[Proj] +towgs84 approximation error

Thomas Knudsen knudsen.thomas at gmail.com
Wed Mar 22 19:13:32 EST 2017

Hello Jochem,

Perhaps this: https://github.com/OSGeo/proj.4/blob/master/src/PJ_helmert.c
may be of interest to you. And although it is still poorly documented and
probably not available in currently distributed versions of qgis, any
comments you may offer on the functionality, with and without using the
+approx flag will be much appreciated

2017-03-22 22:41 GMT+01:00 Jochem <jochem.lesparre at kadaster.nl>:

> Hello,
> 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
> approximation.
> What can I do to help to fix this?
> Regards, Jochem
> --
> View this message in context: http://osgeo-org.1560.x6.nabble.com/towgs84-
> approximation-error-tp5313738.html
> Sent from the PROJ.4 mailing list archive at Nabble.com.
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