Hellenic Geodetic Reference System 1987

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The Hellenic Geodetic Reference System 1987 or HGRS87 (Greek: Ελληνικό Γεωδετικό Σύστημα Αναφοράς 1987 or ΕΓΣΑ'87) is a geodetic system commonly used in Greece. The system specifies a local geodetic datum and a projection system. In some documents it is called Greek Geodetic Reference System 1987 or GGRS87.

HGRS87 datum

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{{#coordinates:38.078400|23.932939||||||| | |name= }}). The central pedestal (CP) at this location has by definition HGRS87 coordinates 380 4'  33.8000" N - 230 55'  51.0000"E, N = +7 m.

Although HGRS87 uses the GRS80 ellipsoid, the origin is shifted relative to the GRS80 geocenter, so that the ellipsoidal surface is best for Greece.[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. The initial uncertainty was estimated as 0.1 ppm (1x10−7).[2] However there are considerable tectonic movements that move parts of Greece towards different directions, causing incompatibilities between surveys taking place at different times.[3]

Migration to HTRS07

HGRS87 replaced the earlier de facto geodetic system which was based on the Bessel ellipsoid, with an accurate determination of the geodetic coordinates at the cental premisses of the National Observatory of Athens supplemented by an accurately measured azimuth from the same location to Mount Parnes. Cartographic projections for civilian use were based on the Hatt projection system, with different projection parameters for each 1:100000 map.

While HGRS87 is still widely used for most civilian uses,[4] it is partly replaced by the new Hellenic Terrestrial Reference System 2007 or HTRS07. HTRS07, which was specified for the Hellenic Positioning System (HEPOS) project, is GPS based and is compatible with European Terrestrial Reference System 1989 (ETRS89). HTRS07 is currently used for the cadastral surveys.

HGRS80 projection

HGRS87 also specifies a transverse Mercator cartographic projection (TM) with m0=0.9996, covering six degrees of longitude either side of 24 degrees east (18-30 degrees east). This way all Greek territory (stretching to approximately 90 of longitude) is projected in one zone. References are in meters. Northings are counted from the equator. A false easting of 500000 m is assigned to the central meridian (24 degrees east), so eastings are always positive.[5]

See also

References

  1. ↑ Script error
  2. ↑ Script error
  3. ↑ Script error
  4. ↑ Maps issued by the Hellenic Military Geographical Service for civilian use are based on HGRS87, while maps for military use are based on ED50 with NATO UTM projection.
  5. ↑ Script error

Further reading


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