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

[Mark Cave-Ayland]

Karel Maesen wrote:
> 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


Hi Karel,

Bearing in mind that not all versions of PROJ.4 contain all datums, the 
easiest way would be to open an issue within the PostGIS bug tracker and 
post the details there.


ATB,

Mark.

-- 
Mark Cave-Ayland
Sirius Corporation - The Open Source Experts
http://www.siriusit.co.uk
T: +44 870 608 0063