[Proj] towgs84 - problem with datum rotation parameters

Paul Kelly paul-grass at stjohnspoint.co.uk
Fri Aug 6 06:57:10 EDT 2004

On Fri, 6 Aug 2004, VAN DAELE, Toon wrote:

> Hi all,
> For the transformation of WGS84 to BD72 (belgian datum), I'm using
> towgs84 with the 7 parameter transformation. The published parameters
> are (national geographical institute, NGI):
> dX=-99.059, dY=53.322, dZ=-112.486, rX=-.419, rY=.83, rZ=-1.885,
> ds=0.99999
> With these parameters the result are about 70 meters away from the
> locations I got with the transformation tool from the NGI. After a few
> hours of trying I found out the calculation is perfect (differences less
> than a centimeter) when only the sings of the rotation parameters are
> changed, thus:
> dX=-99.059, dY=53.322, dZ=-112.486, rX=.419, rY=-.83, rZ=1.885,
> ds=0.99999
> I've checked this with several widely spread control points, and it's
> perfect. Is there an explanation why the rotation parameters could be
> defined in the opposite direction? Have other people experienced the
> same problem with the 7 parameter transformation?

Yes the two different rotation direction conventions is a common and 
well-documented problem. The CRS website is a good reliable source:
gives the parameters as in your second example except that the sign of the 
scale (the last parameter) is reversed, i.e. -1. Does this make any 

The parameters used in GRASS
for the bel72 datum are the same as those on the CRS website, but if you 
can confirm that it still works with the scale factor sign change then I 
will update the parameters in the GRASS datum list to the more precise 
values you have given.

Paul Kelly

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