What is Scale and Types of Scales in Surveying .

A scale is the ratio of the distance marked on the plan to the corresponding distance on the ground. A good draughtsman can plot a length to accuracy within 0.25 mm. Types of Scales are generally classified as large, medium and small. There are generally 5 types of scales used in surveying which we will discuss below.

• 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.

Purpose of Survey	Scale	R.F
Building Site	1 cm = 10 m	1:1000
Town Planning, Reservoir planning, etc	1 cm = 50 cm to 100 m	1:5000 to 10000
Route Surveys	1 cm = 10 m to 60 m	1:1000 to 1:6000
Longitudional Sections.	1 cm = 10 m 1 cm = 1 m	1:1000 1:100
Cross- Sections	1 cm = 1 m	1:100
Land Surveys/ Cadastral Surveys	1 cm = 10 m to 50 m	1:1000 to 1:5000
Topographical Maps	1 cm = 0.25 km to 2.5 km	1:25000 to 1:250000
Geographical Maps	1 cm = 5 km to 150 km	1:500000 to 1:15000000
Mine Surveys	1 cm = 10 m to 25 m	1:1000 to 1:2500
Forest Maps	1 cm = 250 m	1:25000

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.

Also Read : What is Geographic Information System (GIS) ? 
Also Read : Global Positioning System { GPS } in Surveying

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.

• 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.

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