[GRASS-dev] Re: [GRASS GIS] #35: Verify and uncomment Swiss datum parameters

[GRASS GIS]

#35: Verify and uncomment Swiss datum parameters
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  Reporter:  neteler             |       Owner:  grass-dev at lists.osgeo.org
      Type:  defect              |      Status:  closed                   
  Priority:  major               |   Milestone:  6.4.0                    
 Component:  Projections/Datums  |     Version:  unspecified              
Resolution:  fixed               |    Keywords:  CH1903, Swiss            
  Platform:  Unspecified         |         Cpu:  Unspecified              
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Comment (by pkelly):

 My understanding: ETRS89 is fixed to the European/Eurasian plate. It was
 the same as WGS84 in 1989 and since then has been drifting by a centimetre
 or two per year. But the ellipsoid is the same (well, given the
 GRS80/WGS84 difference which is miniscule). Presumably it's quite easy to
 compensate for the drift between ETRS89 and WGS84 when interpreting GPS
 co-ordinates (do some GPS receivers do this? I have no idea), and then
 after that it is simple to apply a localised country datum transformation
 grid or parameters, which don't change over time.

 The interesting thing about CH1903+ is that it is only compatible with
 ETRS89 in the sense that a simple rotation brings it into alignment
 (3-parameter transform) and there is negligible distortion. But to convert
 from CH1903 to ETRS89 you first need to apply the gridshift transform to
 remove the localised surveying distortions and get to CH1903+, and then
 apply the rotation separately. However the structure of PROJ.4 only allows
 for the concept of a gridshift file converting to WGS84.

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Ticket URL: <https://trac.osgeo.org/grass/ticket/35#comment:12>
GRASS GIS <http://grass.osgeo.org>