Since the area that is surveyed is vast, it is never possible to make the drawings to full size, and therefore, drawings of an area are made to a smaller scale. The operation is generally known as drawing to scale. Types of survey scales can be represented by the following two methods:
- Numerical scales
- Graphical scales
Numerical survey scales are further subdivided into (i) engineer’s scales, and (ii) fraction scales.
(i) Engineer’s scale
One cm in the plan represents some whole number of meters the ground, such as 1 cm = 12m, 1 cm = 100 m, etc. This type of scale is known as the engineer’s scale.
(ii) Fraction scale
One unit of length on the plan represents some number of the same unit of length on the ground, such as 1:500 or 1/500, 1:10000 or 1/10000, etc. This type of scale is known as the fraction scale. The ratio of map (or drawing or plan) distance to the corresponding ground distance is known as the representative fraction (abbreviated as R.F.).
The representative fraction can be easily found for an engineer’s scale. For example, if the scale is 1 cm = 50m, then
R.F. = 1/50*100 = 1/5000
In a similar way, the fraction scale can be converted to engineer’s scale. For example, if the fraction scale is 1:10,000, then
1 unit on plan = 10,000 units on the ground
so, 1 cm on plan = 10,000 cm on the ground
or, 1 cm on plan = 100 m on the ground
Hence, the engineer’s scale is 1 cm = 100 m
A graphical scale is a line subdivided into plan distances corresponding to some convenient units of length on the ground.