Surveying (Part 12)
Two cases may arise
(I) An observation is made on the bright portion
(II) An observation is made on the bright line
(I) When the observation is made on the bright portion
b = Phase correction angel COP
where r = radius of the signal b = angle between sun and the line OP
II. When the observation is made on the Bright line Ð SCO = 180 (q -b)
The phase correction is applied algebrically to the observed angle according to the relative positions of the sun and the signal, i.e., it is the +ve and zero.
Measurement of Base line The accuracy of any order of triangulation depends upon the accuracy of the measurement of its base line.
Hence in triangulation a base line is of prime importance.
Selection of the site for a Base line
1. The ground of the sight should be fairly level or uniformal.
Number of zeros: To eliminate the error due to inaccurate graduatios of the horizontal circle, the measures of the horizontal angles are taken in two or three different zones, i.e.,
Type of Triangulations stations
(i) Main station
(ii) Subsidiary stations
(iii) Satellite stations or Eccentric or false stations
(iv) Pivot stations
Signals and Towers
(a) Luminous signal, and
(b) Opaque signals or non-luminous signal
where L = measured length of the line
l = designed length of the tape
Ca = correction fro absolute length
C = correction to be applied to the tape.
The sign of the correction Ca will be the same as that of C.
(2) Correction of Alignment
Cn = (l1+l2) - (l1 cos q1 + l2 cos q2) = l1 (1– cos q1)+l2 (1–cosq2).
It is always substractive If A and B are not intervisible, then
(3) Correction for shope Cs = L – D
= L – (L2–b2)1/2
It is always substractive
or Slope correction = L–L cos q = L (1– cosq)
4) Correction for Tension :
where,Po = standard pull P = Pull applied during measurement
If applied pull in more, tension correction is + ve, and if it is less, tension correction is ‘–’ ve