INDIRECT DISTANCE MEASUREMENT
INDIRECT DISTANCE MEASUREMENT
The method of direct distance measurement is
unsuitable in difficult terrain and sometimes Impossible when obstructions
occurs. on that condition Indirect Method comes in use.
INDIRECT METHOD OF DISTANCE MEASUREMENT:
Subtense bar (Horizontal stave) :
- In subtense bar method, Horizontal angle can't be change. It remain constant.
- It always gives horizontal angle (in m).
- Accuracy of Horizontal distance & horizontal angle depend up on accuracy of angular measurement
here,
I = Theodolite station
O = subtense bar station (Mid point of subtense bar)
AB = L = length of subtense bar
AO = BO = L/2 = Half length of substense bar
IO =D = distance between two point (theodolite and subtense bar)
θ = Horizontal angle between two indicators
The triangle formed at half length of subtense bar and point of theodolite is right angled triangles IOA & IOB.
Now,
taking right angle triangle IOA,
Tan θ/2 = (l/2) /D (By Pythagoras Theorem)
D = (l/2) / Tan θ/2D = (l/2) cot θ/2
E.D.M.:( Electronic Distance Measurement)
E.D.M. is modern developed surveying instrument by which we can measure the distance with very high accuracy. We can achieve accuracy up to ± 5 mm per Km. It is based on generation, propagation, reflection, and subsequent repetion of electromagnetic waves.
On the basis of carrier used in EDM, it can be classified into three categories:-
- light wave E.D.M.
- Infra red E.D.M.
- Micro wave E.D.M.
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