Geodetic reference frames transformations

Local geodetic datums have been developed in the past, in order to satisfy the surveying and mapping requirements of countries all over the world. The broad use of GPS observations, as part of the surveying routine, has shifted the interest from the local to the world geodetic reference systems. The transformation of coordinates between geodetic systems has always been of interest, but the new needs have made it more important. Three dimensional similarity transformations are being used in Geodesy in order to transform coordinates between three dimensional geodetic datums, although the two dimensional approach is often followed, especially for small networks. This paper deals with the congruency of the two approaches as well as with the tolerances with respect to the size of networks, solved within the Hellenic Geodetic Reference System (HGRS '87).