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Osmarender uses the Mercator projection, but in a simplified way: Complex maths is hard to do in XSL so Osmarender just calculates the projection once for the middle point of the map and then uses an affine transformation for all nodes in the map. Also, because there are no trigonometric functions available in XSL, Osmarender uses a simplified way of calculating the projection.

See also: Osmarender bug. (that page should probably be folded in here)