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I had a conversation with a co worker that was asking about an OL map I am developing. She asked me what the 'projection' was and I said EPSG:4326. She didn't know what that means, so I said it's based on WGS84, to which she replied: that is a mercator, what is the projection? Can someone clear this up for me please?

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EPSG:4326 is not a projection "strictly speaking". It is a geographic coordinate system (also called lat-long) coordinate system. The reference datum is WGS 84, which is the most widely used datum nowadays (because of the GPS). So for many people, when you say "Lat/Long" they will understand.

In order to display Lat/long data on a flat screen, you need some kind of projection. The default "projection" associated with WGS84 is the Plate Carrée (which consists in diplaying lat long coordinates as if they were X Y coordinates.

Mercator is the name of a type of projection. When people do not specify which mercator they are using, it is more and more "WGS84 Web Mercator(auxiliary sphere)" (EPSG:3857 or, unofficially, EPSG:900913) which is used in many popular web mapping applications (Google/Bing/OpenStreetMap/etc).

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