Tachometric survey

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 Tachometric survey 

* Tachometry is a branch of surveying in which horizontal and vertical distances are determined by taking angular observations with an instrument known as tachometer.
• Advantages of tachometry:

(i) The chaining operation is completely eliminated in this type of survey. '

(ii) It is commonly used in location survey for railways, roads, reservoirs etc.
(i) It is adopted in rough and difficult terrain where direct leveling and chaining are either not possible or very tedious.
(ii) It requires much less time.

  • Anallatic lens: 

An additional convex lens is provided between the eye piece and the object glass at a fixed distance from the object glass. This convex lens is known as an anallatic lens such a lens is provided in the external focusing telescope


  • The formula for finding the horizontal distance from the instrument station by fixed hair method to staff rod - .

(i) when the line of sight is horizontal  and staff is held vertically ,

                       D = f/i ×S+(f+d)

(ii) (a) When line of sight is inclined but staff is held vertically and considering angle f of elevation,

         D =f/i × S cos²θ +(f+ d ) cosθ

(b) line of sight is inclined but staff is held vertically and considering angle of depression,

          D =f/i XS cos²θ+ (f+ d ) sinθ

Where,
    D = horizontal distance between the instrument station to leveling staff or stadia rod,

     f= focal length of object glass,

     i = length of real image when anallatic lens is provided
     s = staff intercept
     d = distance between optical centre of the object glass and vertical axis of the instrument
    θ = angle of elevation or depressio

The quantities f/i and (f+d ) are known as tachometric constant . f/i is called multiplying constant ans (f + d) is called additive constant .

. Principle of tacheometry:

The Principle of tacheometry is based on the property of isosceles triangle, where the ratio of the base from the apex and the length of base is always constant.
Here o1 a1 a2 , o1 b1 b2 and o1 c1 c2 all are the isosceles triangles where  D1, D2 and D3 are the distances from the apices to the bases S1 , S2, S3 are the lengths of the bases (staff intercepts).

           
          D1/S1 = D2/S2 = D3/S3 = f/i

According to this principle
      f/i = multiplying constant
       f = focal length of objective
       i = stadia intercept .

                           

   





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