- 行业: Earth science
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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 astronomic azimuth taken as the astronomic azimuth of a side in first order (primary) triangulation.
Industry:Earth science
An isostatic gravity correction äg <sub>AH</sub> giving the amount by which calculated gravity must be increased because the base of the crust extends below its average depth into the denser mantle in some regions (such as under mountainous regions) or lies considerably above it and is replaced by mantle in other regions (as under oceans). The isostatic theory of Airy and Heiskanen has the following postulates for use in computations: (a) the density of the crust is a constant 2670 kg/m3; (b) the density of the oceans is a constant 1030 kg/m3; (c) the density of the mantle is a constant 3270 kg/m3. The Airy Heiskanen gravity correction is the difference between the acceleration caused by attraction of matter extending from the bottom of the crust down to the depth of compensation and having the same density as the crust, and the acceleration caused by the attraction of matter actually in the same region and having a density 3270 kg/m3. It is usually calculated by taking the sum of the effects of cylindrical shells, or sectors patterned according to the Hayford zones, and extending from the average depth of the base of the crust to the actual location of the base. For distances out to 167. 7 km from a point, a cylinder of inner radius r1 and outer radius r2 and cut into n equal, vertical prisms contributes the amount äg <sub>AH</sub>(force per unit mass or acceleration, depending on units used): äg <sub>AH</sub> = (2ð G Äñ/n)((r<sub>1</sub>² + D<sub>1</sub>²)1/2 (r<sub>2</sub>² + D<sub>1</sub>²)½ (r<sub>1</sub>² +D<sub>2</sub>²)1/2 + (r<sub>2</sub>² + D<sub>2</sub>²)1/2), in which D<sub>1</sub> and D<sub>2</sub> denote D + d and D, respectively, if the prism is not in an oceanic region and denote D and D d', respectively, if the prism is in an oceanic region. In oceanic regions, the compensating layer begins at depth D d' and ends at depth D; in other regions, it begins at depth D and extends to depth D + d. G is the gravitational constant, Äñ is the difference between the density of the crustal material and the density of the liquid or plastic material supporting the crust. Heiskanen used values of 20, 30, 40, 60, 80 and 100 km for D. The formula assumes that the reference surface is flat. This assumption is not satisfactory if the effect of prisms more than 167. 7 km from the point of interest must be considered. This effect must be calculated using a spherical or ellipsoidal reference surface.
Industry:Earth science
(1) An old French unit of area. The size of the arpent depends on its origin and on local custom. The value 0. 8507 acres has been used for surveys in Arkansas and Missouri. For surveys in Louisiana, Mississippi, Alabama, and northwestern Florida, when no alternative definition for the arpent has been known, the value 0. 84725 acres has been used. The arpent was used, in the U. S. A, in surveys of land granted by the French crown. (2) An old French unit of distance, taken as the length of a side of a square region one arpent in area. Its values in other units, corresponding to the values given in (1) for area, are:<br>
Industry:Earth science
In law, the act of conveying, or the instrument or other legal evidence of the conveyance of, real property. It is usually called common assurance.
Industry:Earth science
(1) The angle from the line of apsides of an elliptical orbit to the radius vector from the attracting focus to a point moving on the ellipse with angular velocity equal to the average velocity of the actual, moving body. It is related to the eccentric anomaly by Kepler's equation. It is related to the period P of the body and to the time t since the body passed through pericenter by the formula (M) = 2πt/P.
(2) In undisturbed elliptical motion, the product of the average angular speed times the interval of time since the body passed pericenter center. Thus the mean anomaly is the angle from pericenter to a hypothetical body moving with a constant angular speed which is equal to average angular speed.
Industry:Earth science
A rotational ellipsoid used in adjustments of horizontal control in Britain. For the triangulation of 1936 to 1950, the dimensions were taken to be:<br>
Industry:Earth science
A plane angle with vertex at the surface of the Earth (or, more usually, at a point on an ellipsoid representing the surface of the Earth).
Industry:Earth science
The angle between half power intensities in the lobes of an antenna pattern.
Industry:Earth science
