[postgis-users] Coordinate transformation problem

[Arnaud Lesauvage]

Hi list !

I am trying to convert data from Belgian Lambert 1972 (SRID 
31370) to WGS84 UTM (zone 31N) (SRID 32631).
The result of Transform(my_lambert_geom,32631) gives my a 
noticeably misplaced result.
The data is approximatively shifted of 100 meters !

(see attached picture, grey is from the Lambert dataset, red 
is from the UTM31N dataset, blue is the UTM31N data shifted 
to fit to the Lambert data)

The Lambert data comes from the Belgian Register, and has a 
.prj file attached to it. I checked the .prj, and it 
corresponds to what I can see in my spatial_ref_sys table.
The WGS84 UTM31N data comes from TeleAtlas (I have 
confirmation that it is in UTM 31N).

How can I explain this shift ? I understand that converting 
conic to geodetic projections implies some distortion, but 
100 meters is quite huge !
What is wrong here ?

Thanks for your help !
--
Arnaud
-------------- next part --------------
A non-text attachment was scrubbed...
Name: LambertBelgian1972_WGS84UTM31N.PNG
Type: image/png
Size: 8167 bytes
Desc: not available
URL: <http://lists.osgeo.org/pipermail/postgis-users/attachments/20061024/a7704fe8/attachment.png>