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The Hellenic Geodetic Reference System 1987 or HGRS87 ((ギリシア語:Ελληνικό Γεωδετικό Σύστημα Αναφοράς 1987 or ΕΓΣΑ'87)) is a geodetic system 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, with an accurate determination of the geodetic coordinates at the central premises 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. 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「Hellenic Geodetic Reference System 1987」の詳細全文を読む スポンサード リンク
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