The National Geospatial-Intelligence Agency (NGA) has an advisory notice posted on its site warning about errors users of the popular Web Mercator projection stating, “the use of Web Mercator and other non-WGS 84 spatial reference systems may cause geo-location / geo-coordinate errors up to 40,000 meters.”
The NGA further advised, “the community to use DoD approved World Geodetic System 1984 (WGS 84) applications for all mission critical activities.”
The NGA lays out its explanation of why using a Web Mercator projection instead of the recommended WGS 84 in a PDF entitled, “WGS 84 and the Web Mercator Projection.”
WGS 84 is ellipsoid based and maintains Conformal property whereas the Spherical based Web Mercator is non-confomal. While both projections look the same when viewed at the level of the world, examining the shift in accuracy at a larger scale highlights the spatial inaccuracies of Web Mercator.
The document provides a detailed mathematical explanation of the difference between the two.
Web Mercator is a population projection used by many online web mapping applications, most notably Google Maps, Bing Maps, and ArcGIS Online.
Esri has released its response to the advisory in a blog post: What does the NGA “Web Mercator” advisory mean for Esri Defense and Intelligence users?