I am creating a geo database for an archaeological excavation. We want to put data from a local system and from UTM35 in the same QGIS-project. I have Ground Control Points in both systems.
I setted the project to UTM35(EPSG 32635) and now want to create a custom coordinate reference system so that every layer in the local system could be converted on the fly to UTM35.
To calculate transformation parameters I used pyproj. I transformed the coordinates of the ground control points of both systems (local und UTM35) to ECEF WGS84 cartesian. To transform the local points to ECEF, I just assumed the cassini projection (Soldner). Second I calculated the 7 Parameters of the Helmert Transformation (with the code of Gabriel De Luca). I now have the parameters to transform between two ellipsoids.
translation (in m):
X: -195.06498913
Y: 495.68894225
Z: 28.56591918
rotation(counterclockwise):
X: 0.017997038245226384 (rad) ; 3712.1556066732 (arcsec)
Y: 0.00945795348286504 (rad) ; 1950.8429426372 (arcsec)
Z: 0.015533377928717144 (rad) ; 3203.9891888298 (arcsec)
scale factor: 0.9999905702146588 in ppm: 1.0000009999905701
I tried this:
+proj=cass +lat_0=37.53084417 +lon_0=27.27634361 +alpha=0 +x_0=2000 +y_0=2000 +ellps=WGS84 +towgs84=-195.06498913,495.68894225,28.56591918,3712.1556066732,1950.8429426372,3203.9891888298,1.0000009999905701 +units=m +no_defs
and this:
+proj=cass +lat_0=37.53084417 +lon_0=27.27634361 +alpha=0 +x_0=2000 +y_0=2000 +ellps=WGS84 +units=m +no_defs +x=-195.06498913 +y=495.68894225 +z=28.56591918+ s=1.0000009999905701 +rx=3712.1556066732 +ry=1950.8429426372 +rz=3203.9891888298
But the result is not as expected. If I import the local ground control points with it, they don't match with the UTM points. There is a distance of 1 up to 20 meters between them.
Everytime I click enter in the custom coordinate menu, QGIS changes the rotation angles to this:
+rx=0.0000049992 +ry=0.0000026272+ rz=0.0000043148
Why does QGIS do that?
Maybe I should use WKT instead.