Theory and reality of direct georeferencing in national coordinates

This paper focuses on the transformation of the sensor's exterior orientation parameters obtained by direct georeferencing (DG) to national reference frames and their subsequent projection for mapping purposes. The required steps are explained and the emphasis is put on the minimization of distortions arising from the curvature of the earth and the inevitable length distortion of the map projection. We recall first the theoretical possibilities of dealing with the vertical distortions due to DG and then introduce a specifically developed technique for their compensation. Afterwards, we demonstrate the influence of such compensation on real examples of small- and large-scale mapping projects. Finally, we offer a comparison with two leading methods and show how these could benefit from extended modeling of the height correction.