Baseline Measurement, Baseline Extension Methods & Equipments
In triangulation the base line is of prime importance (b/c). it is the only hor distance to measured. It should be measured very accurately since the accuracy of the computed sides of triangulation system depends on it. Length of base line varies form a fraction of (0.5-10)km and a fradion of a mile to 10 miles.
According to the grades of Triangulation
In selecting site for a base line, the following requirement should be considered.
A series of triangles connecting a base line to the main triangulation is called a base net. The base should be gradually expended by triangulation as shown figure (i) and (ii)
Equipments for baseline Measurements
Field Work & Procedure:
It consists of
Base line is divided into section of 0.8km to 1.2km [1/2 to ¾ mile in length]
Another method of measuring the base is to measure the distance b/t the fine marks on two successive tripods.
Reduction to Mean Sea Level:
In geodetic work all the horizontal distances are reduced to their equivalent distance as mean sea level called geodetic distances. If the length of the base is reduced to its equivalent at mean sea level, the computed lengths of all other lines of triangulation sys will corresponds to this level. The mean elev of the base must therefore be determined.
This correction is required for comparison of various bases.
Cmsl = correction to length L
Proof: Let ‘l’ be the length of the base reduced to M.S.L
L = L - Lh/R hence Cmsl = LH/R
The angles measured at the triangulation stations are the horizontal angles and are not effected by the diff of elev of these stations.
Extension of Baseline:
The length of a base line is usually not greater then (10-20) km i.e, (8-10) miles. As it is not possible to obtain a possible side for a long base, the usually practice is to measure a short base and extend it by means of well conditioned triangles.
Suppose it is required to prolong a base line AB. Let CB, the extremities of the base, is fixed accurately in line AB prolonged by a theologize centered over station A or B such that E and F are dearly visible from it and well shaped triangles are formed.
Another and more common method of extension of a base line is shown in the figure. In this method the base a gradually enlarged through the medium of well proportioned triangles. The base AB is extended to CD be selection suitable stations C and D on the opposite side of AB.