[Proj] Belge 1972 / Belgian Lambert 72 (31370) - towgs84parameters

Jan Hartmann j.l.h.hartmann at uva.nl
Fri Jan 22 06:42:59 EST 2010

Hi Thibaut, your problem doesn't have anything to do with the two 
different Belgian parameters; the error in your picture is too big. It 
looks as if the WGS84 datum shift has not been applied at all. You could 
test this by projecting the points from the Belgian Lambert projection 
to Google without any towgs parameter.


On 22-1-2010 9:37, Thibaut Gheysen wrote:
> Thanks Jan and Mikael for your responses.
> As suggested, I have forwarded this bug to the QGIS developer mailing 
> list.
> Thibaut.
> 2010/1/21 Jan Hartmann <j.l.h.hartmann at uva.nl 
> <mailto:j.l.h.hartmann at uva.nl>>
>     On 21-Jan-10 12:52, Mikael Rittri wrote:
>>     Jan Hartmann wrote:
>>     > No, if QGIS uses PROJ, this is just an error.
>>     Okay, you may be right that QGIS does not use the file
>>     gcs.override.csv.
>>     But I see that the nad/epsg file of PROJ.4 contains the same
>>     erroneous
>>     +towgs84 parameters for Belge 1972 as the gcs.override.csv.
>>     (At least PROJ version 4.6.1).
>     I have been quoting from PROJ 4.7. The older towgs parameter is
>     not exactly erroneous, it's just a bit les exact
>>     > PROJ and EPSG use opposite rotational formulas, and PROJ uses
>>     degrees,  EPSG radians.
>>     I don't agree in the general case.  PROJ uses the Position Vector
>>     Transform, while EPSG is neutral on the rotation sign convention:
>>     they use the same sign convention as the original source.
>>     And PROJ uses arc seconds for rotations, while EPSG is neutral
>>     on the angle unit: they use the same angle unit as the original
>>     source
>>     (usually arc seconds, but sometimes microradians or radians).
>>          For the EPSG transforms you quote, EPSG use arc-seconds
>>     for the rotations, but either the Position Vector Transform or the
>>     Coordinate Frame Rotation depending on whether they got the
>>     transform from Eurogeographics or directly from Belgium.
>     My information was for the Dutch and Belgian cases, as from the
>     official documents. I don't know on what principles EPSG operates,
>     I guess they just take it as they get it. It is not an easy-to-use
>     database.
>     Jan
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