up the topology rules we want to check for. Click on the Configure button in Topology Checker Panel to open the Topology Rule Settings dialog. Here, we can manage all the topology rules for our project data. For example, in the following screenshot, you can see the rules we might want to configure for our polygon layer, including these:

Once the rules are set up, we can close the settings dialog and click on the Validate All button in Topology Checker Panel to start running the topology rule checks. If you have been careful while creating the polygons, the checker will not find any errors and the status at the bottom of Topology Checker Panel will display this message: 0 errors were found. Let's change that by introducing some topology errors.

For example, if we move one vertex so that two polygons end up overlapping each other and then click on Validate All, we get the error shown in the next screenshot. Note that the error type and the affected layer and feature are displayed in Topology Checker Panel. Additionally, since the Show errors option is enabled, the problematic geometry part is highlighted in red on the map:

Of course, it is also possible to create rules that describe the relationship between features in different layers. For example, the following screenshot shows a point and a polygon layer where the rules state that each point should be inside a polygon and each polygon should contain a point:

Selecting an error from the list of errors in the panel centres the map on the problematic location so that we can start fixing it, for example, by moving the lone point into the empty polygon.

Sometimes, fixing all errors manually can be a lot of work. Luckily, certain errors can be addressed automatically. For example, the common error of self-intersecting polygons, which cause invalid geometry errors (as illustrated in the following screenshot), is often the result of intersecting polygon nodes or edges. These issues can often be resolved using a buffer tool (for example, Fixed distance buffer in the Processing Toolbox, which we will discuss in more detail in Chapter 4, Spatial Analysis) with the buffer Distance set to 0. Buffering will, for example, fix the self-intersecting polygon on the left-hand side of the following screenshot by removing the self-intersecting nodes and constructing a valid polygon with a hole (as depicted on the right-hand side):

Another common issue that can be fixed automatically is so-called sliver polygons. These are small, and often quite thin, polygons that can be the result of spatial processes such as intersection operations. To get rid of these sliver polygons, we can use the v.clean tool with the Cleaning tool option set to rmarea (meaning "remove area"), which is also available through the Processing Toolbox. In the example shown in this screenshot, the Threshold value of 10000 tells the tool to remove all polygons with an area less than 10,000 square meters by merging them with the neighboring polygon with the longest common boundary:

In QGIS, we can use the Topology Checker plugin; it is installed by default and is accessible via the Vector menu Topology Checker entry (if you cannot find the menu entry, you might have to enable the plugin in Plugin Manager). When the plugin is activated, it adds a Topology Checker Panel to the QGIS window. This panel can be used to configure and run different topology checks and will list the detected errors.

To see the Topology Checker in action, we create a temporary scratch layer with polygon geometries and digitize some polygons, as shown in the following screenshot. Make sure you use snapping to create polygons that touch but don't overlap. These could, for example, represent a group of row houses. When the polygons are ready, we can set up the topology rules we want to check for. Click on the Configure button in Topology Checker Panel to open the Topology Rule Settings dialog. Here, we can manage all the topology rules for our project data. For example, in the following screenshot, you can see the rules we might want to configure for our polygon layer, including these:

Once the rules are set up, we can close the settings dialog and click on the Validate All button in Topology Checker Panel to start running the topology rule checks. If you have been careful while creating the polygons, the checker will not find any errors and the status at the bottom of Topology Checker Panel will display this message: 0 errors were found. Let's change that by introducing some topology errors.

For example, if we move one vertex so that two polygons end up overlapping each other and then click on Validate All, we get the error shown in the next screenshot. Note that the error type and the affected layer and feature are displayed in Topology Checker Panel. Additionally, since the Show errors option is enabled, the problematic geometry part is highlighted in red on the map:

Of course, it is also possible to create rules that describe the relationship between features in different layers. For example, the following screenshot shows a point and a polygon layer where the rules state that each point should be inside a polygon and each polygon should contain a point:

Selecting an error from the list of errors in the panel centres the map on the problematic location so that we can start fixing it, for example, by moving the lone point into the empty polygon.

Sometimes, fixing all errors manually can be a lot of work. Luckily, certain errors can be addressed automatically. For example, the common error of self-intersecting polygons, which cause invalid geometry errors (as illustrated in the following screenshot), is often the result of intersecting polygon nodes or edges. These issues can often be resolved using a buffer tool (for example, Fixed distance buffer in the Processing Toolbox, which we will discuss in more detail in Chapter 4, Spatial Analysis) with the buffer Distance set to 0. Buffering will, for example, fix the self-intersecting polygon on the left-hand side of the following screenshot by removing the self-intersecting nodes and constructing a valid polygon with a hole (as depicted on the right-hand side):

Another common issue that can be fixed automatically is so-called sliver polygons. These are small, and often quite thin, polygons that can be the result of spatial processes such as intersection operations. To get rid of these sliver polygons, we can use the v.clean tool with the Cleaning tool option set to rmarea (meaning "remove area"), which is also available through the Processing Toolbox. In the example shown in this screenshot, the Threshold value of 10000 tells the tool to remove all polygons with an area less than 10,000 square meters by merging them with the neighboring polygon with the longest common boundary:

Sometimes, fixing all errors manually can be a lot of work. Luckily, certain errors can be addressed automatically. For example, the common error of self-intersecting polygons, which cause invalid geometry errors (as illustrated in the following screenshot), is often the result of intersecting polygon nodes or edges. These issues can often be resolved using a buffer tool (for example, Fixed distance buffer in the Processing Toolbox, which we will discuss in more detail in Chapter 4, Spatial Analysis) with the buffer Distance set to 0. Buffering will, for example, fix the self-intersecting polygon on the left-hand side of the following screenshot by removing the self-intersecting nodes and constructing a valid polygon with a hole (as depicted on the right-hand side):

Another common issue that can be fixed automatically is so-called sliver polygons. These are small, and often quite thin, polygons that can be the result of spatial processes such as intersection operations. To get rid of these sliver polygons, we can use the v.clean tool with the Cleaning tool option set to rmarea (meaning "remove area"), which is also available through the Processing Toolbox. In the example shown in this screenshot, the Threshold value of 10000 tells the tool to remove all polygons with an area less than 10,000 square meters by merging them with the neighboring polygon with the longest common boundary:

Tools for converting between raster and vector formats can be accessed by going to Raster | Conversion. These tools are called Rasterize (Vector to raster) and Polygonize (Raster to vector). Like the raster clipper tool that we used before, these tools are also based on GDAL and display the command at the bottom of the dialog.

Polygonize converts a raster into a polygon layer. Depending on the size of the raster, the conversion can take some time. When the process is finished, QGIS will notify us with a popup. For a quick test, we can, for example, convert the reclassified landcover raster to polygons. The resulting vector polygon layer contains multiple polygonal features with a single attribute, which we name lc; it depends on the original raster value, as shown in the following screenshot:

Using the Rasterize tool is very similar to using the Polygonize tool. The only difference is that we get to specify the size of the resulting raster in pixels/cells. We can also specify the attribute field, which will provide input for the raster cell value, as shown in the next screenshot. In this case, the cat attribute of our alaska.shp dataset is rather meaningless, but you get the idea of how the tool works:

Whenever we get a new dataset, it is useful to examine the layer statistics to get an idea of the data it contains, such as the minimum and maximum values, number of features, and much more. QGIS offers a variety of tools to explore these values.

Raster layer statistics are readily available in the Layer Properties dialog, specifically in the following tabs:

For vector layers, we can get summary statistics using two tools in Vector | Analysis Tools:

In both tools, we can easily copy the results using Ctrl + C and paste them in a text file or spreadsheet. The following image shows examples of exploring the contents of our airports sample dataset:

An alternative to the Basics statistics tool is the Statistics Panel, which you can activate by going to View | Panels | Statistics Panel. As shown in the following screenshot, this panel can be customized to show exactly those statistics that you are interested in:

Instead of computing raster statistics for the entire layer, it is sometimes necessary to compute statistics for selected regions. This is what the Zonal statistics plugin is good for. This plugin is installed by default and can be enabled in the Plugin Manager.

For example, we can compute elevation statistics for areas around each airport using srtm_05_01.tif and airports.shp from our sample data:

Heatmaps are great for visualizing a distribution of points. To create them, QGIS provides a simple-to-use Heatmap Plugin, which we have to activate in the Plugin Manager, and then we can access it by going to Raster | Heatmap | Heatmap. The plugin offers different Kernel shapes to choose from. The kernel is a moving window of a specific size and shape that moves over an area of points to calculate their local density. Additionally, the plugin allows us to control the output heatmap raster size in cells (using the Rows and Columns settings) as well as the cell size.

The following screenshot shows us how to create a heatmap of our airports.shp sample with a kernel radius of 300,000 layer units, which in the case of our airport data is in feet:

By default, the heatmap output will be rendered using the Singleband gray render type (with low raster values in black and high values in white). To change the style to something similar to what you saw in the previous screenshot, you can do the following:

Tools for converting between raster and vector formats can be accessed by going to Raster | Conversion. These tools are called Rasterize (Vector to raster) and Polygonize (Raster to vector). Like the raster clipper tool that we used before, these tools are also based on GDAL and display the command at the bottom of the dialog.

Polygonize converts a raster into a polygon layer. Depending on the size of the raster, the conversion can take some time. When the process is finished, QGIS will notify us with a popup. For a quick test, we can, for example, convert the reclassified landcover raster to polygons. The resulting vector polygon layer contains multiple polygonal features with a single attribute, which we name lc; it depends on the original raster value, as shown in the following screenshot:

Using the Rasterize tool is very similar to using the Polygonize tool. The only difference is that we get to specify the size of the resulting raster in pixels/cells. We can also specify the attribute field, which will provide input for the raster cell value, as shown in the next screenshot. In this case, the cat attribute of our alaska.shp dataset is rather meaningless, but you get the idea of how the tool works:

Whenever we get a new dataset, it is useful to examine the layer statistics to get an idea of the data it contains, such as the minimum and maximum values, number of features, and much more. QGIS offers a variety of tools to explore these values.

Raster layer statistics are readily available in the Layer Properties dialog, specifically in the following tabs:

For vector layers, we can get summary statistics using two tools in Vector | Analysis Tools:

In both tools, we can easily copy the results using Ctrl + C and paste them in a text file or spreadsheet. The following image shows examples of exploring the contents of our airports sample dataset:

An alternative to the Basics statistics tool is the Statistics Panel, which you can activate by going to View | Panels | Statistics Panel. As shown in the following screenshot, this panel can be customized to show exactly those statistics that you are interested in:

Instead of computing raster statistics for the entire layer, it is sometimes necessary to compute statistics for selected regions. This is what the Zonal statistics plugin is good for. This plugin is installed by default and can be enabled in the Plugin Manager.

For example, we can compute elevation statistics for areas around each airport using srtm_05_01.tif and airports.shp from our sample data:

Heatmaps are great for visualizing a distribution of points. To create them, QGIS provides a simple-to-use Heatmap Plugin, which we have to activate in the Plugin Manager, and then we can access it by going to Raster | Heatmap | Heatmap. The plugin offers different Kernel shapes to choose from. The kernel is a moving window of a specific size and shape that moves over an area of points to calculate their local density. Additionally, the plugin allows us to control the output heatmap raster size in cells (using the Rows and Columns settings) as well as the cell size.

The following screenshot shows us how to create a heatmap of our airports.shp sample with a kernel radius of 300,000 layer units, which in the case of our airport data is in feet:

By default, the heatmap output will be rendered using the Singleband gray render type (with low raster values in black and high values in white). To change the style to something similar to what you saw in the previous screenshot, you can do the following:

Whenever we get a new dataset, it is useful to examine the layer statistics to get an idea of the data it contains, such as the minimum and maximum values, number of features, and much more. QGIS offers a variety of tools to explore these values.

Raster layer statistics are readily available in the Layer Properties dialog, specifically in the following tabs:

For vector layers, we can get summary statistics using two tools in Vector | Analysis Tools:

In both tools, we can easily copy the results using Ctrl + C and paste them in a text file or spreadsheet. The following image shows examples of exploring the contents of our airports sample dataset:

An alternative to the Basics statistics tool is the Statistics Panel, which you can activate by going to View | Panels | Statistics Panel. As shown in the following screenshot, this panel can be customized to show exactly those statistics that you are interested in:

Instead of computing raster statistics for the entire layer, it is sometimes necessary to compute statistics for selected regions. This is what the Zonal statistics plugin is good for. This plugin is installed by default and can be enabled in the Plugin Manager.

For example, we can compute elevation statistics for areas around each airport using srtm_05_01.tif and airports.shp from our sample data:

Heatmaps are great for visualizing a distribution of points. To create them, QGIS provides a simple-to-use Heatmap Plugin, which we have to activate in the Plugin Manager, and then we can access it by going to Raster | Heatmap | Heatmap. The plugin offers different Kernel shapes to choose from. The kernel is a moving window of a specific size and shape that moves over an area of points to calculate their local density. Additionally, the plugin allows us to control the output heatmap raster size in cells (using the Rows and Columns settings) as well as the cell size.

The following screenshot shows us how to create a heatmap of our airports.shp sample with a kernel radius of 300,000 layer units, which in the case of our airport data is in feet:

By default, the heatmap output will be rendered using the Singleband gray render type (with low raster values in black and high values in white). To change the style to something similar to what you saw in the previous screenshot, you can do the following:

Instead of computing raster statistics for the entire layer, it is sometimes necessary to compute statistics for selected regions. This is what the Zonal statistics plugin is good for. This plugin is installed by default and can be enabled in the Plugin Manager.

For example, we can compute elevation statistics for areas around each airport using srtm_05_01.tif and airports.shp from our sample data:

Heatmaps are great for visualizing a distribution of points. To create them, QGIS provides a simple-to-use Heatmap Plugin, which we have to activate in the Plugin Manager, and then we can access it by going to Raster | Heatmap | Heatmap. The plugin offers different Kernel shapes to choose from. The kernel is a moving window of a specific size and shape that moves over an area of points to calculate their local density. Additionally, the plugin allows us to control the output heatmap raster size in cells (using the Rows and Columns settings) as well as the cell size.

The following screenshot shows us how to create a heatmap of our airports.shp sample with a kernel radius of 300,000 layer units, which in the case of our airport data is in feet:

By default, the heatmap output will be rendered using the Singleband gray render type (with low raster values in black and high values in white). To change the style to something similar to what you saw in the previous screenshot, you can do the following:

Heatmaps are great for visualizing a distribution of points. To create them, QGIS provides a simple-to-use Heatmap Plugin, which we have to activate in the Plugin Manager, and then we can access it by going to Raster | Heatmap | Heatmap. The plugin offers different Kernel shapes to choose from. The kernel is a moving window of a specific size and shape that moves over an area of points to calculate their local density. Additionally, the plugin allows us to control the output heatmap raster size in cells (using the Rows and Columns settings) as well as the cell size.

The following screenshot shows us how to create a heatmap of our airports.shp sample with a kernel radius of 300,000 layer units, which in the case of our airport data is in feet:

By default, the heatmap output will be rendered using the Singleband gray render type (with low raster values in black and high values in white). To change the style to something similar to what you saw in the previous screenshot, you can do the following:

Finding nearest neighbors, for example, the airport nearest to a populated place, is a common task in geoprocessing. To find the nearest neighbor and create connections between input features and their nearest neighbor in another layer, we can use the Distance to nearest hub tool.

As shown in the next screenshot, we use the populated places as Source points layer and the airports as the Destination hubs layer. The Hub layer name attribute will be added to the result's attribute table to identify the nearest feature. Therefore, we select NAME to add the airport name to the populated places. There are two options for Output shape type:

It is recommended that you use Layer units as Measurement unit to avoid potential issues with wrong measurements:

It is often necessary to be able to convert between points, lines, and polygons, for example, to create lines from a series of points, or to extract the nodes of polygons and create a new point layer out of them. There are many tools that cover these different use cases. The following table provides an overview of the tools that are available in the Processing toolbox for conversion between points, lines, and polygons:

In general, it is easier to convert more complex representations to simpler ones (polygons to lines, polygons to points, or lines to points) than conversion in the other direction (points to lines, points to polygons, or lines to polygons). Here is a short overview of these tools:

One common spatial analysis task is to identify features in the proximity of certain other features. One example would be to find all airports near rivers. Using airports.shp and majrivers.shp from our sample data, we can find airports within 5,000 feet of a river by using a combination of the Fixed distance buffer and Select by location tools. Use the search box to find the tools in the Processing Toolbox. The tool configurations for this example are shown in the following screenshot:

After buffering the airport point locations, the Select by location tool selects all the airport buffers that intersect a river. As a result, 14 out of the 76 airports are selected. This information is displayed in the information area at the bottom of the QGIS main window, as shown in this screenshot:

If you ever forget which settings you used or need to check whether you have used the correct input layer, you can go to Processing | History. The ALGORITHM section lists all the algorithms that we have been running as well as the used settings, as shown in the following screenshot:

The commands listed under ALGORITHM can also be used to call Processing tools from the QGIS Python console, which can be activated by going to Plugins | Python Console. The Python commands shown in the following screenshot run the buffer algorithm (processing.runalg) and load the result into the map (processing.load):

Another common task is to sample a raster at specific point locations. Using Processing, we can solve this problem with a GRASS tool called v.sample. To use GRASS tools, make sure that GRASS is installed and Processing is configured correctly under Processing | Options and configuration. On an OSGeo4W default system, the configuration will look like what is shown here:

For this exercise, let's imagine we want to sample the landcover layer at the airport locations of our sample data. All we have to do is specify the vector layer containing the sample points and the raster layer that should be sampled. For this example, we can leave all other settings at their default values, as shown in the following screenshot. The tool not only samples the raster but also compares point attributes with the sampled raster value. However, we don't need this comparison in our current example:

Mapping the density of points using a hexagonal grid has become quite a popular alternative to creating heatmaps. Processing offers us a fast way to create such an analysis. There is already a pre-made script called Hex grid from layer bounds, which is available through the Processing scripts collection and can be downloaded using the Get scripts from on-line scripts collection tool. As you can see in the following screenshot, you just need to enable the script by ticking the checkbox and clicking OK:

Then, we can use this script to create a hexagonal grid that covers all points in the input layer. The dataset of populated places (popp.shp), is a good sample dataset for this exercise. Once the grid is ready, we can run Count points in polygon to calculate the statistics. The number of points will be stored in the NUMPOINTS column if you use the settings shown in the following screenshot:

Another spatial analysis task we often encounter is calculating area shares within a certain region, for example, landcover shares along one specific river. Using majrivers.shp and trees.shp, we can calculate the share of wooded area in a 10,000-foot-wide strip of land along the Susitna River:

Sometimes, we want to run the same tool repeatedly but with slightly different settings. For this use case, Processing offers the Batch Processing functionality. Let's use this tool to extract some samples from our airports layer using the Random extract tool:

Using the graphical modeler, we can turn entire geoprocessing and analysis workflows into automated models. We can then use these models to run complex geoprocessing tasks that involve multiple different tools in one go. To create a model, we go to Processing | Graphical modeler to open the modeler, where we can select from different Inputs and Algorithms for our model.

Let's create a model that automates the creation of hexagonal heatmaps!

Another useful feature is that we can specify a layer style that needs to be applied to the processing results automatically. This default style can be set using Edit rendering styles for outputs in the context menu of the created model in the toolbox, as shown in the following screenshot:

Models can easily be copied from one QGIS installation to another and shared with other users. To ensure the usability of the model, it is a good idea to write a short documentation. Processing provides a convenient Help editor; it can be accessed by clicking on the Edit model help button in the Processing modeler, as shown in this screenshot:

By default, the .model files are stored in your user directory. On Windows, it is C:\Users\<your_user_name>\.qgis2\processing\models, and on Linux and OS X, it is ~/.qgis2/processing/models.

You can copy these files and share them with others. To load a model from a file, use the loading tool by going to Models | Tools | Add model from file in the Processing Toolbox.

Finding nearest neighbors, for example, the airport nearest to a populated place, is a common task in geoprocessing. To find the nearest neighbor and create connections between input features and their nearest neighbor in another layer, we can use the Distance to nearest hub tool.

As shown in the next screenshot, we use the populated places as Source points layer and the airports as the Destination hubs layer. The Hub layer name attribute will be added to the result's attribute table to identify the nearest feature. Therefore, we select NAME to add the airport name to the populated places. There are two options for Output shape type:

It is recommended that you use Layer units as Measurement unit to avoid potential issues with wrong measurements:

It is often necessary to be able to convert between points, lines, and polygons, for example, to create lines from a series of points, or to extract the nodes of polygons and create a new point layer out of them. There are many tools that cover these different use cases. The following table provides an overview of the tools that are available in the Processing toolbox for conversion between points, lines, and polygons:

In general, it is easier to convert more complex representations to simpler ones (polygons to lines, polygons to points, or lines to points) than conversion in the other direction (points to lines, points to polygons, or lines to polygons). Here is a short overview of these tools:

One common spatial analysis task is to identify features in the proximity of certain other features. One example would be to find all airports near rivers. Using airports.shp and majrivers.shp from our sample data, we can find airports within 5,000 feet of a river by using a combination of the Fixed distance buffer and Select by location tools. Use the search box to find the tools in the Processing Toolbox. The tool configurations for this example are shown in the following screenshot:

After buffering the airport point locations, the Select by location tool selects all the airport buffers that intersect a river. As a result, 14 out of the 76 airports are selected. This information is displayed in the information area at the bottom of the QGIS main window, as shown in this screenshot:

If you ever forget which settings you used or need to check whether you have used the correct input layer, you can go to Processing | History. The ALGORITHM section lists all the algorithms that we have been running as well as the used settings, as shown in the following screenshot:

The commands listed under ALGORITHM can also be used to call Processing tools from the QGIS Python console, which can be activated by going to Plugins | Python Console. The Python commands shown in the following screenshot run the buffer algorithm (processing.runalg) and load the result into the map (processing.load):

Another common task is to sample a raster at specific point locations. Using Processing, we can solve this problem with a GRASS tool called v.sample. To use GRASS tools, make sure that GRASS is installed and Processing is configured correctly under Processing | Options and configuration. On an OSGeo4W default system, the configuration will look like what is shown here:

For this exercise, let's imagine we want to sample the landcover layer at the airport locations of our sample data. All we have to do is specify the vector layer containing the sample points and the raster layer that should be sampled. For this example, we can leave all other settings at their default values, as shown in the following screenshot. The tool not only samples the raster but also compares point attributes with the sampled raster value. However, we don't need this comparison in our current example:

Mapping the density of points using a hexagonal grid has become quite a popular alternative to creating heatmaps. Processing offers us a fast way to create such an analysis. There is already a pre-made script called Hex grid from layer bounds, which is available through the Processing scripts collection and can be downloaded using the Get scripts from on-line scripts collection tool. As you can see in the following screenshot, you just need to enable the script by ticking the checkbox and clicking OK:

Then, we can use this script to create a hexagonal grid that covers all points in the input layer. The dataset of populated places (popp.shp), is a good sample dataset for this exercise. Once the grid is ready, we can run Count points in polygon to calculate the statistics. The number of points will be stored in the NUMPOINTS column if you use the settings shown in the following screenshot:

Another spatial analysis task we often encounter is calculating area shares within a certain region, for example, landcover shares along one specific river. Using majrivers.shp and trees.shp, we can calculate the share of wooded area in a 10,000-foot-wide strip of land along the Susitna River:

Sometimes, we want to run the same tool repeatedly but with slightly different settings. For this use case, Processing offers the Batch Processing functionality. Let's use this tool to extract some samples from our airports layer using the Random extract tool: