Web Mercator projection
It rose to prominence when Google Maps adopted it in 2005.
[citation needed] The discrepancy is imperceptible at the global scale but causes maps of local areas to deviate slightly from true ellipsoidal Mercator maps at the same scale.
General lack of understanding that the Web Mercator differs from standard Mercator usage has caused considerable confusion and misuse.
[5][6] For all these reasons, the United States Department of Defense through the National Geospatial-Intelligence Agency has declared this map projection to be unacceptable for any official use.
[5] Unlike most map projections for the sphere, the Web Mercator uses the equatorial radius of the WGS 84 spheroid, rather than some compromise between the equatorial and polar radii.
Formulas for the Web Mercator are fundamentally the same as for the standard spherical Mercator, but before applying zoom, the "world coordinates" are adjusted such that the upper left corner is (0, 0) and the lower right corner is (
Services such as Google Maps cut off coverage at 85.051129° north and south.
This is not a limitation for street maps, which is the primary purpose for such services.
The value 85.051129° is the latitude at which the full projected map becomes a square, and is computed as
EPSG's definition says the projection "uses spherical development of ellipsoidal coordinates".
[10] Web Mercator is a spherical Mercator projection, and so it has the same properties as a spherical Mercator: north is up everywhere, meridians are equally spaced vertical lines, angles are locally correct (assuming spherical coordinates), and areas inflate with distance from the equator such that the polar regions are grossly exaggerated.
Lines deviate because Web Mercator specifies that coordinates be given as surveyed on the WGS 84 ellipsoidal model.
This is standard practice on the standard spherical Mercator projection, but unlike Web Mercator, the spherical Mercator is not normally used for maps of local areas, such as street maps, and so the accuracy of positions needed for plotting is typically less than the angular deviation caused by using spherical formulas.
The benefit the Web Mercator gains is that the spherical form is much simpler to calculate than the ellipsoidal form, and so requires only a fraction of the computing resources.
[11] Due to slow adoption by the EPSG registry, the Web Mercator is represented by several different names and spatial reference system identifiers (SRIDs), including EPSG:900913, EPSG:3785 and EPSG:3857, the latter being the official EPSG identifier since 2009.
[12] The projected coordinate reference system originally lacked an official spatial reference identifier (SRID), and the Geodesy subcommittee of the OGP's Geomatics committee (also known as EPSG) refused to provide it with one, declaring "We have reviewed the coordinate reference system used by Microsoft, Google, etc.
We will not devalue the EPSG dataset by including such inappropriate geodesy and cartography.
It was originally defined by Christopher Schmidt in his Technical Ramblings blog[14] and became codified in OpenLayers 2,[15] which, technically, would make OpenLayers the SRID authority.
Later that year, EPSG provided an updated identifier, EPSG:3857 with the official name "WGS 84 / Pseudo-Mercator".
Although the projection is closely associated with Google, Microsoft is listed as the "information source" in EPSG's standards.
[17] The projection covers the Earth from −180° to 180° longitude, and 85.05° north and south.
Using well-known text representation of coordinate reference systems (WKT), EPSG:3857 is defined as follows:[12]
