[Proj] Cartesian (geocentric) transformations

[Richard didier]

Le Lundi 29 Novembre 2004 11:56, Richard didier a écrit :
> Hi all,
>
> Using the following :
>
> cs2cs \
> +proj=geocent +towgs84=0.0000,0.0000,0.0000 \
> +a=6378137.0000 +b=6356752.3141 +units=m +no_defs \
> +to \
> +proj=geocent
> +towgs84=789.5240,-626.4860,-89.9040,0.6006,76.7946,-10.5788,-32.324100 \
> +a=6378388.0000 +b=6356911.9461 +units=m +no_def
> <<EOF
> 3356123.5400 1303218.3090 5247430.6050
> 3353657.1750 1303862.6620 5249102.0550
> EOF
>
> I got :
> 3353421.83      1304074.31 5248935.61
> 3350954.73      1304718.57 5250606.19
>
> While I'm expecting :
> 3353421.02      1304074.55 5248934.99 (not too bad)
> 3353740.30      1303962.22 5249236.89 (very bad)
>
> any idea ?

I'm replying to myself :-)

The second transformation seems not to have the right parameters !

It should be read like :
cs2cs \
+proj=geocent +towgs84=0.0000,0.0000,0.0000 \
+a=6378137.0000 +b=6356752.3141 +units=m +no_defs \
+to \
+proj=geocent
+towgs84=-80.283,-107.80,-136.031,0.34527,0.0,5.395935,0.185 \
+a=6378388.0000 +b=6356911.9461 +units=m +no_def
<<EOF
3353657.1750 1303862.6620 5249102.0550
EOF
I then got :
3353770.95      1303891.27 5249234.93
which is not too bad because of the 7 parameters that I have back-computed !

didier
-- 
Didier RICHARD, chef de projet "Diffusion des données numériques" 
Institut Geographique National           tel: +33/0 1.43.98.83.23
2/4 Avenue Pasteur                       fax: +33/0 1.43.98.81.71
F-94165 St-Mande Cedex                e-mail: Didier.Richard at ign.fr