[Gdal-dev] GDAL with VB.net (VS 2005 Express)

Frank Warmerdam warmerdam at pobox.com
Wed Mar 28 16:22:11 EDT 2007

Milo van der Linden wrote:
> My question goes further. I use Transformpoint to transform a point like 
> (123.01, 234.02,0) where Z is 0.
> My source projection is Dutch: "Rijksdriehoekstelsel" a stereographic 
> projection based upon the Bessel Ellipsoide
> My target projection is lat/lon WGS84, the projection used by Google Earth.
> Because of the difference in ellipsoids, my Z value changes too. the 
> original 0 becomes a negative value in my case because the bessel 
> ellipsoide is "flatter" then the WGS84 ellipsoid. However, my original Z 
> value is relative to the terrain which is ok. It doesn't need to be 
> transformed. Is there a parameter I can use to prevent it from 
> transforming?


In an earlier message you used:

   +proj=latlong +datum=WGS84 +no_defs


   +proj=somerc +lat_0=47.14439372222222 +lon_0=19.04857177777778 \
         +x_0=650000 +y_0=200000 +ellps=GRS67 +units=m +no_defs

you don't include any information in your somerc projection indicating
how you want to transform to wgs84.  So by default PROJ.4 will convert
your location to geocentric coordinates on the GRS67 ellipse, and
then back to lat/long/elev on WGS84.

If you know proper datum shift parameters, add them to your somerc
declaration (eg . +towgs84=10,434,9)

If you don't know them, and you want to avoid a change of ellipsoid
calculation internally just use "+proj=latlong +ellps=GRS67" for
your geographic coordinate system.

Best regards,
I set the clouds in motion - turn up   | Frank Warmerdam, warmerdam at pobox.com
light and sound - activate the windows | http://pobox.com/~warmerdam
and watch the world go round - Rush    | President OSGeo, http://osgeo.org

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