# Eccentricity Definitions for Land Surveyors

**eccentric reduction—**The correction which must be applied to a direction observed by an instrument, with either the instrument or signal, or both being eccentric. To reduce the observed value to what it would have been if there had been no eccentricity.

**eccentric signal—**A signal (target) which is not in the same vertical line with the station which it represents.

**eccentric station—**See *station, eccentric; reduction to center.*

**eccentricity-**^{1}Amount of deviation to center.** ^{2} **The eccentricity of a rotational ellipsoid is given by ” , where

*a*and

*b*are the lengths of the equatorial and polar semi-axes, respectively.

^{3}[SURVEYING] An effect due to one or a combination of conditions: a straight line through the ends of the magnetic needle fails to pass through the center of rotation of the needle; the center of rotation of the needle is not coincident with the center of figure of the graduated circle; the line of sight fails to pass through the vertical axis of a surveyor’s compass.

**eccentricity of alidade—**The distance between the center of figure of the index points on an alidade and the center of figure of the graduated circle. The index points (of vernier or micrometer microscope) are carried by the alidade, and any eccentricity of alidade combines with the eccentricity of circle to form the eccentricity of instrument.

**eccentricity of circle—**The distance between the center of figure of a graduated circle and its center of rotation. Eccentricity of circle is usually expressed in terms of its equivalence in seconds of arc on the circle. It may be made quite small by instrument-shop adjustment. Its effect on an observed direction is eliminated by reading the circle at equidistant points around its circumference. See also *eccentricity of instrument.*

**eccentricity of instrument—**The combination of eccentricity of circle and eccentricity of alidade. The effect of eccentricity of instrument on an observed direction is eliminated by having the verniers or micrometer microscopes with which the circle is read spaced at equal distances around the circle.

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

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