# Distance Definitions for Land Surveyors

**distance, double zenith—**A value of twice the zenith distance of an object, obtained by observation and not by mathematical process.

**distance, horizontal—**The distance, measured on a horizontal line, between the plumb lines of two points.

**distance, external—**The distance from the intersection of the tangents (vertex) of a circular curve to its midpoint, measured along the line extending between the vertex and its center of radius.

**distance, parenthetical **[PLSS]—Regular and fractional portions of lines constituting the boundaries of the quarter-quarter sections and fractional subdivisions bounded thereby; noted on plats in parentheses. Parentheses are used where the record is not supplied by the field notes, indicating that the distance was not measured on the ground. The figures show what was used in the calculation of areas. The same lengths are to be adopted proportionately whenever there is a need for an establishment of sixteenth-section corners on the section boundaries, and for the control points of a subdivision of sections. See also *protraction *[PLSS]*; protraction diagram *[PLSS].

**distance, principal—**The perpendicular distance from the internal perspective center to the plane of a particular length corrected for the enlargement (or reduction) ratio and the film (or paper) shrinkage (or expansion). The line defining the principal distance maintains the same perspective angles at the internal perspective center to points on the finished negative or print, as existed in the taking camera at the moment of exposure. This is a geometrical property of each particular finished negative or print.

**distance angle—**See *angle, distance.*

**distance prorate rule—**See *balancing a survey; distance prorate rule.*

**distance-ratio method—**A method of adjusting the results of a trilateration by assuming that the ratio of distances measured from a station does not change and adjusting the ratios of distances rather than the distances themselves. Its usefulness depends on the fact that the ratio of two distances measured from the same point at times dose together is less sensitive to the effects of atmospheric refraction than are the distances themselves.

**distance wedge**—Principle embodied in some instruments, or in attachments to surveying instruments, where a wedge is placed in front of a portion of the objective that causes the line of sight to be deflected by a known angle and intersects a horizontal staff allowing the distance to be determined from this measured displacement. A reduction tacheometer utilizes a distance wedge, but also has provision for further deflection of the line of sight with telescope inclination.

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

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