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
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