The Hellenic Geodetic Reference System 1987 or HGRS87 ( el, Ελληνικό Γεωδετικό Σύστημα Αναφοράς 1987 or ΕΓΣΑ'87) is a

geodetic system A geodetic datum or geodetic system (also: geodetic reference datum, geodetic reference system, or geodetic reference frame) is a global datum reference or reference frame for precisely representing the position of locations on Earth or other plan ...

commonly used in Greece ( SRID=2100). 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 (). The central pedestal (CP) at this location has by definition HGRS87 coordinates 38° 4' 33.8000" N - 23° 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. 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). However, there are considerable tectonic movements that move parts of Greece towards different directions, causing incompatibilities between surveys taking place at different times. HGRS87 replaced an earlier ''de facto'' geodetic system. The datum of that system was based on the

Bessel ellipsoid The Bessel ellipsoid (or Bessel 1841) is an important reference ellipsoid of geodesy. It is currently used by several countries for their national geodetic surveys, but will be replaced in the next decades by modern ellipsoids of satellite geode ...

, 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. Cartographic projections for civilian use were based on the Hatt projection system, with different projection parameters for each 1:100000 map.

HGRS87 projection

HGRS87 also specifies a transverse Mercator

cartographic projection In cartography, map projection is the term used to describe a broad set of transformations employed to represent the two-dimensional curved surface of a globe on a plane. In a map projection, coordinates, often expressed as latitude and longitud ...

(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 9° of longitude) is projected in one zone. References are in meters. Northings are counted from the

equator The equator is a circle of latitude, about in circumference, that divides Earth into the Northern and Southern hemispheres. It is an imaginary line located at 0 degrees latitude, halfway between the North and South poles. The term can als ...

. A false easting of 500000 m is assigned to the central meridian (24° east), so eastings are always positive. Conversion from geographical coordinates in GRS80 to a projection in HGRS87 is supported by PROJ with appropriate parameters. The conversion is performed with the following code (shell command line): proj +proj=tmerc +lat_0=0 +lon_0=24 +k=0.9996 +x_0=500000 +y_0=0 +ellps=GRS80 +towgs84=-199.87,74.79,246.62,0,0,0,0 +units=m +no_defs

Migration to HTRS07

While HGRS87 is still widely used for most civilian uses,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 UTM may refer to: Computing * Unified threat management, an approach to network security * Universal Turing machine, a theoretical computer * Urchin Tracking Module, a Web analytics package that served as the base for Google Analytics * Usabil ...

/ MGRS projection. it is partly replaced by the new Hellenic Terrestrial Reference System 2007 or HTRS07 ( SRID=96758). 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.

Converters

Online conversion among WGS84 / EGSA87 / HRTS07 / HATTMatlab & C functions converting between WGS84 / EGSA87
(Manolis Lourakis)

HGRS87 datum

HGRS87 projection

Migration to HTRS07

Converters

See also

References

Further reading