One of the most frequent questions I get when it comes to ArcGIS Online spatial analysis is, “What projection is used in the analysis calculations?”. It is a legitimate question to ask! In ArcGIS Online, like in most online mapping platforms, Web Mercator is the default coordinate system. As many of you know, Web Mercator is not designed to minimize distortions for distance and area calculations. To address this concern is my motivation for writing this blog post.
The good news is that you don’t have to perform spatial analysis in the same coordinate system used by the web map. ArcGIS Online handles projection for you during analysis behind the scenes. Let us elaborate more on this.
The Default Behavior
In ArcGIS Online Map Viewer all layers in a web map inherit the coordinate system from the basemap. Web Mercator is the coordinate system of the default basemaps. However, this does not mean that data used for analysis must have the same projection as the basemap. Thanks to the ArcGIS “Projection on the Fly” algorithm, when a feature service with a different coordinate system and projection is added, it is re-projected on the fly onto the web map. It is important to point out that “Projection on the Fly” does not change the original data’s projection. It merely displays the features in the web map coordinate system for visualization purposes.
When you run the spatial analysis tools in ArcGIS Online, the original coordinate system of the input feature layer is used for the calculations. Note that the input for analysis tools in ArcGIS Online can also be feature collections. Items such as CSV files and map notes can be added to a web map as feature collections. The published feature collections have the same coordinate system as the basemap. When the input features are not projected (that is, when coordinates are given in latitudes and longitudes), distances in tools such as “Find Hot Spots” are computed using chordal measurements. Within a small area (for example, a 700 km wide region), chordal distances provide a good proxy (within 0.05%) for relative geodesic distances between features and allows for faster computation within the tool applications.
In ArcGIS Online the analysis output result layers are stored with the same projection as the input. Some online analysis tools such as merge, overlay, and find locations require multiple input layers. In these cases the output result layer will use the same projection as the input target layer or the first input layer. The only exception is the “Aggregate Points” tool. Since the output layer from “Aggregate Points” is a polygon layer, the projection for the output layer will be the same as the input polygon layer when there is one provided.
Overriding the Default Projection
Overriding the default behavior for the results layer’s coordinate systems is possible when calling the ArcGIS Online spatial analysis services via the REST API. With the REST API you can specify an output spatial reference system.
Using Projected Basemaps
There are times when you may want to use a basemap in a local projection or a coordinate system that is more tailored to your data instead of the default Web Mercator. This can be accomplished with a few extra steps in ArcGIS Pro. You can create your own basemap with whatever data and coordinate system you like and publish it to ArcGIS Online. An administrator can configure the custom basemap gallery for your organization to enable everyone in the organization to use basemaps with coordinate systems other than Web Mercator (learn more about creating projected basemaps)
The Geodesic Buffer
Buffer is a commonly used analysis workflow in ArcGIS Online to generate an area around input features at a given distance. When performing a buffer operation with a dataset that has features covering a large region or when using a very large buffer distance, it is important to remember that projection distortion can have a serious effect on the results. The ArcGIS Online “Create Buffers” tool creates geodesic buffer by default. The advantage is that this will always produce results that are geographically accurate because geodesic buffers are not affected by the distortions introduced by a projected coordinate system.
The Equal Sized Bins
Tools like “Generate Tessellations” and “Aggregate Points” or “Summarize Within” with custom binning options involve an operation to create equal sized area bins in the web map. To ensure that the areas of the bins are equal in size and appropriate for the analysis extent, these analysis tools choose an appropriate equal-area projection behind the scenes. In the next step, the generated bins are projected back to the original input spatial reference. Finally, when the result layer with the equal area bins is added to the web map, it is projected on the fly to Web Mercator for display (the default) or to the projection of your custom basemap.
A Web Mercator projection may cause the shapes of the bins to appear distorted, especially for large bins or bins near the polar regions. However, these distortions are part of the display only and do not reflect an inaccurate analysis. This is an example when the Web Mercator basemap just isn’t the best for visualization. You may want to consider a basemap in an equal area projection instead.
In summary, although the default ArcGIS Online web map uses the Web Mercator coordinate system, ArcGIS Online spatial analysis tools support measurements in local projections and geodesic distance. With some extra steps, you can even make your own non-Mercator web maps.