#35: Verify and uncomment Swiss datum parameters ---------------------------------+------------------------------------------ Reporter: neteler | Owner: [email protected] Type: defect | Status: closed Priority: major | Milestone: 6.4.0 Component: Projections/Datums | Version: unspecified Resolution: fixed | Keywords: CH1903, Swiss Platform: Unspecified | Cpu: Unspecified ---------------------------------+------------------------------------------ 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. -- Ticket URL: <https://trac.osgeo.org/grass/ticket/35#comment:12> GRASS GIS <http://grass.osgeo.org>
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