[postgis-devel] Problem with projection 31370 / 31300 (Belgian Lambert72)

[Karel Maesen]

Hi,

I recently did a project for a Belgian client. I ran into a problem  
with the definitions for the Belgian Lambert72 CRS in the  
spatial_ref_sys table (srid=31370 and 31300). Neither definition has  
a correct proj4text CRS description string. In both cases the datum  
transformation instruction is missing.

Lambert72 uses the BD72 datum and not WGS84.  Due to the missing  
datum transformation instruction, transforming Lambert72 to WGS84 in  
Postgis results in geographic coordinates relative to the BD72 datum,  
in stead of WGS84. This results in deviations in the order of 100-200  
meter.

I determined that the following string needs to be added to the  
proj4text:  
+towgs84=-106.868628,52.297783,-103.723893,0.336570,-0.456955,1.842183,- 
1

See the page at http://www.ngi.be/NL/NL4-4.shtm for reference.  (Note  
that the parameters indicated on this page are from ETRS89 to BD72.  
They need to be inversed for the +towgs84 instruction. )

Could this be fixed in future versions?

Regards,

Karel Maesen
Geovise BVBA