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What map projections are used in The National Map tiled base map services and dynamic overlay services?

The projection used for all tiled base map services in The National Map is the World Geodetic System 1984 (WGS 84) Web Mercator (Auxiliary Sphere) so that base map services can be used in combination with other common viewers. See Spatial Reference SR-ORG:6928 and Spatial Reference SR-ORG:7483.

Dynamic overlay services are in WGS84 (see Spatial Reference EPSG:4326) but can be reprojected as needed. In the past, USGS map services were in WGS 84 with units of geographic decimal degrees, but the WGS 84 Web Mercator (Auxiliary Sphere) projection was implemented to allow users the greatest level of freedom in mixing these services with other commonly used web map applications that exist today.