Reference Ellipsoid
an earth ellipsoid with certain dimensions and a certain position in the body of the earth. It is used as an auxiliary mathematical surface to which the results of all geodetic measurements on the surface of the earth are referred and on which the points of a geodetic control network are thereby projected. Topographic surveys and maps compiled from such surveys are also referred to the surface of the reference ellipsoid. In order to refer geodetic measurements to the surface of the reference ellipsoid, we must know the height of the earth’s surface above the surface of the reference ellipsoid and the deflection of plumb line at all points at which the measurements are made. The height of the earth’s surface above the surface of the reference ellipsoid is determined by means of geometric and astronomical-gravimetric leveling. On topographic maps, however, the height of the earth’s surface is indicated with respect to sea level.
The dimensions of the reference ellipsoid are given in terms of its semimajor axis a and semiminor axis b or in terms of the semimajor axis and the flattening, or ellipticity, α, which is defined by the equation
The position of the reference ellipsoid within the earth is defined by what are called initial geodetic data, that is, the geodetic coordinates of the perpendicular projection of some point on the earth’s surface onto the reference ellipsoid, the geodetic azimuth of some direction from the point, and the height of the geoid above the reference ellipsoid at the point.
Different reference ellipsoids are used in geodetic and cartographic work in different countries. The USSR and other socialist countries use the Krasovskii ellipsoid as the reference ellipsoid. This ellipsoid is characterized by the quantities
a = 6, 378, 245 m
α = 1:298.3
The position, or orientation, of the Krasovskii ellipsoid within the earth is determined by the geodetic coordinates of the center of the Great Round Hall of the Pulkovo Observatory:
latitude B= 59°46ʹ18ʺ.55
longitude L = 30°19ʹ42ʺ.09
the azimuth to the control point Bugry is
A = 121°10ʹ38ʺ.79
The height of the geoid above the reference ellipsoid at Pulkovo is assumed equal to zero.
REFERENCES
Krasovskii, F. N. Rukovodstvo po vysshei geodezii, part 2. Moscow, 1942.Izotov, A. A. “Novye iskhodnye geodezicheskie daty SSSR.” In Sbornik nauchno-tekhnicheskikh i proizvodstvennykh statei po geodezii, kartografii, topografii, aeros” emke i gravimetrii, fase. 17. Moscow, 1948.
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