- Branche: Earth science
- Number of terms: 93452
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Founded in 1941, the American Congress on Surveying and Mapping (ACSM) is an international association representing the interests of professionals in surveying, mapping and communicating spatial data relating to the Earth's surface. Today, ACSM's members include more than 7,000 surveyors, ...
The whole and fractional number of sidereal days (determine by the equinox of date, that have elapsed at Greenwich since the beginning of the sidereal day that was in progress at Julian Date 0.0 i.e., at noon, 1 January 4713 B.C. For example, 1980 January 0.5 U.T. Has the Greenwich sidereal date 2 450 931.77
Industry:Earth science
The difference between a value of gravity measured at a point on or outside the Earth's surface and the value of gravity calculated for that or for some other specified point.
Industry:Earth science
A development by Helmert (1901) of the formula for gravity acceleration, using the measurements of gravity available at the time but not using any pre assigned value of the Earth's flattening. Referred to the Potsdam value of gravity, Helmert's gravity formula of 1901 is τ <sub>o</sub> = 978.030 (1 + 0.005302 sin²φ - 0.000007 sin²2φ). While the International Gravity Formula is based on a pre assigned value of the Earth's flattening, the above formula is not but was used in determining a value for the flattening, namely, 1/298.2. The coefficients 978.030 (τ <sub>e</sub>) and 0.005302 (β <sub>1</sub>) were determined from the measurements. Attempts made to determine the coefficient of sin²2φ in the same way were abandoned because of the large probable errors that resulted, and the value 0.000007 was based on Darwin's and Weichert's theories of the figure assumed, at equilibrium, by a rotating, liquid body of the size of the Earth. Such a body, unless homogeneous, will not be exactly ellipsoidal in shape but will, in middle latitudes, lie below an ellipsoid (rotational) having the same lengths of major and minor axes. The amount of this depression will depend upon the law used to express the variation of density with respect to distance from the Earth's center. Darwin and Weichert assumed different laws but both found a depression of 3 meters at latitude 45<sup>o</sup>. This depression corresponds to the difference between 0.0000059 and 0.000007, the corresponding coefficients in the International and the Helmert gravity formulae.
Industry:Earth science
A system of meridians and parallels used to locate points on the Earth's surface. The proper name for such a system is geographic coordinate system.
Industry:Earth science
Trilateration in which the distances between stations have been determined by using Hiran.
Industry:Earth science
A formula relating the difference between a star's right ascension á and that star's observed time T of transit across the meridian to the correction (clock correction) ΔT to the observed time, the astronomical latitude Φ of the observer, the star's declination δ and the azimuth constant a, the level constant b and the collimation constant c. It is usually written as α = T + ΔT + b sec Φ + n (tan δ - tan Φ) + c sec Φ, in which n ≡ b sin Φ - a cos Φ
Industry:Earth science
Worldwide relative measurements of gravitational acceleration, used in geodetic studies of the Earth.
Industry:Earth science
A plane containing three ground marks which are also control points. A hill plane may be, but rarely is, a horizontal plane.
Industry:Earth science
A test for the accuracy of graduations on a graduated circle (e.g., a horizontal circle) by measuring repeatedly the angle between two points (e.g. Collimators) subtending a suitable angle (about 40<sup>o</sup>); a number rounds on each face are taken for each angular increment from 0<sup>o</sup> to 360<sup>o</sup>
Industry:Earth science
