[MapProxy] Distortions in EPSG 4236 -> 900913 reprojection

Oliver Tonnhofer olt at omniscale.de
Thu Dec 2 05:25:37 EST 2010

Hi Andreas,

On 01.12.2010, at 20:09, Andreas Trawoeger wrote:
> 2010/11/29 Oliver Tonnhofer <olt at omniscale.de>:
>> I can reproduce the error. libproj wraps around at the date line and transforms a point at 180.001 to -179.999 (in 900913). That messes up the pixel coordinates for the image transformation. Unfortunately, I can't just hack in a quick fix, because it can affect other transformations as well. I'll look into that the next time I touch the transformation code.
> I don't want to count how much time I spend trying to fix or mitigate
> this problem. The bad news is that at first I ended up having exactly
> the same problem while trying to transform from EPSG 900913 -> 4326
> which was one the first thing I tried :-((

It wasn't obvious for me too. I had to dive into the debugger to find it out.

> The good news is that their is an astonishingly simple fix: Simply add
> '+over' to your PROJ.4 EPSG:900913 definition [0].

Ah, good to know that.


Oliver Tonnhofer <olt at omniscale.de>
Omniscale - Dominik Helle, Oliver Tonnhofer GbR
Nadorster Str. 60, 26123 Oldenburg
Tel: +49(0)441/9392774-2 (Fax: 9)

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