- 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, ...
A formula giving the value of gravity as a function of geodetic or geocentric latitude (and possibly also of geodetic or geocentric longitude and/or height), and considered standard by a large number of geodesists. Commonly called a normal gravity formula if the formula applies to the gravity field of a rotational ellipsoid on which the gravity potential has one specified value and which rotates as the same rate as the Earth. Standard gravity formulae are used because differences between actual gravity and theoretical gravity are more easily handled than are the actual values themselves and because users of gravity data can more conveniently compare results if a common reference formula is used. The term normal has entered American usage through European geodesy, in which normal has the meaning standard. Four gravity formulae: the Helmert 1901 gravity formula, the International Gravity Formula, Gravity Formula 1967 and gravity Formula 1980, have commonly been considered standard gravity formulae by geodesists. These usually have one of three forms: τ = τ <sub>o</sub> (1 + Β 1 sin²φ + β 2 sin4φ) ,<br>
Industry:Earth science
The difference between the magnitude of a measured value of gravity at a point on the earth's surface and a corresponding, theoretical value of gravity at that same point.
Industry:Earth science
A table giving the deformation of the geoid and the effect of this deformation on gravity, computed for masses of unit density extending to various distances above and below the surface of the geoid. Fundamental gravity tables serve as the basis for preparing special tables using particular assumptions about density, isostasy, and so on. Several such fundamental tables have been prepared, each designed for a particular effect of such assumptions. (a) The tables of Cassinis, Dore and Ballarin (1937) determine the direct effect, on gravity, of masses of unit density extending to various distances above and below the geoid. This direct effect is known as the Hayford effect; it neglects the differences of elevation between the reference ellipsoid and the geoid. (b) The tables of Lambert and Darling (1938) determine the indirect effect of masses of unit density extending to various distances above and below the geoid. This indirect effect is known as the Bowie effect; it takes into account the difference of elevation between the reference ellipsoid and the geoid. (c) The tables of Darling (1949) give the horizontal effect (deflection of the vertical) of masses of unit density extending to various distances above and below the geoid.
Industry:Earth science
An instrument for measuring the rate of change of gravity with a vertical or horizontal change of location. The torsion balance is at present the most sensitive and widely used type of gravity gradiometer. It relies on the static balancing of gravity gradient by torsion on a wire. However, instruments depending on dynamical principles and involving rapidly revolving masses have been invented. Such instruments are sturdier than the torsion balance, require less time to make a measurement, and can be moved rapidly about while in operation.
Industry:Earth science
A geometric figure (pyramid) whose base is the triangle formed from three control points on the ground and whose apex is the perspective center of the camera at the instant the photograph showing the control points was taken.
Industry:Earth science
A matrix such that the elements of the transposed matrix are equal to the complex conjugates of the corresponding elements of the original matrix.
Industry:Earth science
A grid extending beyond its normal limits on a map so as to cover maps of regions governed by a different grid. Normally, large scale maps of regions that lie within approximately 25 km of the dividing line between two grids will bear an overlapping grid indicated by short lines (ticks) drawn outward from the neatline of the map.
Industry:Earth science
The sum of the elevation of a point and the geodetic height of the geoid above the reference ellipsoid at the corresponding point on the geoid. Ellipsoidal height is neither a height nor an elevation, but the quantity is sometimes convenient as a substitute for the actual vertical distance above a terrestrial ellipsoid.
Industry:Earth science
A graphical representation of a frequency function, in which the frequency of occurrence is indicated by the height of a rectangle whose base is proportional to the interval within which the events occur with the indicated frequency.
Industry:Earth science
(1) An instrument used in determining altitudes of points on the Earth's surface in relation to sea level by determining atmospheric pressure through observation of the boiling point of water at each point. Explorers have made much use of this method for determining the heights or altitudes of mountaintops. (2) A differential barometric altimeter. I.e., a barometric altimeter indicating changes in altitude with respect to a surface of constant pressure. (3) An instrument used in forestry for determining heights of trees. (4) A person who measures elevations or the land, or who constructs maps or models to show elevations of land. This use is rare.
Industry:Earth science
