base—The fundamental part of something; basic principle; essence; foundation; basis; groundwork.

base, broken—A base line for triangulation consisting of two or more lines that form a continuous traverse and have approximately the same general direction.

base angle—The horizontal angle between the base line and the orienting line in artillery fire measured from the base line in the same direction as angles are measured by the sight on the gun.

base-height ratio—The ratio between the actual length of the air base and the height above the ground from which’ a stereoscopic pair of aerial photographs were taken; K-factor.

base line, baseline–¹ A surveyed line established with more than usual care, to which surveys are referred for coordination and correlation. ² In GPS, a line between two stations at which simultaneous GPS observations have been made. ³ The change in X, Y, Z coordinates between these two stations; baseline vector. ⁴ In construction, the center line of location of a railway or highway; the base line of location. A reference line for the construction of a bridge or other structure. ⁵ See air base [PHOTOGRAMMETRY].

baseline, independent [GPS]—Baseline determined form independent observing sessions.

base line, triangulation—The side of one of a series of connected triangles, the length of which is measured with prescribed accuracy and precision, and from which the lengths of the other triangle sides are obtained by computation. Important factors in the accuracy and precision of base measurements are the use of standardized Invar tapes, controlled conditions of support and tension, and corrections for temperatures, inclination, and alinement. See also Appendix A, Standards for Geodetic Control Surveys.

base line of location—See base line (definition 4).

base map—See map, base.

base net—A part of a triangulation net starting with a measured base and expanding that relatively short distance to a larger distance comparable to the average length of the triangulation sides. Often little linear progress along the net is achieved in a base net; the primary purpose is usually an expansion of triangle sides.

base rate—An established and usually guaranteed rate of pay per unit of time (as per hour or day) or for production at the standard rate.

bas-relief-¹A photographic print having the appearance of sculpture made from a positive transparency and its negative in contact but with the images not quite coinciding. ² A sculptural relief in which the projection from the surrounding surface is slight and no part of the modeled form is undercut.

base station-¹A station whose position is known, so that measurements made elsewhere can be referred to the value obtained at the base station. ² A receiver set up on such a station, used to make differential corrections for positions obtained by a roving receiver.

base tape—A tape or band of metal or alloy, so designed and graduated and of such excellent workmanship that it is suitable for measuring the lengths of lines (base lines) for controlling triangulation, and for measuring the lengths of first- and second-order traverse lines. In use, a base tape is subject to certain physical conditions which influence its effective length, and which must be taken into account in computing the length of a measured base line. These conditions include tension, temperature, method of support, sag, grade, alignment, and standardization length. See tape corrections. At one time, base tapes were made of steel, a metal having a high coefficient of thermal expansion. They are now made of Invar, Nilvar, or some other alloy having a very small coefficient of thermal expansion. See also Invar; tape.

base tilt—See orientation, basal.

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

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