# Tangent Definitions for Land Surveyors

**tangent- **^{1 }A straight line that touches a given curve at one and only one point and does not intersect it. ^{2 } In route alignment, a) that part of alignment from one point of intersection (P.I.) to the next PI.; b) the distance from the point of tangency (PT.) of one curve to the point of curvature (PC.) of the next curve; c) the distances from the P.C. to P.I. and the P.I. to the P.T. of a curve; semi-tangent. ^{3 } [USPLS] The line of a great circle normal to the meridian at a selected corner on a base line, standard parallel, or latitudinal township boundary.

**tangent method **[USPLS]—A method of determining the parallel of latitude for the survey of a base line or standard parallel by offsets from a great circle line initiated at an established township corner and tangent to the base line or standard parallel at that corner. The tangent great circle is projected at an angle of 90° from the meridian at the township corner from which it starts, and proper offsets are re-measured north from the tangent to the parallel upon which the corners are established. See also *offset *[USPLS]*; secant method* [USPLS]*.*

**tangent plane****—**A plane that touches a curved surface of double curvature at one and only one point or that touches a curved surface of single curvature along one or more parallel straight lines which are elements of the surface, without intersecting the surface. In geodetic work, a plane tangent to the ellipsoid at any point is perpendicular to the normal at that point. Tangent planes have been used in computing map projections for small areas (local systems of map projections).

**tangent spiral point—**See *curve, tangent spiral point.*

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

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