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

Jan Hartmann j.l.h.hartmann at uva.nl
Fri Jan 22 09:27:15 EST 2010

It really looks like a QGIS problem. Perhaps you should send a 
bug-report to QGIS, that the correct +towgs84 parameter is the one which 
displays your points correctly. If possible, add both images to it.


On 22-1-2010 13:34, Thibaut Gheysen wrote:
> Hi Jan,
> When I replace towgs84 parameters in QGIS by 
> '+towgs84=-99.059,53.322,-112.486,0.419,-0.83,1.885,-0.999999' the 
> points are correctly placed (red points in the image). When I remove 
> towgs84 parameters, I have a error similar to this with original QGIS 
> parameters but the points are not placed at same place 
> (http://www.fsagx.ac.be/gf/outilslogiciels/Garbel/proj4-2.jpg). I have 
> thus supposed that error come from towgs84 parameters but I'm not a 
> proj.4 specialist at all.
> Thibaut.
> 2010/1/22 Jan Hartmann <j.l.h.hartmann at uva.nl 
> <mailto:j.l.h.hartmann at uva.nl>>
>     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.
>     Jan
>     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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