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**Surveying (Part 16)**

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.

**Select 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 graduations of the horizontal circle, the measures of the horizontal angles are taken in two or three different zones, i.e., Face Ð 90º , Face L 180º Face L 270º.

**Type of Triangulation stations **

(i) Main station

(ii) Subsidiary stations

(iii) Satellite staions or Eccentric or false stations

(iv) Pivot stations

Signals and Towers The signals may be classified as

(a) Luminous signal, and

(b) Opaque signals or non-luminous signal

- Base line is measured 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 lenght of the tape

C_{a} = correction for 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

C_{n} = (l_{1}+l_{2}) – (l_{1} cosq_{1} + l_{2} cos q_{2})

= l_{1} (1– cosq_{1})+ l_{2} (1- cosq_{2})

It is always substractive

If A and B are not intervisible, then

(3) Correction for slope

Cs = L – D = L – (L^{2} –b^{2})1/2

It is always substractive

or slope correction =L-L cos q = L (1-cosq)

DOAP, tan

0r

and

where, r = radius of the signal q = angle between the sun and the line Op.**Reconnance **

Preliminary field inspection of the entire area to be concerned by triangulation is known as reconnance.

**It includes the following operations:**

(i) Proper examination of the terrian

(ii) Selection of suitable position for base lines

(iii) Selection of suitable positions of triangulation stations.

(iv) Determination of intervisibility of triangulation stations.

**Determination of intervisibility of stations **If there is no obstruction due to intervening ground, the distance of visible horizon from the station of known elevation above mean sea level, may be calculated with the formula. h = 0.06735 D^{2}

- While deciding the intervisibility of various stations, the line of sight should be at least 3 metres above the point of tangency of the earth’s surface to avoid grazing sights.
- Intervisibility of stations intervened by ground profiles

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