HGRS87 specifies a non-geocentric datum that is tied to the coordinates of the key geodetic station at the Dionysos Satellite Observatory (DSO) northeast of Athens (38°04′42″N 23°55′59″E / 38.078400°N 23.932939°E / 38.078400; 23.932939).
[1] The specified offsets relative to WGS84 (WGS84-HGRS87) are: Δx = -199.87 m, Δy = 74.79 m, Δz = 246.62 m. The HGRS87 datum is implemented by a first order geodetic network, which consists of approximately 30 triangulation stations throughout Greece and is maintained by the Hellenic Military Geographical Service.
[2] However, there are considerable tectonic movements that move parts of Greece towards different directions, causing incompatibilities between surveys taking place at different times.
The datum of that system was based on the Bessel ellipsoid, with an accurate determination of the geodetic coordinates at the central premises of the National Observatory of Athens 37° 58' 20.1" N - 23° 42' 58.5"E with current Google Earth TM coordinates:37° 58' 20.20" N - 23° 43' 05.36"E and supplemented by an accurately measured azimuth from the observatory to Mount Parnes.
[6] The conversion is performed with the following code (shell command line): While HGRS87 is still widely used for most civilian uses,[7] it is partly replaced by the new Hellenic Terrestrial Reference System 2007 or HTRS07 (SRID=96758).