[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):

 ok, I was a bit confused when updating the upcoming release notes if to
 call it an update to CH1903 or CH1903+. From comment:2 I took the new
 3-param terms in r38376 to belong to CH1903+.

 Yes I was confused too, which is why I asked on the PROJ.4 list for more
 clarification. My understanding now is that the 3-parameter transformation
 is an (almost) exact transformation from CH1903+ to ETRS89 and has the
 same accuracy over all of Switzerland. Since CH1903 and CH1903+ are very
 close and only ever differ from each other by a couple of metres (the
 difference varies across Switzerland), it is reasonable to consider CH1903
 as an approximation of CH1903+ and hence the parameters are also valid for
 CH1903 to ETRS89 (with an accuracy of a couple of metres). Indeed as the
 difference between ETRS89 and WGS84 (a few tens of centimetres) is much
 smaller than the accuracy of the parameters as applied to CH1903, they can
 be equally considered an accurate transformation from CH1903 to WGS84.

   so will the 3-param terms drift at the same rate as the plate?

 As mentioned above for CH1903, for the foreseeable future the difference
 between WGS84 and ETRS89 due to drifting of the plate is much smaller than
 the error in the 3-parameter transformation. So for CH1903 the accuracy of
 the 3-parameter transformation (when used to transform to WGS84) can be
 considered constant over time. (Obviously applied to ETRS89 the accuracy
 is actually constant over time.)

 When the 3-parameter transform is applied to the CH1903+ to WGS84
 transformation you are right that the accuracy will gradually degrade over
 time (a few centimetres per year). Again, applied to the CH1903+ to ETRS89
 transformation it is constant.

 Now I'm just waiting for someone to tell me I've missed an important
 detail and got this all wrong! ;) I think we need a new library in the
 free software world that separates the datum from the projection handles
 all this stuff properly for best accuracy.

 GRASS has never had an entry in the datum.table for CH1903+, only CH1903.
 All I did in the SVN commit was to correct the 3-parameter transform for
 CH1903 (and it was a very small change). I guess we could add CH1903+ in
 with the same parameter set, but I wasn't sure if + signs would be allowed
 in the datum name and that could take a while to check so didn't want to
 rush into making that change. Ideally we would support CH1903 and CH1903+
 separately, using the gridshift file for CH1903, but the way PROJ.4
 currently works we can't support that...

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Ticket URL: <https://trac.osgeo.org/grass/ticket/35#comment:14>
GRASS GIS <http://grass.osgeo.org>