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This tutorial will highlight the Select Layer by Location's sensitivity to projections when adopting planar methods. It will also showcase alternative (safer) selection methods. This tutorial was created using ArcGIS Pro version 3.3.
Before tackling this tutorial, you will need to download and install a dataset following these instructions:
Create a folder called projections1 somewhere in your course working directory (e.g. C:\Users\yourname\Documents\ArcGISPro_tutorials\proj1\).
Download the data for this exercise then extract the contents of projections.zip into your newly created projections1 folder.
Open the project file
Open the project file
Open the coordinates1_bardcollege.aprx file.
The project consists of three maps--each map displaying the same earthquake and Bard College locations in different projected coordinate systems. In the examples that follow, you will compare the results from a spatial selection whereby you will select all earthquakes within 2000 miles from Bard College.
The projections used in this example are Mercator, Natural Earth, and an equidistant Azimuthal projection centered on Bard College.
Identifying a map frame's coordinate system
Identifying a map frame's coordinate system
Each map frame is adopting a different coordinate system. You can identify the map frame's coordinate system from its Properties window.
In the Mercator map frame, right-click on Mercator and select Properties. Alternatively, you can double-click on Mercator to bring up its properties.
In the Properties window, select the Coordinate Systems link. This will reveal the name of the coordinate system used (Mercator in this example).
The name may not be terribly informative. To reveal more information about this coordinate system click on the Details link above the coordinate system name.
The Details window indicates that this is a projected coordinate system based off of a Mercator projection (one of many cylindrical projections). It is built off of a WGS 1984 Geographic coordinate system (and by extension, a WGS 1984 datum). Its mapping unit is in meters.
On your own, explore the other two map frame coordinate systems in this project.
They should all be projected coordinate systems built off of different projection methods.
However, they all share the same underlying geographic coordinate system--that being the WGS 1984 geographic coordinate system.
Identifying a layer's coordinate system
Identifying a layer's coordinate system
Layers taking part in a map frame do not need to share the same coordinate system as that of its map frame. However, it's usually good practice to have both the map frame and its layers adopt the same coordinate system.
A layer's coordinate system can be assessed from its properties.
In the Mercator map frame, right-click on the Bard layer and select Properties.
In the Properties window, select Source and expand the Spatial Reference block.
Note that the layer shares the same coordinate system as that of its parent map frame.
On your own, explore some of the other layer's coordinate systems in the different map frames. You should note that each layer shares its parent's coordinate system.
Selecting points by location
Selecting points by location
Next, you will select all earthquakes centered within 2000 miles of Bard College using the Mercator projection.
Make sure that you have the Mercator map selected.
Open the Select Layer by Location geoprocessing tool.
Select all quakes points within 2000 US Survey Miles from the Bard point feature.
Click OK.
Zoom in on the US.
You should end up with 912 selected quake points.
On your own, perform the same selection for the two other maps in the project document. You should end up with the following number of selections:
Mercator: 912
Natural Earth: 1231
Azimuthal: 1434
Selections on a planar coordinate system
Selections on a planar coordinate system
Three different map projections generate three different sets of selections! So which one is correct?
You must first remember that a (planar) map projection is an attempt at representing a three dimensional surface on a two dimensional plane. As such, distortions are to be expected. You must also remember that different projections distort the earths' surface features differently.
To visualize the grid defined by a map's coordinate system, you can overlay its grid. Note that you cannot overlay a grid in the Map view. Instead, you need to create a new Layout.
Click on the Insert tab then click on New Layout.
You'll be presented with different templates. Select the ANSI:Landscape Letter template.
Next, you will embed a snapshot of the Mercator Map view window in the new layout.
While making sure that you are in the Layout pane, click on Map Frame from the Insert tab.
Select one of the Map views (in this example, the Mercator map is added to the layout).
Place the map in the empty Layout window (you will need to drag a marquee box to define the map's extent).
Once in place, you can turn layers on and off as you would on the Map pane. But note that checking layers on or off in the Layout's Contents pane will not be reflected in the Map's Contents pane.
Note that your map may still show the selected points. The adjoining figure shows the map without the selected points.
Next, make sure that the Map frame is selected in the Layout window. You can select it by clicking it once.
Once selected, you should see the Grid tool activated in the Insert tab ribbon. Click on Grid and select one of the Measured Grid templates (DO NOT select one of the graticule templates).
This will add the Map's coordinate reference grid to the map layout.
For example, the following figure shows the Mercator projection grid used in calculating distances between points (note that the background layers are turned off to place emphasis on the point features). In this PCS the coordinate values are in meters (the most common mapping unit used in projections).
If you run a selection in ArcGIS, it is calculating the distances between points using this representation of the features. Now, compare the above representation with that of the other two map projections shown below.
Natural Earth Projection
Natural Earth Projection
Azimuthal Projection
Azimuthal Projection
If you look carefully, you'll note that both the points layout and their coordinate values differ between projections. This results in different measurements between the same set of points.
The following examples show the distance measurements you should get between the Bard point feature and the third closest quake point feature when measuring on the Mercatur map, note the third closest point changes in the other maps but the images below display the distance to the same point identified as the third closest in the Mercatur map.
To measure the distances between points, switch to the Mercator Map view.
Right-click the Bard College point feature class and select Zoom To Layer.
You can then use the Measure tool from the Map's ribbon. Make sure to select Planar as the measurement method.
Mercator Natural earth Azimuthal
Planar vs Geodesic Measurements
Planar vs Geodesic Measurements
The distance measurements made so far have been planar (i.e. measurements made on a "flat" surface). Such measurements should be made with projections that preserve the intended spatial property (distance in our example). But even the projections designed to preserve distance or area may not do so exactly. Errors in such measurements may not, for example, be uniform across the entirety of the map extent.
Another option, when available, is to make use of geodesic measurements. These measurements ignore the projections used in the map and make the measurement directly off of the modeled sphere or spheroid. This eliminates any errors associated with a map projection.
In each of the three maps, change the selection method from "within a distance" to "within a distance geodesic".
Re-run each the selection.
You should have the same number of selected quakes in each map projection: 1435. This is to be expected given that the (planar) map projections in each map is ignored in the selection process.
Oddly enough, the projection that offers the most distorted "look" of the earth's surface, the Azimuthal projection, is the one that did a good job in properly identifying the number of earthquakes. This projection is designed to preserve distance from the center of the map to any other feature on that map. The projection's center was set to Bard College thus preserving the distance from Bard to all quake point features. But note that the Azimuthal projection is not to be used when measuring distances between point pairs where neither one of the points is centered on the projection.
So when in doubt as to whether or not a projection preserves a desired spatial property, make use of a geodesic measurement if that option is available in the geoprocessing tools. This applies to both area and distance measurements.
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