# Correction Definitions for Land Surveyors

**correction—**A quantity which is applied to an observation or function thereof to diminish or eliminate the effects of errors and obtain an improved value of the observation or function. It is also applied to reduce an observation to some arbitrary standard.

**correction, alignment—**A correction applied to the measured length of a line to allow for a tape not being held exactly in a vertical plane containing the line. Although the vertical planes at the two ends of a line which contain the direction of the line at those points are not coincident, they are practically so for all surveying purposes, and any method of aligning a base between its end points will serve in determining an alignment correction.

**correction, anamorphic—**The change made in the scale of a picture in one direction, while the scale in the other direction is unchanged; this corrects for differences in scale introduced when the original picture was taken.

**correction, arc-to-cord—**See *t-minus-T correction.*

**correction, collimation **[LEVELING]—That correction which is applied to an observed difference in elevation to correct for the error introduced by the fact that the line of sights through the leveling instrument is not absolutely horizontal when the bubble is centered in its vial.

**correction, grade—**A correction applied in taping to a distance measured on a slope to reduce it to a horizontal distance between the vertical lines through its end points. See also *correction, slope.*

**correction, index **[LEVELING]—That correction which must be applied to an observed difference of elevation to eliminate the error introduced into the observations when the zero of the graduations on one or both leveling rods does not coincide exactly with the actual bottom surface of the rod.

**correction, length—**The difference between the nominal length of a tape and its effective length under conditions of standardization. When a tape is standardized, its length is determined at a definite temperature under prescribed conditions of tension and support. For use in the field, its coefficient of thermal expansion must be known and the prescribed conditions of tension and support must be approximately satisfied.

**correction, orthometric—**The quantity added to a difference of elevation to correct for the error introduced when level surfaces at different elevations are not parallel; orthometric leveling correction.

**correction, rod **[LEVELING]—That correction which is applied to an observed difference of elevation to correct for the error introduced when the temperature at which the leveling rods are used in the field is different from the temperature at which they were standardized. See also *standardization.*

**correction, sag—**The difference between the effective length of a tape (or part of a tape) when supported continuously throughout its length and when supported at a limited number of independent points.

**correction, slope-1 **In hydrographic surveying, the correction applied to soundings erroneously positioned as a result of an echo-sounder receiving its initial return from a point upslope from its recorded position. **2 **In land surveying, correction applied to a distance measured on a slope to reduce it to a horizontal distance between vertical lines through its end points; correction for inclination of tape, grade correction.

**correction, temperature—**The quantity applied to the nominal length of a tape to allow for a change in its effective length because of its being used at a temperature other than that for which its standard length is given.

**correction, tension—**The correction applied to the nominal length of a tape to allow for a change in effective length because of its being used at a tension other than that for which its standard length is known. See also *tape, normal tension.*

**correction for inclination of the horizontal axis—**A correction applied to a measured horizontal angle to eliminate any error that may have been caused by the horizontal axis of the instrument not having been exactly horizontal.

**correction line—**See *parallel, standard *[USPLS]*.*

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

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