Hellenic Geodetic Reference System 1987
Encyclopedia
The Hellenic Geodetic Reference System 1987 or HGRS87 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.
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 station
s 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.
, 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 objervatory 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, 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
.
. A false easting of 500000 m is assigned to the central meridian (24 degrees east), so eastings are always positive.
Geodetic system
Geodetic systems or geodetic data are used in geodesy, navigation, surveying by cartographers and satellite navigation systems to translate positions indicated on their products to their real position on earth....
commonly used in Greece
Greece
Greece , officially the Hellenic Republic , and historically Hellas or the Republic of Greece in English, is a country in southeastern Europe....
. 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 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. 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
Geodetic network
A geodetic network is a network of triangles which are measured exactly by techniques of terrestrial surveying or by satellite geodesy.In "classical geodesy" this is done by triangulation, based on measurements of angles and of some spare distances; the precise orientation to the geographic north...
, which consists of approximately 30 triangulation station
Triangulation station
A triangulation station, also known as a triangulation pillar, trigonometrical station, trigonometrical point, trig station, trig beacon or trig point, and sometimes informally as a trig, is a fixed surveying station, used in geodetic surveying and other surveying projects in its vicinity...
s throughout Greece and is maintained by the Hellenic Military Geographical Service
Hellenic Military Geographical Service
The Hellenic Military Geographical Service or HMGS -History:Established in 1889 as the “Geodetic Mission” with the purpose of compiling the “Topographic and Cadastral Map of the Country”, the service acquired its current name in 1926...
. 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.
Migration to HTRS07
HGRS87 replaced an earlier de facto geodetic system. The datum of that system was based on the Bessel ellipsoidBessel ellipsoid
The Bessel ellipsoid is an important reference ellipsoid of geodesy. It is currently used by several countries for their national geodetic surveys, in Europe and on other continents, but will be replaced in the next decades by modern ellipsoids of satellite geodesy.The Bessel ellipsoid was derived...
, with an accurate determination of the geodetic coordinates at the cental premisses of the National Observatory of Athens
National Observatory of Athens
The National Observatory of Athens is a research institute in Athens, Greece. Founded in 1842, it is the oldest research foundation in Greece, as it was the first scientific research institute built after Greece became independent in 1829, and one of the oldest research institutes in Southern...
supplemented by an accurately measured azimuth from the objervatory to Mount Parnes
Parnitha
Mount Parnitha is a densely forested mountain range north of Athens, the highest on the peninsula of Attica, with an elevation of 1,413 m and a summit known as Karavola...
. 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, 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
European Terrestrial Reference System 1989
The European Terrestrial Reference System 1989 is a ECEF geodetic Cartesian reference frame, in which the Eurasian Plate as a whole is static...
(ETRS89). HTRS07 is currently used for the cadastral surveys
Cadastre
A cadastre , using a cadastral survey or cadastral map, is a comprehensive register of the metes-and-bounds real property of a country...
.
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 equatorEquator
An equator is the intersection of a sphere's surface with the plane perpendicular to the sphere's axis of rotation and containing the sphere's center of mass....
. A false easting of 500000 m is assigned to the central meridian (24 degrees east), so eastings are always positive.
Further reading
- Συντεταγμένες ΕΓΣΑ’87 σε δέκτες GPS Programming popular GPS receivers to display coordinates using the HGRS87 datum (in Greek).
- Coordinate Reference Systems GR
- Map Projections GR)|year=1995|author=Hellenic Mapping & Cadastral Organisation|month=October|language=Greek|publisher=Hellenic Mapping & Cadastral Organization (HEMCO)|location=Athens, Greece}})|publisher=Hellenic Mapping & Cadastral Organization (HEMCO) |location=Athens, Greece}}