[Qgis-user] What's wrong with PRJ file for CRS 3857?

[Andre Joost]

Am 16.09.2015 um 22:00 schrieb Redoute:
> Am 16.09.2015 um 20:31 schrieb Andre Joost:
>
> Thank you for your explanations. The discussion if Web Mercator is
> conformal or non conformal seems academical to me.

I added that part to show that this projection is not conformal with 
usual projection definitions that were established in the last centuries 
by geographers.

>  The central point however seems to be
>
>> The proj.4 definition is:
>> which uses a sphere, while the WKT definition in the .prj file is using
>> the ellipsoid:
>
> Are you saying that it is not possible to fully describe this projection
> as WKT without using a "PROJ4 extension" (or to reference auth ids that
> use a "PROJ4 extension")?

No, both WKT definition and proj.4 definition are not capable of 
defining the transition from ellipsoid to spheroid in the way Google 
uses it. The mathematics in the code are just a dirty hack. The 
EPSG:3395 projection was one wrong step on the way to turn Google 
Mercator into something that can be used in GIS software (or GIS towards 
Google Mercator). EPSG refused to accept it a long time, hence 
EPSG:900913 was invented, which is replaced by EPSG:3875 by now.

>
> And this extension is that new/uncommon, so that it cannot be written
> into the primary prj file? Meaning that exchanging CRS 3857 shapefiles
> between different applications, e. g. from QGIS to CartoDB, will ever
> result in heavy projection errors?

ESRI defined the .prj file structure way before Google Mercator was 
known, but they do not add the EPSG code by default. It is allowed to 
expand the WKT definition by adding the proj.4 string and/or theEPSG 
code, but you can not be sure that the other software does it. That is 
why QGIS adds its .qpj file.

HTH,
André Joost