Parametrization of the Hungarian stereographic map sheets

A topographic map series of Northern Transylvania was drawn in the 1940s. Georeferencing them needs information about their coordinate systems. These maps used two reference systems known as the Budapest and the Marosvásárhely Stereographic projections. The datum transformation parameters of the former had already been determined in previous studies, but they had to be adjusted slightly to use a common Ferro–Greenwich difference for both systems. Parameters of the Marosvásárhely system were only available as a Molodenskiy transformation determined with insufficient accuracy. In this study, we recalculate the Molodenskiy parameters with higher accuracy and determine six parameters from the usual 7-parameter Helmert (aka. Burša–Wolf) transformation. The ideas given in this paper can be useful in other similar cases when the information about base point coordinates on the old system is very limited: We had access only to the ellipsoidal coordinates of the origin and the projected coordinates of one single point. The accuracy of the resulting transformation was evaluated by aligning the old map sheets with recent base maps.