Map Projections in ArcGIS Pro
The Earth is a spheroid - it is round like a ball or sphere but flattened by slightly by the centrifugal force of rotation. The Earth has a circumference of around 24,900 miles, but is around 27 miles wider than it is tall.
The Earth exists in three-dimensions but, other than globes, most representations of the earth, such as printed maps or web maps, are two dimensional. A projection is a set of mathematical transformations used to represent the three-dimensional world in two dimensions.
This tutorial introduces changing and getting information about projections in ArcGIS Pro.
Setting and Changing Projections in ArcGIS Pro
Projections can be changed on an ArcGIS Pro map by right-clicking on the layer in the Contents pane and selecting Properties -> Coordinate Systems.
The dialog box will show you the current projection and allow you to search or browse for a new projection.
The projections dialog gives you two sets of coordinate systems to choose from:
- Geographic coordinate systems indicate where features are on the surface of the planet.
- Projected coordinate systems indicate where features are on maps.
The video below demonstrates how to change a world map from its existing projection (Web Mercator - EPSG 3857) to the common Robinson projection (EPSG 54030). Because different projections often have similar names, if you want a specific projection and you have a WKID number to search on, that will help you find that specific projection. In this case, we use 54030 for the Robinson projection.
Getting Projection Information in ArcGIS Pro
While the software gives you the option to display a map in different geographic coordinate systems, you will normally only create maps using projected coordinate systems. The geographic coordinate system used to create the projected coordinate system is implied when you choose a projected coordinate system.
You can find the geographic coordinate system used in a projection by selecting the Details option in the ArcGIS Pro coordinate systems dialog.
For example, this video shows that the Robinson projected coordinate system (ESRI 54030) uses the WGS 1984 datum (EPSG 4326) as its geographic coordinate system.
Grids and Graticules in Map Layout
A common feature of maps is a grid of lines that show map coordinates.
A graticule is a grid of lines that shows intervals of latitude and longitude on a map.
The video below shows how to add a graticule to a world map. Note a distinctive characteristic of this Mercator projection is that lines of latitude get further apart as you move away from the equator.
By contrast, in ArcGIS Pro terms, a measured grid is a grid of lines that displays intervals in the projected coordinate system used in the map.
Measured grid lines are more appropriate than graticule lines when working with a projection that represents actual distances across the surface of the earth, such as in a state plane coordinate system. Graticules are more appropriate on maps that cover large areas (like the whole world or large countries) where information about latitude and longitude may be helpful for interpreting locations on the map.
Graticules and measured grids should be used with caution as they can add clutter to a map, especially if there is a base map or complex labeling. Graticules and measured grids should generally be avoided unless absolutely needed to convey useful information to the map reader.
The video below shows how to add a measured grid to a map layout for a map displayed in the Illinois East State Plane Coordinate System.
Euclidian Distance with Planar Coordinates
The State Plane Coordinate Systems treat the earth as a flat plane (hence, the name) and can therefore be conveniently used for calculating distance.
The example below uses two points in Chicago: The Sears Tower (41.8789, -87.6358) and the Cloud Gate sculpture in Millennium Park (41.882686, -87.623325). Chicago is in the Illinois East State Plane Coordinate System zone, and the WKID for the coordinate system in US feet is EPSG 3435.
The easting in a State Plane projection is the position east-west (similar to longitude), and the northing is the location north-south (similar to latitude). Because this is a US Feet State Plane variant, those numbers are in feet.
Using ArcGIS Pro as shown in the video, you can change the map projection to State Plane and then get the northing and easting for each point. By subtracting to get the east-west distance and the north-south distance between the points, you get the two sides of a triangle. You can then use the Pythagorean theorem to calculate the Euclidean distance (hypotenuse) between the two points in feet.
Note that you cannot do this with latitudes and longitudes because those are angular measurements, and the ground distance represented by a degree is different on different parts of the planet.
Sears Tower: 1174208 East, 1899233 North Cloud Gate: 1177593 East, 1900640 North East/west distance = 1177593 - 1174208 = 3385 feet apart east/west North/south distance = 1900640 - 1899233 = 1407 feet apart north/south square_root(3385^2 + 1407^2) = square_root(11458225 + 1979649) = 3666 feet