- Industry: 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, ...
(1) Naturally occurring earth whose temperature is below 0<sup>o</sup> winter and summer, irrespective of the state of any moisture present. (2) Permanently frozen subsoil.
Industry:Earth science
An arbitrary point to the south and west of a grid overlaying a map, from which the lines of the grid are numbered and from which distances are taken as positive eastward and northward on the grid.
Industry:Earth science
(1) An accurate portrayal of the surface of the Earth in the region being mapped. (2) A portrayal of the most prominent of a dense group of similar features.
Industry:Earth science
The process of determining the distance between two points on the ground by counting the number of paces taken in walking from the one point to the other in a straight line.
Industry:Earth science
Convergent photographs so taken that both photographs show the horizon.
Industry:Earth science
A map projection from an ellipsoid to a developable surface, such that corresponding points on the two surfaces lie on a common line through a single, specified point (the perspective center). A perspective map projection is equivalent, geometrically, to the process of drawing straight lines radially from the selected point (called the perspective center or center of projection) through points on the ellipsoid, to the developable surface. Of the various developable surfaces, only the plane, the cylinder and the cone have been used extensively. Each of these surfaces characterizes an important family of map projections using that surface. (a) The simplest family is that in which the ellipsoid is mapped onto a plane. The plane is usually tangent to the ellipsoid at the center of the region to be mapped. If the ellipsoid is a sphere, the center of projection is usually some selected point on that diameter which passes through the point of tangency. If the center of projection is at the center of the sphere, the gnomonic map projection results; if it is at the opposite end of the diameter from the point of tangency, the stereographic map projection results; and if it is at an infinite distance along that diameter from the point of tangency, the orthographic map projection results. Special names have been given to perspective map projections mapping the sphere onto a plane tangent at a pole, according to the distance of the center of projection from the center of the sphere. Denoting that distance by D and denoting the radius of the sphere by R, the principal named varieties are: Clarke's twilight map projection (D = 1.4 R); James's map projection (D = 1.367R); La Hire's map projection (D = R + ½ R √2); Lowry's map projection (D = 1.69 R); the Parent II map-projection (D = R √3); and the Parent-III-map projection (D = 2.105 R). Airy's map-projection is often considered part of the family of perspective map projections because the principle Airy used in deriving it was also used in deriving some truly perspective map projections. However, it is not, itself, a perspective map projection. (b) Perspective map projections using the cylinder as the developable surface are next up in the order of complexity. Although any ellipsoid can in principle be mapped onto a cylinder by a perspective map projection, this is rarely done. The projection is almost always from a sphere to the cylinder. The cylinder is usually tangent to the sphere and the center of projection is at the center of the sphere. If the cylinder is tangent at the equator, meridians map into equally spaced, parallel, straight lines; the parallels of latitude map into parallel, straight lines; and the two families of straight lines are perpendicular to each other. The lines representing parallels of latitude are not spaced evenly; if the center of projection is at the center of the sphere, the distance between them increases as the tangent of the latitude and the line representing the pole is at an infinite distance from the line representing the equator. (This map projection must not be confused with the Mercator map projection which it resembles.) (c) Perspective map projections using the cone as the developable surface are the most complex and, like the perspective map projection onto a cylinder, is almost always done using a sphere as the original surface. The cone is tangent or secant to the sphere and the center of projection is almost always placed on the axis of the cone usually at the center of the sphere.
Industry:Earth science
A clock in which the intervals of time are determined by the period of a pendulum. Energy is fed to the pendulum by a mechanism or electromagnet called the escapement. To a good first approximation, the period is independent of the mass of the pendulum and depends only on the length of the pendulum and on the force of gravity at the clock. Pendulum clocks were formerly the most accurate timepieces known, and national timekeeping services and observatories used such designs as the Rieffler, Schuler and Short clocks until well into the 1950's.
Industry:Earth science
A disk having concentric, graduated circles drawn or engraved on it, and used for measuring angles. The nonius is named for a Portuguese mathematician and geographer, Nunez (1492 - 1577). It was later replaced by the vernier, but as late as 1879, in Germany, a vernier was still called a nonius.
Industry:Earth science
(1) That temperature which would cause a black-body to radiate the same amount of power as the amount present as noise in a particular electrical circuit. (2) The temperature at which the amount of noise which would be generated by heat in a passive circuit is equal to the actual amount of noise at the terminals. Noise temperature is a convenient way of expressing the amount of unwanted power present in the circuit
Industry:Earth science
