[Proj] towgs84 - problem with datum rotation parameters

[VAN DAELE, Toon]

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?

Regards,
Toon


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Toon Van Daele
Instituut voor Natuurbehoud
Kliniekstraat 25, 1070 Brussel
Tel: 02 558 18 10
Fax: 02 558 18 05
GSM: 0498 05 68 90
toon.van.daele at instnat.be
http://www.nara.be
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