Geodetic Datum, Initial
Geodetic Datum, Initial
the set of quantities that determine the position of the reference ellipsoid adopted for treating the geodetic grid of a particular country or group of countries relative to the geoid, that is, the quantities that fix the position of the reference ellipsoid in the body of the earth. An initial geodetic datum consists of the geodetic coordinates (specifically, the latitude B0 and the longitude L0 of one of the reference points of the grid, taken as the initial one), the azimuth A0 of the direction from the initial point to one of the adjoining points of the grid, and the elevation £0 of the initial point above the geoid. The initial geodetic datums are determined after deriving the reference ellipsoid by establishing the astronomical coordinates (ϕ0 and λ0) of the initial point and the astronomical azimuth α of the direction from the initial point to the given adjoining point on the grid and correcting for deviations of the plumb line. The geodetic coordinates of all other points of the grid and their azimuths are then obtained by computation on the basis of the results of geodetic measurements related to the surface of the reference ellipsoid. The geodetic coordinates of the points of the USSR’s astronomical-geodetic grid as well as those of several other countries are computed on the surface of the Krasovskii ellipsoid. The center of the Great Round Hall at the Pulkovo Astronomical Observatory serves as the initial point of the USSR’s geodetic grid, and the following geodetic coordinates are adopted for it: latitude B0 = 59°46’18”, 55; longitude L0 = 30°19’42”, 09; elevation £o is set equal to zero. These data for the USSR were derived by A. A. Izotov and M. S. Molodenskii in 1942. Like the Krasovskii ellipsoid, these initial data are taken as the basis of the uniform state system of coordinates in performing all geodetic and cartographic work in Soviet territory.
Since the early 1960’s the methods of space geodesy have made it possible to use observations from artificial earth satellites to obtain the parameters of the earth’s ellipsoid, representing the earth as a whole, and to develop a uniform worldwide system of geodetic coordinates that join together the varied astronomical-geodetic grids of different continents and countries. This holds great scientific and practical importance for resolving problems in geodesy and numerous allied sciences. The unrelated astronomical-geodetic grids that had been worked out previously, with different initial geodetic data and using different reference ellipsoids, can now be related to a single world-wide system of coordinates on one ellipsoid most suited to the earth as a whole planet or to a single worldwide system of rectangular Cartesian coordinates.
REFERENCESZakatov, P. S. Kurs vysshei geodezii. Moscow, 1964.
Izotov, A. A. “Novye iskhodnye geodezicheskie daty SSSR.” In Sbornik nauchno-tekhnicheskikh i proizvodstvennykh statei po geodezii, kartografii, topografii, aeros”emke i gravimetrii, no. 17. Moscow, 1948.
Standartnaia Zemlia: Geodezicheskie parametry Zemli na 1966 god. (Collection of articles.) Moscow, 1969. (Translated from English.)
G. A. MESHCHERIAKOV