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

The projection used for all tiled base map services is the WGS84 Web Mercator (Auxiliary Sphere) so base map services can be used in combination with other common viewers, such as Google Maps and Microsoft Bing Maps.  For more info: 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 WGS84 with units of geographic decimal degrees, but the WGS84 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.