[Proj] rotated ellipsoid (towgs84)

[Heinrich Schewe]

I try the following datum transformation from WGS84 to a rotated Bessel
ellipsoid. The back-transformation gives a height difference of 0.01 m.

Is this due to the approximate rotation computation (assuming cos = 1,
sin = rad and sin*sin = 0) ?
Are these rotation values too big?
What can I do to get the same values or smaller differences for the
back-transformation?

Heinrich

cs2cs +proj=latlong +datum=WGS84 +to +proj=latlong
+towgs84=0,0,0,6,3,-7,0 +ellps=bessel
48 9 0
48d0'7.982"E    8d59'56.866"N 738.150

cs2cs -I +proj=latlong +datum=WGS84 +to +proj=latlong
+towgs84=0,0,0,6,3,-7,0 +ellps=bessel
48d0'7.982"E    8d59'56.866"N 738.150
48dE    9dN 0.010