Fundamentals Of Surveying (Part-2)

In fundamentals of surveying (part-2), we details analysis about plan & map, scale, representation of scale,shrinkage ratio, type of scales & other.

Contents

Plan & Map

Plan: It is the graphical representation of various features on or near to the earth’s surface as projected on a horizontal plane. Plan represents the area on a horizontal plane. The horizontal distances are measured between the various points on the horizontal plane. Because of small areas involved in surveying as compared to earth’s total area, the areas may be regarded as flat surface and the plan is constructed by orthographic projections. A plan is usually drawn or a relatively large scale.

Map: In this, the scale of graphical projection or horizontal plane is small. Due to small scales a map depicts a large number of details as compared to plan. Some additional features are also shown on map like reliefs, contour lines, undulations etc.

Note

Distance: Linear measurement in the horizontal plane is termed as distance.

Elevation: Linear measurement in the vertical plane is termed as elevation.

SCALE

Scale is a technique through which any ground distance is represented on paper or map.

Representation of Scale

There are many way to representation of scale.

1.Engineer’s Scale

This scale is represented by a statement like  1cm = 10m ( 1cm :10m, $$\frac{1cm}{1m}$$ )etc.

Here 1cm on the plan represents & 10m represents ground distance.

2.Representation Fraction (RF)

Here one unit of length on the plan represents some number of the same units of length on the ground.

Example:

1cm = 10m is represented in RF as 1:1000, $$\frac{1cm}{1000cm}$$ or $$\frac{1}{1000}$$

3.Graphical Scale

If the plan or map is to be used after a few years the numerical scales (ES, RF) may not give accurate results, if the sheet or paper “Shrinks or Expands”.

In such a case, graphical scale is used in which scale is drawn on plan or map, which also shrink or expand proportionally & the distance can thus be found accurately.

Shrinkage Factor or Shrinkage Ratio

The ratio of shrunk length to the actual length is know as Shrinkage Factor (SF) or Shrinkage Ratio (SR).

Shrinkage ratio (SR) or (SF) = $$\frac{\text{Shrunk length}}{\text{Original length}}$$

Shrunk Scale = SF × Original Scale

Error Due to Shrinkage of Map

Correct distance on map in term of original scale =$$\frac{\text{measured distance on map}}{SF}$$

Correct area on map in term of original scale =$$\frac{\text{measured area on map}}{SF^{2}}$$

Correct Volume on map in term of original scale =$$\frac{\text{measured volume on map}}{SF^{3}}$$

Error Due to Wrong Measuring Scale

If a wrong measuring scale is used to measure the length or area of an already drawn line or area on the plan, the measured length or area will not be correct.

Hence it is to be correct as follows

Correct length =$$\frac{\text{RF of wrong scale × measured length}}{\text{RF of correct scale}}$$

Correct area =$$\left [ \frac{\text{RF of wrong scale}}{\text{RF of corrcet scale}} \right ]^{2}\times$$measured length

Type of Scales

Scales may be classified as follows

1. Plane Scale

It is a scale on which it is possible to measure two dimensional only units & tenth. Example (cm & m),  (m & dc).

2.Diagonal Scale

On this scale, it is possible to measure three dimensions unit, tenth, & hundredth. Example (m, dm, cm)

3.Vernier Scale

It is a secondary unit of the main scale which is used to measure the dimension even lesser than the least count of the main scale.There are many type of vernier scale.

A. Direct Vernier

This vernier is calibrated  in the same direction of main scale. This vernier has division slightly smaller than division of main scale.

Let (n-1) division of main scale is divided into n division of vernier scale

nv=(n-1)s

v=$$\frac{(n-1)s}{n}$$

s>v

here:

s= least count of main scale (length of 1 division of main scale)

v= length of one division of vernier scale (not the least count)

Least Count (LC): The least count of the vernier is equal difference in length of one division of the main scale & one division of vernier scale.

LC= s-v = s- $$\frac{(n-1)s}{n}$$

LC= $$\frac{ns-(n-1)s}{n}$$

LC= $$\frac{s}{n}$$

This vernier is calibrated  in the opposite direction of main scale. This vernier has division slightly greater than division of main scale.

Let (n+1) division of main scale is divided into n division of vernier scale

nv=(n+1)s

v=$$\frac{(n+1)s}{n}$$

s<v

here:

s= least count of main scale (length of 1 division of main scale)

v= length of one division of vernier scale (not the least count)

Least Count (LC)

LC= v-s= $$\frac{(n+1)s}{n}$$ -s

LC= $$\frac{(n+1)s-ns}{n}$$

LC= $$\frac{s}{n}$$

C. Extended Vernier

Here  (2n-1) division of main scale is divided into n division of vernier scale.

nv=(2n-1)s

v=$$\frac{(2n-1)s}{n}$$

s<v

Least Count (LC)

LC= 2s-v= 2s-$$\frac{(2n-1)s}{n}$$

LC= $$\frac{2ns-2ns+s}{n}$$

LC= $$\frac{s}{n}$$

D. Double Vernier

When the vernier is calibrated in both direction of index line it is called double vernier. To use double vernier main scale should also be calibrated in both direction.

Here also LC= $$\frac{s}{n}$$

Note: Least Count of theodolite is 20″ (20 second)

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