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**Methods of Contour Surveying**

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There are two methods of contour
surveying:

[1]. Direct
method

[2]. Indirect
method

**Direct Method of Contouring**

It consists in finding
vertical and horizontal controls of the points which lie on the selected
contour line.

For vertical control levelling instrument is
commonly used. A level is set on a commanding position in the area after taking
fly levels from the nearby bench mark. The plane of collimation/height of
instrument is found and the required staff reading for a contour line is
calculated.

The instrument man asks staff man to move up
and down in the area till the required staff reading is found. A surveyor
establishes the horizontal control of that point using his instruments.

After that instrument
man directs the staff man to another point where the same staff reading can be
found. It is followed by establishing horizontal control.

Thus, several points
are established on a contour line on one or two contour lines and suitably
noted down. Plane table survey is ideally suited for this work.

After required points
are established from the instrument setting, the instrument is shifted to
another point to cover more area. The level and survey instrument need not be
shifted at the same time. It is better if both are nearby to communicate
easily.

For getting speed in
levelling some times hand level and Abney levels are also used. This method is
slow, tedious but accurate. It is suitable for small areas.

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Indirect Method of Contouring

In
this method, levels are taken at some selected points and their levels are
reduced. Thus in this method horizontal control is established first and then
the levels of those points found.

After
locating the points on the plan, reduced levels are marked and contour lines
are interpolated between the selected points.

For selecting points
any of the following methods can be used:

[1]. Method
of squares

[2]. Method
of cross-section

[3]. Radial
line method

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Method of Squares

In this method area is
divided into a number of squares and all grid points are marked (

**Fig. Con. 10**).
Commonly
used size of square varies from 5 m × 5 m to 20 m × 20 m. Levels of all grid
points are established by levelling. Then grid square is plotted on the drawing
sheet. Reduced levels of grid points marked and contour lines are drawn by
interpolation (

**Fig. Con. 10**).####
Method of Cross-Section

In this method
cross-sectional points are taken at regular interval. By levelling the reduced
level of all those points are established. The points are marked on the drawing
sheets, their reduced levels (RL) are marked and contour lines interpolated.

**Fig. Con. 11**shows a typical planning of this work. The spacing of cross-section depends upon the nature of the ground, scale of the map and the contour interval required. It varies from 20 m to 100 m. Closer intervals are required if ground level varies abruptly.

The cross- sectional line need not be always
be at right angles to the main line. This method is ideally suited for road and
railway projects.

####
Radial Line Method

(

**Fig. Con. 12**). In this method several radial lines are taken from a point in the area. The direction of each line is noted. On these lines at selected distances points are marked and levels determined. This method is ideally suited for hilly areas. In this survey theodolite with tacheometry facility is commonly used.
For

**interpolating contour points**between the two points any one of the following method may be used:
(a) Estimation

(b) Arithmetic
calculation

(c) Mechanical or
graphical method.

**Mechanical or graphical method of interpolation**consist in linearly interpolating contour points using tracing sheet:

On a tracing sheet several parallel lines
are drawn at regular interval. Every 10th or 5th line is made darker for easy
counting. If RL of A is 97.4 and that of B is 99.2 m. Assume the bottom most
dark line represents 97 m RL and every parallel line is at 0.2 m intervals.
Then hold the second parallel line on A.

Rotate the tracing sheet so that 100.2 the
parallel line passes through point B. Then the intersection of dark lines on AB
represents the points on 98 m and 99 m contours (

**Fig. Con. 13**).
Similarly the contour points along any line
connecting two neighbouring points may be obtained and the points pricked. This
method maintains the accuracy of arithmetic calculations at the same time it is
fast.

####
Drawing
Contours

After locating contour
points smooth contour lines are drawn connecting corresponding points on a
contour line. French curves may be used for drawing smooth lines. A surveyor
should not lose the sight of the characteristic feature on the ground. Every
fifth contour line is made thicker for easy readability. On every contour line
its elevation is written. If the map size is large, it is written at the ends
also.

**Contour Gradient**

Gradient represents the ascending or descending slope of the terrain between two consecutive contour lines. The slope or gradient is usually stated in the format 1 in S, where 1 represents the vertical component of the slope and S its corresponding horizontal component measured in the same unit.

The gradient between two consecutive contour lines can also be
expressed in terms of

**tanθ**as follows:**tanθ = Contour Interval (CI) /Horizontal Equivalent (HE)**… both measured in the same unit.**Location of Contour Gradient**

To locate a rising gradient of
1 in 100 from the station P, a level is set up at a commanding position and
back sight is taken at P. Let the back sight reading be 1.255 m. The staff
reading at any point X on the contour gradient can be calculated from its
distance from P (

**Fig. Con. 14**). For the distance XP of 20 m, the required staff reading would be
To locate the point X on the
ground, the staff man holds the 20 m-mark of the tape, keeping the zero-mark at
P, and moves till the staff reading of 1.055 m is obtained. Likewise, the staff
readings for other points at known distance from P, are calculated, and the
points are located. If the point Q is on the contour of 105 m, its distance
from P would be 500 m in this case. The instruments such as Indian clinometer,
theodolite and Ghat tracer may also be used for tracing the contour gradient on
the ground.