I understand what the difference is between the Web Mercator projection and Web Mercator Auxiliary Sphere (WMAS). I also understand that both Google and Esri have adopted this projection as their primary projection for their web-maps which is why we have specialized functions that not only re-project between all projections, but are specific functions for WM, such as webMercatorToGeographic. So ultimately I was wondering why we use the WMAS projection, and the reason it has become a standard in web mapping. Is it a purely a result of two spatial giants moving in that direction or was it just solely because of accuracy reasons?

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Mercator Projection


4 Answers 4


I'm pretty sure we have Google to thank. Take a look at the original EPSG code WKID for Web Mercator. What does 900913 look like? Helps if you're at least a little l33t.

When Google Maps blew up a few years ago (2005ish), everyone started copying Google. This included Virtual Earth/Bing, Mapquest, Yahoo Maps and eventually Esri. Everyone wanted/needed to be compatible with the most popular web mapping platform. It has been the standard ever since.

Edit: per mkennedy's comment, changed EPSG code to WKID


As mentioned, it is originally from Google Maps and its tiling system. However, there are some pretty basic, fundamental advantages* that make the choice of WMAS a good one, and that is why others have followed suit.

The Bing Maps Tile System page is an excellent resource. It outlines some of the advantages of the projection, e.g. that it is conformal and cylindrical. The fact that the area of interest can fit nicely into one square tile at the smallest scale is a nice property when dealing with the tiling system. It also mentions the reason for using a sphere: easier calculations -- which in the early 2000s was an important factor when dealing with JavaScript applications.

*Note that the advantages here are defined in the context of bringing web maps to the masses.


Just to add a bit, think of the shape of a building footprint. Google Maps (and Bing) commonly show data in cities with building footprints and other items with expected shapes such as property lines.

In a Geographic coordinate system (lat/lon), building footprints that normally have 90 degree corners look very strange and squashed - the angles are not preserved. The coordinate system they chose to use had to be applicable for the vast majority of the populated world, giving a decent appearing view of the map. Area measurement was not the priority - just the appearance of entities in the map. For a whole earth coordinate system, a general Mercator projection works well for appearance due to conformality (angles are preserved). They further simplified the work by assuming that the world is a sphere instead of an ellipse, which makes the math for distance measurement much easier.

So, in short, it's chosen for looks and for easier math.


Just a additional 2-bits of thought... the drive for 'Mashups' the spawned off of google made them a defacto-standard, and since they support the majority of the populated world in there mapping we see WMAS become a common baseline. It was not really a decision by the industry as a whole; but a drive to embrace and extend to support the widest audience.

Google did it, Microsoft extended it, ESRI embraced it and now we see a common starting point. Not bad, could there have been better answers, sure, but for the amount that we as the industry have to invest in having a free pretty good global-cached basemap, we really can't complain too much.

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