maybe this can help (CH1903 to WGS84):
http://www.swisstopo.admin.ch/internet/swisstopo/en/home/products/software/products/skripts.html
GRASS GIS wrote:
#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.
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--
Dr. Eng. Massimiliano Cannata
Responsabile Area Geomatica
Istituto Scienze della Terra
Scuola Universitaria Professionale della Svizzera Italiana
Via Trevano, c.p. 72
CH-6952 Canobbio-Lugano
Tel: +41 (0)58 666 62 14
Fax +41 (0)58 666 62 09
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