A scale is the ratio of the distance marked on the plan to the corresponding distance on the ground.
Types of Scales are generally classified as large, medium and small.
- Large scale : 1 cm = 10 m or less than 10 m.
- Medium scale : 1 cm = 10 m to 100 m.
- Small scale : 1 cm = 100 m or more than 100 m.
Representation of a Scale :
A. Engineer’s scale : 1 cm = 50 m
B. Representative Fraction (R.F) :
It is the ratio of the distance on the map to the corresponding distance on the ground taken as same units. Scale of 1 cm = 50 m, 1 cm on the map represents 50 m (5000 cm) on the ground. Therefore, the representative fraction (R.F.) is 1/5000 or 1: 5000.
C. Graphical scale :
A graphical scale is a line drawn on the map so that its map distance corresponds to a convenient units of length on the ground.
It has the advantage over the numerical scales that the distances on th....Show More
A scale is the ratio of the distance marked on the plan to the corresponding distance on the ground.
Types of Scales are generally classified as large, medium and small.
- Large scale : 1 cm = 10 m or less than 10 m.
- Medium scale : 1 cm = 10 m to 100 m.
- Small scale : 1 cm = 100 m or more than 100 m.
Representation of a Scale :
A. Engineer’s scale : 1 cm = 50 m
B. Representative Fraction (R.F) :
It is the ratio of the distance on the map to the corresponding distance on the ground taken as same units. Scale of 1 cm = 50 m, 1 cm on the map represents 50 m (5000 cm) on the ground. Therefore, the representative fraction (R.F.) is 1/5000 or 1: 5000.
C. Graphical scale :
A graphical scale is a line drawn on the map so that its map distance corresponds to a convenient units of length on the ground.
It has the advantage over the numerical scales that the distances on the maps can be determined by actual scaling even when the map has shrunk.
Types of Measuring Scales in Surveying
Plane Scale :
It is possible to measure two successive dimensions only.
Diagonal Scale :
It is possible to measure three successive dimensions.
Chord scale :
It is used to set out angles without using a protractor.
Vernier Scale :
It is a device for measuring accurately the fractional part of the smallest division on a graduated scale. It divided into,
- Direct Vernier : ‘n’ divisions on the vernier scale are equal in length to (n – 1) divisions on the main scale. Thus ‘n’ divisions of vernier = {n -1) of main scale :
∴ n ‘v’ = (n-1) ‘s’ or v = {(n-1)/n}*s
Where, n = total No. of divisions on vernier scale, v = length of one division on the vernier, s = length of one division on the main scale. The least count (L.C) is, therefore given by
L.C = s – v.
L.C = s – {(n-1)/n}*s.
L.C = s/n.
The verniers, which extend in the same direction graduation of their main scale increases and smallest division is shorter than the in which the smallest division of their main scale, are called as direct vernier.
- Retrograde Vernier :
- The verniers, which extend in the opposite direction of their main scale increases and smallest division is longer than the smallest division of their main scales, are called as retrograde vernier.
‘n’ divisions of the vernier scale are equal to ‘(n+1)’ divisions on the main scale.
∴ n ‘v’ = (n+1) ‘s’
- Extended Vernier : ‘n’ divisions of the vernier scale are equal in length to (2n – 1) divisions of the main scale. Therefore,
∴ n ‘v’ = (2n-1)s
∴ v = {(2n-1)/n}s - Double Vernier : It is used when the graduations on the main scale are numbered in both directions. It is a combination of both direct and retrograde verniers.
- Double folded vernier : Its length is half of corresponding double verniers – economy of space.
Micro scale :
It is a device which enables a measurement to be taken to a still finer degree of accuracy.
Note:
Least count (LC):- The difference between smallest division of main scale & vernier scale is scale by called least count. It is obtained by dividing the value of one division of primary the total number of divisions of the vernier scale. If S is the one division of primary scale and n is the total number of divisions of vernier scale;
then
Least count `=S/n`
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