[Proj] Swiss CORINE: Bessel 1841 Hotine Oblique Mercator Azimuth Natural Origin?

[Paul Kelly]

On Sat, 11 Jul 2009, Ziegler Stefan wrote:

> Hi Markus
>
> please check in the proj4 epsg-file the definition of 21781. It needs
> the +towgs84 parameter:
>
> +proj=somerc +lat_0=46.95240555555556 +lon_0=7.439583333333333
> +x_0=600000 +y_0=200000 +ellps=bessel
> +towgs84=674.374,15.056,405.346,0,0,0,0 +units=m +no_defs
>
> The 200 meters shift sounds a bit like a forgotten towgs84 parameter but 
> that's just a guess. Does the Grass ticket still needs some 
> clarification?

What would be really interesting to understand would be the origins of the 
parameters given in the August 2001 PE & RS Grids & Datums Column on 
Switzerland (<http://www.asprs.org/resources/grids/08-2001-swiss.pdf>) as 
they are what the parameters in GRASS were originally based on. The 
article states:

"Transforming the CH1903+ Datum the to CHTRS95 Datum (WGS84 ellipsoid) 
then is accomplished by dX = +674.253m, dY = +015.053m, dZ = +405.324m, 
according to the Swiss Federal Office of Topography."

This is what GRASS has been using up to now. So why the subtle difference 
in the parameters from the set previously given?

In addition the article goes on to state:

"According to the Swiss Federal Office of Topography, the seven-parameter 
Datum shift from CH1903+ to WGS84 is: dX = +660.077m +/- 4.055m, dY = 
+013.551m +/- 4.816m, dZ = +369.344m +/- 3.914m, alpha = 2.484 cc +/- 
0.417cc, beta = 1.783 cc +/- 0.455cc, gamma = 2.939 cc +/- 0.411cc, and M 
= 1.000000566 +/- 0.000000052."

What do these parameters relate to? I'm very confused.

The other point worth mentioning is that we won't be able to use the NTv2 
grid file in GRASS since (as previously discussed on this list) it 
transforms from CH1903 to CH1903+, rather than to WGS84 (or a closely
"WGS84-compatible" datum) as most gridshift files do.

Best regards

Paul