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What does 'Authority: EPSG' mean in an ArcGIS Desktop .prj file?
EPSG was the acronym for the European Petroleum Survey Group. It is now known as the Geomatics Committee of the International Association of Oil and Gas Producers (OGP).
OGP is a consortium of companies involved in exploration and extraction world-wide. As a service to their members, the organization has compiled a dataset of coordinate system-related information for the entire world. The dataset includes coordinate systems, datum transformations, projections, units, spheroids and all other related information. The information comes from petroleum companies, as well as from national government agencies responsible for geodesy in that specific country. An example is the National Geodetic Survey of the United States.
The EPSG Geodetic Parameter Dataset is available as a free download at http://www.epsg.org, and is accepted as a world-wide clearinghouse for coordinate system and transformation information. There is an online registry at EPSG Geodetic Parameter Registry
A new version of the dataset is published between two and six times each year, depending on the amount of new information the organization receives and wishes to include in a new version of the database.
Esri obtains the majority of the information used in our projection engine library from the dataset. That is the 'authority' for the object, and the number is the code number EPSG has assigned in their dataset to that specific object information.
However, from time to time Esri also receives coordinate system-related information that has not been published in the EPSG dataset directly from organizations, government agencies, or companies. In those cases, Esri will include that information in our projection utilities. The 'authority' for the object is 'ESRI', and the associated code number is assigned by Esri, not EPSG.
WKID stands for 'Well-known ID' for the object definition. A well-known ID is used in ArcObjects SDK and other programming languages as a shortcut to do coordinate system-related tasks outside of the ArcGIS Desktop GUI interface, as well as in reference to data that is uploaded to a web service, so that people can download the data and display it in their ArcMap documents and other applications.
Questions about coding this type of application are handled by the SDK team in Esri Support Services. A complete list of the WKIDs supported by Esri is available in the Developer Help on the Esri Support Center.