Prism Definitions for Land Surveyors

prism-1 A polyhedron having two faces that are polygons in parallel planes while the other faces are parallelograms. 2 A transparent body bounded in part by two plane faces that are not pantile! used to deviate or disperse a beam of light; an electric or magnetic field similarly used for a beam of electrons. 3 Something that refracts light or produces an effect suggestive of a spectrum.

prism, reflecting—A prism in which deviation of a light beam is produced by reflection within the prism. Almost all prisms used in photogrammetric instruments are of this type.

prism, Roelof’s—A prism made to be attached to the front of the objective lens-system of a theodolite’s telescope and permitting precise pointing at the sun’s center by creating four overlapping images of the sun. ’11 images overlap in such a way that all four images intersect at a common point on the optical axis when the telescope is pointed at the center the sun.

prism level—See level, prism.

prismatic compass—See compass, prismatic.

prism correction—Correction of the difference between the measured distance and the true distance between the prism and the observation point. This difference is caused by the reflection of light rays and their change of speed within the prism, or because the center of the prism does not coincide with the vertical axis of the setup.

prismoid—Any solid bounded by planes. and whose end-faces are parallel. It is also usually understood to include figures whose bounding surfaces are warped surfaces.

prismoidal formula—Used for computing volumes of cuts and fills in earthwork, generally in railroad or highway grading.

V1/6 (A1  + 4Am  + A2 )

where V is volume, 1 denotes the length of a uniformly tapering part of the cut or fill, and A r A., and A2, are cross-sectional areas of the ends and middle.

Source: NSPS “Definitions of Surveying and Related Terms“, used with permission.

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