QGIS By Example

Plugin management implies their activation, installation, update, and removal, which are performed by navigating to Plugins | Manage and Install Plugins. There are several tabs in the dialog window. Clicking on an individual plugin in any of these tabs shows you detailed information: whether the plugin is experimental or not, the functionality, rating, author, version, and also some links to its homepage, code repository, and tracker:

To install a plugin, go back to the Not installed tab, select the plugin you want to install, and click on Install plugin. After that, the installation starts, and upon its completion, the message about successful installation is displayed. The installed plugin is activated by default and either appears in the Plugins menu, or is added to another relevant menu (for example, Vector or Raster). Also, some plugins appear as separate toolbars and panels that can be placed and turned on or off manually.

Finding and choosing a plugin that fits your needs might be difficult because their number is over 400 and is constantly growing. To explore the world of QGIS plugins, you can start from http://plugins.qgis.org/, which contains information about popular and featured plugins. If you are looking for a specific function but don't know the exact plugin name, we advise you to search by using tag words in the Manage and Install Plugins dialog window, as shown on the following screenshot:

Most of our data is in the ESRI shapefile format, which is one of the most common vector data file formats. There are a few alternative ways of loading a shapefile into QGIS. You can open the dialog window by going to Layer | Add Layer | Add Vector Layer, or by clicking on the relevant button on the Manage Layers toolbar, or by using the Ctrl + Shift + V keyboard shortcut.

In the Add vector layer dialog window, configure the following items:

Alternatively, you can use the Browser panel to navigate to the working directory. Select the files you want to load (either one or several, by holding down the Ctrl key), and then just drag and drop them onto the map area. If you want to simplify the navigation, add the folder you are currently working with to Favourites from its right-click menu shortcut.

The loaded files are assigned random colors. Don't worry about that as of now; we will cover layer symbology in Chapter 2, Visualizing and Styling the Data.

The procedure of adding raster files is similar to what was described earlier, so we will briefly go through it for the example of the common raster file format GeoTIFF:

ESRI Personal GeoDatabase is an original ArcGIS data format where all of the database's content is held in a single .mdb Microsoft Access file. It is widely used to store data in a single data file instead of multiple files of different formats.

Adding an ESRI file geodatabase to QGIS is similar to adding any other vector format:

A Comma-separated values (CSV) file is another popular data file format. In fact, it is just a spreadsheet with field values delimited by commas. There are various delimiters possible instead of commas (for example, tabs, spaces, colons and so on). Very often, these tables contain spatial data in the form of positional attributes represented by point longitude or latitude (XY) coordinates, or well-known text (WKT) geometry that describes points, lines, or polygons.

In our dataset, we have a CSV file called noise.csv. It contains the details of noise complaints registered mostly by the New York City Police Department. To add this file as a spatial layer, follow these steps:

As GPS receivers became portable and relatively cheap, GPS tracking became a ubiquitous and widely used technique of collecting data during a field survey, or simply tracking your own routes while running or cycling. Depending on the receiver's capabilities, in addition to spatial coordinates, a lot of information can be registered and written—for example, time, elevation, and so on. Registered information is stored in points that represent location changes (way, track, or route points), a planned route (if it exists), and a track of movement. There are numerous formats for storing GPS data. As primary data format, QGIS uses the standard interchange GPX (GPS eXchange) format.

We will load the nyc_marathon.gpx file from our dataset. You can load a .gpx file as a vector layer by going to Layer | Add Layer | Add Vector Layer, by clicking on the relevant button in the Manage Layers toolbar, or by using the Ctrl + Shift + V keyboard shortcut. Go to the data directory and apply the GPS eXchange format (GPX) file type filter. Select the file (or files) you want to add and click on Open. Alternatively, you can drag and drop files from the Browser panel.

As GPS data consists of points, routes, and tracks, the window pops up where you choose exactly which feature you want to open. You can select one or more features by clicking on a row and holding down the Ctrl key. If you are not sure exactly which feature you need, use Select All. Every feature type will be presented in a separate layer. By turning layers on and off, you can check whether they contain data or not; for example, the file we work with contains only tracks and track points. You can remove unnecessary empty layers by right-clicking on the layer contextual shortcut Remove.

OpenStreetMap (OSM) is a crowdsourcing project aimed at the development of an open map of the world. Worldwide, the OSM community uses high-resolution remote sensing imagery, GPS surveying, and local knowledge to make the map as accurate and detailed as possible. As the OSM data is licensed under Open Data Commons' Open Database License (ODbL), it has recently become a widely used source of spatial data. Understanding the importance of unrestricted access to spatial data, QGIS supports OSM, providing integrated access and support for its data.

The QGIS core functionality is available from the OpenStreetMap menu under Vector. Here, you can find all the required tools to download the data and export it into conventional spatial data formats, but if you are new to the QGIS and OSM data concepts, you may find the process tedious. This is why we recommend that you use external QuickOSM plugin capabilities to download the OSM data.

After installation as described in the Extending functionality through plugins section, the plugin's main functionality can be accessed by going to Web | QuickOSM | QuickOSM. The Quick query tab of the main window contains everything you need to perform a query and get the data into QGIS. First, define a key=value pair from the drop-down lists, or just type it in manually.

Then, set up the area of interest extent by location name, map canvas, or layer. If the layer drop-down list is inactive, click on the button beside it to refresh it. In the Advanced section, you can turn on/off different geometry types. By default, all geometry types are turned on, so all the relevant points, lines, and polygons will be downloaded. Browse to the directory you want to save the data in. In the following screenshot, you can see the example of the QuickOSM query used to download bicycle parking point data.

After clicking on the Run query button, the shapefile with the data will be loaded into the map canvas.

When you load your data into QGIS, it tries to automatically define information about the projection from the accompanying projection description file and set it to a newly added layer. You can check the layer's projection information in the Layer Properties dialog. To open it, double-click on the layer name in Layers panel, or use the right-click contextual shortcut Properties. Information about the projection is shown in the Coordinate reference system section of the General tab, like this:

There are numerous projection description file formats, and they mostly depend on the data format and origin. One of the most common among them is a .prj file. This file format sometimes provides information about the projection in a reduced form, and QGIS cannot define it properly. If information about the projection is absent or incomplete, you will see that the projection description is missing or looks like this: USER:100000 - * Generated CRS (+proj=etc.). It means that QGIS didn't find an appropriate projection within its own database and added it as a custom CRS. If you are sure that the projection is standard but QGIS fails to interpret it properly because of a reduced description, specify it manually. There are different ways to assess the Coordinate Reference System Selector dialog as follows:

In the Coordinate reference system selector dialog, choose the necessary CRS from the list and click on OK. Note that you can use a filter by key words or the EPSG code to hide unnecessary projections. With time, often used projections will be added to the list of the recently used CRS.

Note that specifying the projection manually from the Properties dialog is a temporary solution; it does not persist after closing a file and/or exiting QGIS. This means that whenever you are loading this file into QGIS, you will have to specify the projection again. If you are going to work with the file in other projects, it is better to use the Define Current Projection tool by going to Vector | Data Management Tools | Define Current Projection. In this dialog, you can choose a predefined spatial reference system from the Coordinate reference system Selector dialog or Import spatial reference system from existing layer with a known CRS. In both the cases, the existing .prj file will be overwritten by QGIS with all the necessary parameters, and you will not have to adjust the projection manually next time.

All of this is true when you import files from external sources. If you create a shapefile in QGIS, both .prj and .qpj files are created, so you don't need to define the projection manually. The files projection will be interpreted correctly in the other GIS applications that read it from .prj files.

You can also maintain QGIS behavior when loading or creating a new layer that has no CRS information by going to Settings | Options | CRS, where three alternatives of CRS for new layers are available:

Working with projections can be tricky, unless you recognize the difference between the specify and define projection options. It's all about assigning information about the projection properly, but when you specify it, the result is temporary within a current working session. Once you define a projection, it becomes permanent unless redefined again. In both cases, if you choose the wrong projection, you will not be able to see your data on the map (at least in the area where you expect it to be). Usually it is good practice to specify the projection at first, comparing your data to the correctly projected dataset. If everything is okay, define it to make projection assignment permanent.

When you load QGIS, its map canvas projection is, by default, set to EPSG:4326 – WGS 84. This behavior is defined by the global default projection, which can be changed by going to Settings | Options | CRS | Always start new projects with this CRS.

However, when you start adding data, projection identifier automatically changes to the projection of the first loaded layer. So, by default, project projection is defined by the first loaded layer. Data source projection and project projection are not always the same. If they are different, QGIS can apply the so-called on-the-fly transformation. This means that it reads the data projection, and if it differs, automatically aligns it with the project projection, applying the necessary transformations. There are a few ways to specify the projection for the project and enable on-the-fly transformation:

In the last way, the difference is very subtle and is mainly obvious when adding the first layer to the project. In the case of automatically enabled reprojection, the default map canvas CRS will be superseded by the first layer projection, and the following layers will be automatically adjusted to it too. In the case of reprojection by default, all layers will be reprojected into the project initial CRS (this is usually default global projection, unless you've changed it).

After on-the-fly transformation is enabled, you will see an OTF abbreviation near the current CRS on the status bar, and the icon will become solid.

If you want to quickly change the project CRS according to a specific layer in the Layers panel, right-click on a layer and choose the Set Project CRS from Layer shortcut.