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**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

- Base line is measurred by standard tape and after that following connections are applied to the measured base length.

(1) Correction for standard length

where L = measured length of the line

l = designed length of the tape

C_{a }= correction fro absolute length

C = correction to be applied to the tape.

The sign of the correction C_{a} 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 C_{s} = L – D

= L – (L^{2}–b^{2})1/2

It is always substractive

or Slope correction = L–L cos q = L (1– cosq)

4) Correction for Tension :

where,P_{o }= 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

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