base line

base line

(1) An imaginary line connecting two points and used as a referent for measurements. Also spelled baseline.

(2) A surveyed line established with more than usual care, to which surveys are referred for coordination and correlation. Also spelled baseline. Base lines are established for specific purposes. The more important ones are defined as follows. (a) The side of one of a series of connected triangles, the length of which is measured with prescribed ratio of standard deviation of the average to the length, and from which the lengths of the sides of the other triangles are obtained by computation. Base lines are classified according to the character of the network in which they are to be included. The instruments and methods to be used in their measurement are such that the pre-scribed ratios of standard deviation of the average to the length are not exceeded. The ratios specified by the U.S. Federal Geodetic Control Committee (1974) were as follows: for inclusion in

first order class I triangulation 1 : 1 000 000

second order class I triangulation 1 : 900 000

second order class II triangulation 1 : 800 000

third order class I triangulation 1 : 500 000

third order class II triangulation 1 : 250 000

A new set of standards for geodetic networks was published by the National Geodetic Survey in 1984 That set contains no standards for the accuracy of base lines It does contain specifications for the errors allowable in measurements of distance using various kinds of instruments (other than tape or wire) Because the new set supersedes the set of 1980, there are no federal standards of accuracy for base lines per se

(a) A line serving as basis for measurement of other lines or the angles between them This definition is clearer than that given in

(b) and is recommended for geodetic purposes

(c) The center line (central line) of location of a railway or highway It is often called the base line of location or baseline of location

(d) A line used as reference for distances and directions in the construction of a bridge or other structure

(e) A line extending east and west along the astronomic parallel passing through the initial point, along which standard township, section, and quarter section corners are established (USPLS) It serves as the starting line for the survey of the meridional township boundaries and section lines Auxiliary governing lines, known as standard parallels or as correction lines, are established along the astronomic parallel usually at intervals of 24 miles north or south of the base line In some of the very early surveys, the base line is referred to as the basis parallel

(f) The line from which the outer limits of the territorial sea and other offshore zones are measured; the dividing line between inland waters and the territorial sea This definition is part of the international Law of the Sea.

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