a) The S-42 Reference System
The S-42 served as reference frame for countries of former Warsaw Pact.
In the former "Geodetic Surveys of Socialistic Countries" Hungary has been involved to the
S-42 Reference System introduced by the former USSR for common use in member countries.
As commonly known the system is of arbitrary positioned, its Datum point is Pulkovo and the reference surface is the Krassovskij ellipsoid. The Unified AstroGeodetic Network consisting of the subnetworks of the member countries have been commonly adjusted two times: in 1958 and 1983. Each member country's subset of the adjustment has been respectively distributed to the owners.
Due to the above fact all points of the Hungarian control network at 1983 status level have coordinates both in HD-72 and S-42 reference systems. The S-42 coordinates have been classified till the end of 1980's. After introduction of HD-72 by the civilian National Mapping Agency, the S-42 coordinates have been used by the defence organizations only, except for some research purposes.
Transformation between the HD-72 and S-42 can be realised only experimentally, based on the common network points.
b) The Gauss-Krüger projection system in Hungary
This is the projection system having the S-42 bases and the Krassovskij reference surface.
It has been classified till the end of 1980's.
After introduction of EOV projection by the civilian National Mapping Agency, the Gauss-Krüger coordinates have been used by the defence mapping organizations only for their topographic maps of different scales.
Transformation between the EOV and Gauss-Krüger projection systems can be realised only experimentally, based on the common points.
c) Other projection systems used in Hungary
In large area of Hungary till now so called pre-EOV projection systems are in use.
They are as follows: Stereographic projection, Cylindric-North, Cylindric-Central and
Cylindric-South projections as well as some local systems.
The reference surface for them is the Bessel ellipsoid.
No explicit mathematical expression of transformation can be given to perform transformation from any of the above projection system to the EOV. The transformation can be realised only experimentally, based on affine transformation in local areas by use of common points.