FUNDAMENTALS OF SURVEYING : Special Forms of Vernier

Special Forms of Vernier Extended Vernier:

> Sometimes it may happen that divisions on main scale are very close, and then it will be difficult to judge the exact graduations where coincidence occurs.

> Construction is such that n divisions on vernier are equal to (2n —1) divisions on main scale.

nV = (2n —1) S

> Thus the Least Count is difference between two main scale divisions & one vernier scale division

Least Count LC S V

, = —

n

> Generally used in astronomical sextant. Double Folded Vernier:

It is adopted when the length ofthe corresponding double vernier would be so great as to make it impracticable.

Errors Due to Use of Wrong Scale

RF of Wrong Scale

Correct Length =        X Measured Length
RF of Correct Scale

2

  RF of Wrong Scale

Correct Area =                                    XMeasured Area
 RF of Correct Scale 

Errors due to Shrinkage of Scale:

> Suppose, a graphical scale is not drawn on the plan or map and sheet on which plan or map is drawn may shrink due to variation in atmospheric conditions.

> Let the original scale is known (as it will be stated on the sheet). The distance between any two points on the plan can be measured with the help of stated scale and actual distance of known points can be measured on the ground.

Shrunk Length

Therefore, Shrinkage Factor or Shrinkage Ratio =__________________

Original Length

Shrunk Scale = Shrinkage Factor×Original Scale Shrunk RF= Shrinkage Factor × Original RF

Correct Distance =	measured distance
	shrinkage factor

Correct Area =	measured area
	( shrinkage factor)2