Chapter 6. Projections and Coordinate System Basics

In this chapter, we will cover the following recipes:

• Determining the coordinate system for an existing map
• Setting the coordinate system for a new map
• Changing the coordinate system for a map
• Defining a coordinate system for data
• Projecting data to different coordinate systems

Introduction

One of the things that makes GIS such a powerful visualization and analytical tool is the ability to overlay multiple layers of information together in a map. Putting our GIS data in real-world coordinate systems and projections is how we are able to do this. By placing our data in a real-world system, we tie it to the Earth. This allows us to locate features anywhere on the Earth’s surface and then bring them into a map so we can see how those features are related spatially.

There are two basic types of coordinate systems we can use in ArcGIS Pro, geographic and projected. A geographic coordinate system is based on a 3D model of the earth, called the ellipsoid or spheroid. The ellipsoid is then tied back to the physical Earth by the datum. Geographic coordinate systems use degrees as their primary unit of measurement.

Each degree can then be broken down into different sub-units, such as decimal degrees, minutes, decimal minutes, or minutes, and seconds. You will often hear geographic...

Determining the coordinate system for an existing map

Every map and scene you create in ArcGIS Pro makes use of a coordinate system in order to display data in a location. However, it not only can impact how and where features are displayed, but also what you can do with the map. The coordinate system used by a map can impact if you can share it as a web map via ArcGIS Online, how well layers overlay one another, and more. So, it is important to know what coordinate system your map is using.

The first layer you add to the map will define the coordinate system for that map. But what if you are working with an existing map, maybe one that was created by someone else? How do you determine the coordinate system for the existing map you are working with?

In this recipe, you will learn how to determine which coordinate system has been assigned to an existing map.   

Getting ready

This recipe does require the sample data be installed on the computer. You are not required to have completed any previous...

Setting the coordinate system for a new map

You now know how to determine the coordinate system assigned to existing maps. But how do you assign one to a new map? That is pretty easy. As was mentioned earlier, the first layer you add to a new map will set the coordinate system for the map.  

In this recipe, you will create a new map in the project you used in the previous recipe. You will then see what coordinate system has been assigned to the new map. Next, you will add a new layer to the map and see what that does to the coordinate system for your map. Lastly, you will add another layer to the map that is in a different coordinate system, and check to see if that changes the coordinate system for your map.  

Getting ready

This recipe does require the sample data be installed on the computer. It is recommended that you complete the previous recipe in this chapter along with the recipes in Chapter 1, ArcGIS Pro Capabilities and Terminology, before you begin this recipe. This will ensure that...

Changing the coordinate system of a map

You now know how to determine the coordinate system for an existing map, and how to set one for a new map. But how do you change the coordinate system for a map or scene?

You might be wondering why you would want to change the coordinate system for a map. There are several reasons why you might want to change the coordinate system assigned to a map. As mentioned in the introduction, some coordinate systems are better suited for specific operations. If you need to measure areas or distances for analysis or editing, a projected coordinate system is best, especially for small areas such as a county, city, or district. If you are trying to show locations of features across large areas, such as a country or the world, then a geographic coordinate system is often best. If you are publishing as a web map then it might require the map be in the WGS 1984 Web Mercator (Auxiliary Sphere) coordinate system to conform with other datasets. So, if you need to do one...

Defining a coordinate system for data

You now know how to assign and change the coordinate system used in a map. Maps are not the only thing that are assigned a coordinate system. Our data is also assigned a coordinate system, or at least it should be. If your data is not assigned a coordinate system then ArcGIS Pro may not be able to display the correct location with all your other data. This is also true if your data is assigned the wrong coordinate system.

Where is the coordinate system information stored within your data? Well, that depends on what type of data you are working with. If it is a feature class stored in a geodatabase, then the coordinate system information is stored within the geodatabase itself. If you are working with shapefiles, then the assigned coordinate system is stored in a file with a .prj file extension.

If you are working with CAD data, meaning .dwg, .dxf, or .dgn files, then you will also need a .prj file in order for ArcGIS Pro to know what coordinate system...

Projecting data to different coordinate systems

You now know how to change the coordinate system for a map, and how to assign one to data that is missing a coordinate system. But how do you change the coordinate system for data? That process is called projecting data.

When you need to move or project data from one coordinate system to another, it requires a lot more than just redefining the assigned coordinate system like you did in the previous recipe. It requires ArcGIS Pro to recalculate all the coordinate values for the features within that dataset. This means all those features will have new coordinates that represent values in the coordinate system it has moved to. To calculate the new coordinate values, ArcGIS Pro must perform many calculations for each feature to account for possible changes in datums, units, reference locations, earth models, and more. So, it can take a long time for larger datasets.

The tool you use to move data from one coordinate system to another is the Project...