[GRASS-dev] [GRASS GIS] #2229: error in projection definition

[GRASS GIS]

#2229: error in projection definition
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  Reporter:  vincent             |      Owner:  grass-dev@…
      Type:  defect              |     Status:  new
  Priority:  normal              |  Milestone:  7.0.5
 Component:  Projections/Datums  |    Version:  svn-releasebranch70
Resolution:                      |   Keywords:  pseudomercator
       CPU:  Unspecified         |   Platform:  Unspecified
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Comment (by mlennert):

 As discussed during the code sprint, the issue is because of the very
 weird nature of this "projection" system:

 As mentioned on [http://epsg.io/3857 epsg.io]:

 "Uses spherical development of ellipsoidal coordinates. Relative to WGS 84
 / World Mercator (CRS code 3395) errors of 0.7 percent in scale and
 differences in northing of up to 43km in the map (equivalent to 21km on
 the ground) may arise."

 It uses the WGS 84 datum (which normally implies the WGS 84 ellipsoid),
 but actually develops the coordinates on a sphere. AFAIU, in GRASS, we
 have not foreseen the option to declare one datum, but using a different
 ellipsoid than the one normally linked to that datum.

 The proj.4 definition of this projection system seems to be somewhat of a
 hack.

 In the discussions, one idea that came up was to provide alternative datum
 transformation parameters for this system, one on the sphere and one on
 the wgs84 ellipsoid.

 The most reasonable conclusion, however, was to not use this system at all
 because of it's weird nature. Several official administrations forbid its
 use...

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Ticket URL: <https://trac.osgeo.org/grass/ticket/2229#comment:19>
GRASS GIS <https://grass.osgeo.org>