I am working with some ArcPy functions and while working with it, I found something that caught my eye.

While working with projections I observed that the most commonly used WGS 1984 has its name as GCS_WGS_1984. As shown in the screenshot below:

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Whereas, some projections do not have such Prefix(GCS).

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This is also followed in ArcGIS Pro.

What could be the possible reason for this variation?

2 Answers 2


I'm a product engineer at Esri who specializes in coordinate (reference) systems and transformations. Most of the coordinate system handling occurs in the Esri projection engine. When a colleague and I first started working on it in the 1990s, I had this weird antipathy to text strings (thanks, UNIX!) and we included underscores in all the names.

Because some 'datums' and 'geographic coordinate systems' had similar/equal names, we prefixed them with D_ and GCS_, respectively, to make sure that we had unique names. The library has changed since then and we no longer worry about similar datum and GCS names so we've stopped adding the prefixes. Coordinate reference systems need to have unique names, but datums are a separate object and don't count. Internally when we compare objects, we ignore the prefixes and underscores in the names.

Beyond the ArcGIS Pro documentation that user30184 linked, we have a public GitHub repo that contains lists of the supported coordinate reference systems, transformations, and other objects here. The information includes at which software release we added each object, although we've recently switched from using ArcGIS Desktop/Server release numbers to ArcGIS Pro's.

  • Thank you for your insights:) Oct 11, 2021 at 18:26

The document Geographic and Vertical Coordinate System Tables
https://pro.arcgis.com/en/pro-app/latest/help/mapping/properties/pdf/geographic_coordinate_systems.pdf shows that all coordinate systems that have GCS_ in the name are geographic coordinate systems but all geographic coordinate systems do not have GCS_ in their names. Probably only ESRI knows if there is some systematic rule that defines when GCS_ is not used.

  • Thank you for the URL @user30184, I am just curious to know what could be the idea behind such naming difference:) Oct 10, 2021 at 13:51

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