- 行业: Earth science
- Number of terms: 93452
- Number of blossaries: 0
- Company Profile:
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, ...
Distance measurement by using light. Classical methods use the fact that light travels in a straight line (except for the effect of refraction) by making the light rays the two sides of a isosceles triangle in which the observer is either at the vertex of the triangle and uses the measured angle there together with the known length of the base, or he is at the base and uses the two opposite angles there together with the known length of the base. This is actually distance determination if the observer actually calculates the distance from the measured angle or angles; it is distance measurement if the instrument used does the calculating and displays the corresponding distance. The stadia method and the subtense-bar method are examples. Newer methods do not use the properties of a triangle but measure the distance directly. The instrument measures the difference between the time or phase at which light is emitted and the time or phase at which the light returns from the distant point and converts this quantity to a distance. The term is also applied to methods in which infrared radiation is used instead of light.
Industry:Earth science
(1) The placing of marks on an instrument or device to represent standard values. (2) The marks, on an instrument or device, which represent standard values. The term is applicable especially to a circle because the number of equally spaced marks placed on the circle determines the size of a unit, rather than the size of a unit determining the spacing and number of marks. The term graduation is almost synonymous with division but is more often applied to the placing of intermediate marks on an instrument or device (tape, thermometer, etc. ) by interpolation.
Industry:Earth science
A method of adjusting the results of a trilateration by assuming that the ratio of distances measured from a station does not change and adjusting the ratios of distances rather than the distances themselves. It is also called the line ratio method, Robertson's method and relative lateration. It was invented by K. D. Robertson. Its usefulness depends on the fact that the ratio of two distances measured from the same point at times close together is less sensitive to the effects of atmospheric refraction than are the distances themselves.
Industry:Earth science
A method of adjusting a triangulation network, using adjustment by conditions, by determining corrections to measured directions. Each angle is considered to be the difference between two directions, for each of which a separate correction is determined. The direction method is used in adjusting triangulation networks composed of overlapping triangles, but for some work, where the network consists of a chain of single triangles, the angle method of adjusting triangulation may be preferred.
Industry:Earth science
A thin prism placed in front of the objective of a telescope and rotatable through an accurately measured angle about a line (horizontal or vertical) transverse to the optical axis. Rotating the prism through a known angle deflects the line of sight through a measurable distance on a graduated staff. For a given angle of rotation, the distance of the observer from the staff is a function of the distance the line of sight is deflected along the staff and can be determined from these two quantities. The principle is used in some tachymeters.
Industry:Earth science
A line segment joining two points of a conic and bisecting each member of a family of parallel chords.
Industry:Earth science
(1) A set of precisely spaced, parallel lines which cause incident electromagnetic radiation to be diffracted either upon reflection or upon transmission between the lines. (2) The ruled piece of metal or other material causing diffraction. Diffraction gratings for radio waves usually consist of closely spaced wires held in a frame. Diffraction gratings for radiation at wavelengths from the infrared to ultraviolet are ruled by a graving tool on a plane or curved sheet of metal, or may be produced by photographing onto a glass plate a set of inked lines on paper. At very short wavelengths, the lattices of crystals serve as diffraction gratings.
Industry:Earth science
A system, used within the U. S. Department of Defense, for indexing all aerial photographs held by the U. S. Government. The flights made for aerial photography are plotted on sheets of acetate covering 1<sup>o</sup> quadrangles of the world at a scale of 1:250 000.
Industry:Earth science
