[OSM-dev] Mapnik / General question on projection

M∡rtin Koppenhoefer dieterdreist at gmail.com
Thu Nov 18 19:45:51 GMT 2010


2010/11/18 Andy Allan <gravitystorm at gmail.com>:

> As far as I'm aware, 900913, 3785 and 3857 are all identical, and
> don't have the difference that you are describing above. From
> spatialreference.org
> From the proj4 project:


> Moreover, you refer to changes made to the generate_image.py file, but
> I don't see such a change:
> http://trac.openstreetmap.org/browser/applications/rendering/mapnik/generate_image.py#L38

yes, sorry, this was probably part of my own tests (and as I used
primarily generate_tiles it didn't harm).


> However, the following two entries at EPSG are conflicting with all
> the above, and do in fact appear to be different:
> http://www.epsg-registry.org/report.htm?type=selection&entity=urn:ogc:def:crs:EPSG::3785&reportDetail=short&style=urn:uuid:report-style:default-with-code&style_name=OGP%20Default%20With%20Code&title=EPSG:3785
> http://www.epsg-registry.org/report.htm?type=selection&entity=urn:ogc:def:crs:EPSG::3857&reportDetail=short&style=urn:uuid:report-style:default-with-code&style_name=OGP%20Default%20With%20Code&title=EPSG:3857


yes, I also found that proj4 treats them equally, but in the end I
thought it would be the best to rely on EPSG for issues on EPSG...


> Ho hum. Maybe take it up with the proj project? Anyway, we use
> Spherical Mercator ie. the one without the +rf=298.25... so I'd find a
> different copy of generate_image.py


Thanks for clarifying this.

I think we should write this in the wiki (basically we shouldn't write
that we are using EPSG 3857 but we could stick to 900913) , as you get
(obviously) different results with and without the ellipsoid. It is
quite confusing indeed, but maybe I was wrong with the 3785 / 3857
idea: I found out that EPSG themselves has 2 versions of EPSG 3857:
one is "not valid" (and identical to what before they called 3785) and
one is "valid" (according to EPSG) which has the ellipsoid with
inverse flattening.

cheers,
Martin



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