[OSM-dev] Mercator projection

Kristian Thy thy at 42.dk
Sat Jan 13 10:29:00 GMT 2007

On Fri, Jan 12, Christopher Schmidt wrote:
> On Sat, Jan 13, 2007 at 03:07:59AM +0100, Kristian Thy wrote:
> > Would it be correct to assume the Mercator projection being used for OSM
> > is epsg:54004?
> EPSG '54004' is not actually a definition. It is one of the codes in the
> "user-defined" range allocated by the EPSG standards organization.

My bad, I keep forgetting the codes in the epsg file distributed with
proj.4 isn't all EPSG :)

> I assume you're using '54004' to describe the 'simple mercator'
> projection used by Google. In that case, the projection for the WMS-C
> layer is EPSG:54004.

I think we're talking about the same projection, but just to make sure,
here's the definition of it from my epsg file:

# World Mercator
<54004> +proj=merc +lat_ts=0 +lon_0=0 +k=1.000000 +x_0=0 +y_0=0 +ellps=WGS84 +datum=WGS84 +units=m  no_defs <>

> The projection for the OSM tiles generated using generate_tiles.py is
> not the same. It assumes a sphere, instead of an ellpisoid, for the
> shape of the planet, so far as I can tell from reading the code, which
> is what causes the difference in the two sets of tiles.

If anybody could put that into proj or WKT params, I'd be a happy camper :)

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