INDIRECT DISTANCE MEASUREMENT

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 INDIRECT DISTANCE MEASUREMENT

We discussed on the topic of direct distance measuring instrument on another post . on this we will discuss on the topic of Indirect distance measuring instrument.

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:

  
In this method different instrument is used to measure the distance by indirectly. In this method we not measure the distance directly but we use to measure the certain data and after that by the help of mathematical calculation we used to find out the distance between the two point.

The instrument used during this method are terms as Indirect Distance measuring Instruments:
1.  Theodolite and Staff
2.  Theodolite and Subtense bar
3. E.D.M.  (Electronic Distance Measurement)


Theodolite and Subtense bar:


Subtense bar (Horizontal stave) :


Subtense bar is a distance measuring instrument. This is a rigid rod, usually made up of a stable material like Invar,  which not changes due to variable in temperature. It is of fixed length (typically 2m). It used to  mount over the tripod at the station to which the distance is described. It is bought to level and and reading are carried out through the telescope which is placed over another station .

A theodolite is used to measure the angle between Indicators which was placed on the two ends of the subtense bar. The distance from the telescope to the subtense bar is the height of a triangle, which is an isosceles triangle whose base is length of the subtense bar and the apex is station at which theodolite setup, and it is determined by trigonometry. 


Note:

  1. In subtense bar method, Horizontal angle can't be change. It remain constant.
  2. It always gives horizontal angle (in m).
  3. 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/2
D = (l/2) cot θ/2

Theodolite and Staff:






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:-

    1.  light wave E.D.M.
    2.  Infra red E.D.M.
    3.  Micro wave E.D.M.

 

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