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Web Mercator
Web Mercator, Google Web Mercator, Spherical Mercator, WGS 84 Web Mercator or WGS 84/Pseudo-Mercator is a variation of the Mercator projection and is the de facto standard for Web mapping applications. It rose to prominence when used in the first Google Maps in 2005. It is used by virtually all major online map providers, including Google Maps, Bing Maps, OpenStreetMap, Mapquest, Mapbox, and many others.〔
〕 Its official EPSG identifier is EPSG:3857, although others have been used historically.
== Properties ==
Web Mercator is a slight variant of the Mercator projection that is used primarily in Web-based mapping programs. It uses the same formulas as the standard Mercator as used for small-scale maps. However, the Web Mercator uses the spherical formulas at all scales whereas large-scale Mercator maps normally use the ellipsoidal form of the projection. 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. This deviation becomes more pronounced further from the equator, and can reach as high as 35 km on the ground.〔(【引用サイトリンク】title=The Google Maps / Bing Maps Spherical Mercator Projection )〕
While the Web Mercator's formulas are for the spherical form of the Mercator, geographical coordinates are required to be in the WGS 84 ellipsoidal datum. This discrepancy causes the projection to be slightly non-conformal. General lack of understanding that the Web Mercator differs from standard Mercator usage has caused considerable confusion and misuse.〔 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.〔(【引用サイトリンク】title=NGA: (U) NGA Advisory Notice on "Web Mercator" (UNCLASSIFIED) )〕
抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』
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