# Projection Definitions for Land Surveyors

**projection-**^{1} The extension of a line beyond the points which determine its character and position. ^{2} The transfer of a series of survey lines to a single theoretical line by a series of lines perpendicular to the theoretical line. In surveying a traverse, a series of measured short lines may be projected onto a single long line, connecting two main survey stations, and the long line is then treated as a measured line of the traverse.

**projection, orthographic**—A perspective azimuthal projection in which the projecting lines, emanating from a point at infinity, are perpendicular to a tangent plane. It is regularly used in mechanical drawings, and when so used, the two vertical planes are revolved about their respective lines of intersection with the horizontal plane so as to show all three views on the plane of the paper. This projection is also used in navigational astronomy for interconverting coordinates of the celestial equator and horizon systems.

**projection, perspective**—The projection of points by straight lines drawn through them from some given point to an intersection with the plane of projection. Unless otherwise indicated, the point of projection is understood to be within a finite distance of the plane of projection. For example, a photograph is formed by a perspective projection of light rays from the rear node of the lens (the point of projection) to the negative (the plane of projection). The term perspective projection is preferable to the term conic map projection, when applied to the geometry of a photograph.

**projector, reflecting** [PHOTOGRAMMETRY]—An instrument used to project the image of an aerial photograph onto a map. By varying the position of the projector lens, the scale of the projected image can be varied, and. by tilting the table top, compensation can be made for any tilt in the photograph.

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

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