[Proj] Meaning of 7-parameter transformation rotations

Martin Vermeer martin.vermeer at hut.fi
Mon Nov 28 18:03:29 EST 2005

I find in the document gen_parms.html the remark

"The seven parameter case uses delta_x, delta_y, delta_z, Rx - rotation
X, Ry -  rotation  Y, Rz - rotation Z, M_BF - Scaling. The three
translation parameters are in meters as in the three parameter case. The
rotational parameters are not in physical units. They are something like the
sine of the rotational angle times the ellipoid axis length, but I don't know
the exact details of how to derive these from a physical description of
the rotation."

However, looking at the example I see


(note the sixth parameter Rz = 0.554)

and for the longitude

4 -> 4d0'0.554"E

where we see a rotation around the z axis of... 0.544". 

Therefore I conclude that the rotation angles are in seconds of arc,
which is indeed common geodetic practice.

- Martin Vermeer

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