[OSM-dev] about mercator projection

Robert (Jamie) Munro rjmunro at arjam.net
Thu Sep 6 18:59:42 BST 2007


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Martín RV (OPENGeoMap) wrote:
> Hi:
> 
> I am an land surveyor and programmer in Spain.
> I would like to talk about the projection used in JOSM and in
> OpenStreetMap. I was studying the projection in Google Earth and World
> Wind, and i think that they have a good choice.
> 
> They work always in the geodetic coordinates-*plate carrée*:
> 
> x=longitude
> y=latitude

I wouldn't call Google Earth "Plate carrée", I'd call it "3D".

Plate carrée projection squashes everything vertically as you move away
from the equator. This doesn't matter too much if you look at the whole
world, but if you zoom in to anywhere not close to the equator, it
really starts to show.

For example, compare Oxford in this Plate carrée projection:
http://tilecache.org/demo.html?lat=51.755&lon=-1.247&zoom=12&layers=00B000000T
(Slightly out of date rendering rules and database)

To Oxford in the current OSM site (mercator projection):
http://www.openstreetmap.org/?lat=51.755&lon=-1.247&zoom=13&layers=B0F

Looking at the Plate carrée map, you might imagine that Cutteslowe (in
the North) is as near as Botley (in the West), but in fact Botley is
much nearer, as you can see in the Mercator map, or in Google Earth etc.

Robert (Jamie) Munro
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