[josm-dev] projection Mercator, set EPSG 3785 instead of EPSG 3857

[Sebastian Klein]

Tobias Wendorff wrote:
> That's the spherical formular. When you draw a point in JOSM (x,y)
> it gets translated back to the *sphere*, not to the ellipsoid.
> This spherical information will be stored to the WGS84-database.

Correct, but this is EPSG:3857 by definition.

It would be EPSG:3785 if you would store it to a database of spherical coordinates.

have a look at the spec again:
<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>
<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>

In 3857 you start with WGS84 coordinates and blindly apply the spherical formula. This is exactly what we do. From a scientific viewpoint, the results are flawed, but it is done anyway because the calculation is simple. EPSG:3857 is called "WGS 84 / Pseudo-Mercator" because this is what you do, when you have WGS84 coordinates (e.g. from GPS) and you are too lazy to do proper reprojection. Instead you use the simples formula that produces somewhat usable results.

In contrast to that, with 3785, you start with spherical coordinates and apply *the same* formula. But we don't have spherical coordinates, so we cannot use this method.


Sebastian