[Proj] +towgs84 approximation error

Jochem jochem.lesparre at kadaster.nl
Wed Mar 22 16:41:29 EST 2017


I joined the mailing list because I noticed that the PROJ.4 library uses
approximate formulas for the 7 parameter coordinate transformation defined
by +towgs84 (technical details: The elements of the rotation matrix are the
rotation angles instead of the product of sines and cosines of the angles).
For really small rotation angles this gives the same result, but not for
larger rotations. 

For my current application I want to transform a map in QGIS with 1 mm
accuracy. However, the approximation causes an error of 4 cm. Since PROJ.4
normally gives results within 0.1 mm accuracy, I did not expect such

What can I do to help to fix this?

Regards, Jochem


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