Practical: Contouring

Introduction:
A contour or a contour line may be defined as the line of intersection of a level surface with
the surface of ground. This means every point on a contour line has the same altitude as that
of the assumed intersecting surface.

The process of tracing contour lines on the surface of the earth is called contouring and the
maps upon which these lines are drawn are called contour maps. a contour map therefore,
gives an ides of the altitudes of the surface feature as well as their relative positions in plan.
Thus a contour map serves the purpose of both, a plan and a section.

Instruments:
1. Dumpy level
2. Theodolite
3. leveling station
4. chain
5. tape
6. pegs

Methods of contour:
There are mainly two methods of locating contours;
(1) Direct method, and

(2) Indirect method.

1. Direct method:
In this method, the contours to be located are directly traced out in the field by locating and
making a number of points on each contour. These points are then surveyed and plotted on
plan and the contours drawn through them. This method is the most accurate but very slow
and tedious as a lot of time is wasted in searching points of the same elevation for a contour.
This is suitable for small areas and where great accuracy is required.

Procedure:
To start with, a temporary B.M. is established near the area to be surveyed with reference to a
permanent B.M. by taking flying levels. The level is then set up in such a position so that the
maximum number of points can be commanded from the instrument station. The height of
instrument is determined by taking a back sight on the B.M. and adding it to the R.L of the
bench mark.
The staff readings required to fix points on the various contours from the height of
instrument. The staff is held on an approximate position of point and then moved up or down
the slope until the desired reading is obtained.

The point is marked with a peg. Similarly various other points are marked on each contour.
The line joining all these points gives the required contour. It may be noted that one contour
is located at a time. Having fixed the contours within the range of the instrument, the level is
shifted and set up in a new position. The new height of instrument and the required staff
readings are then calculated in a similar manner and the process repeated till all the contours
are located.

The position of the contour points are located suitably either simultaneously with levelling or
afterwards. A theodolite or a compass or a plane table traversing is usually adopted for
locating these points. The points are then plotted on the plan and the contours drawn by
joining the corresponding points by dotted curved lines.

Direct method by radial lines:

The contour points are then located and marked on these lines as explained above and their
positions are determined by measuring their distances along the radial lines. they are then
plotted on the plan and the contours drawn by joining all the corresponding points

Indirect method:
in this method, the points located and surveyed are not necessarily on the contour lines but
the spot levels (spot level means the R.L. of a point on the surface of the ground) are taken
along the series of lines laid out over the area.

The spot levels of the several representative points representing hills, depression, ridge and
valley lines, and the changes in the slope all over the area to be contoured are also observed.
Their positions are then plotted on the plan and the contours drawn by interpolation. This
method of contouring is also known as contouring by spot levels.

This method is commonly employed in all kinds of surveys as this is cheaper, quicker and
less tedious as compared with the direct method.
There are mainly three methods contouring under this head:
(i) By Squares.
In this method, the whole area is divided into a number of squares, the sides of which may
vary from 5m to 30m depending upon the nature of the ground and the contour interval. the
squares need not be of the same size throughout, the corners of the squares are pegged out
and the reduced levels of these points are determined with a level.

The important points within the squares may be taken when required and located by
measurements from the corners. The squares are plotted and the reduced levels of the corners
are written on the plan. The contour lines are then interpolated as in fig. 8.9.

(ii) By Cross-Sections
this method is most suitable for survey of long narrow strips such as a road, railway canal etc.
cross -section ore run transverse lo the centre line of the work and representative points are
marked along the lines of cross-section. the cross-section lines need not necessarily be at
right angles to the centre line of the work.

This may be inclined at any angle to the centre line if necessary. the spacing of the cross
-sections depends upon the topography of the country and the nature of the survey, the
common value is 20 to 30m in hilly country and 100m in flat country. the levels of the points
along the section line are plotted on the plan and the contour are then interpolated as usual as
the method can be more clearly understood.
(iii) By Tacheometric Method:
tacheometer is transit theodolite having a diagram fitted with two stadia wires, one above and
other below the central wire. the horizontal distance between the instrument and the staff
-station may be determined by multiplying the difference of the staff readings of the upper
and lower stadia wires with the stadia constant of the instrument, which is usually 100. Thus
the tacheometer is used for both the vertical as well as for the horizontal measurements.

This method is most suitable in hilly areas as the number of stations which can be
commanded by a tacheometer is far more than those by a level and thus the number of
instrument-settings is considerably reduced.

a number of radial lines are laid out at a known angular interval and representative points are
marked by pegs along these radial lines. Their elevations and distances are then calculated
and plotted on the plan and the contour lines are then interpolated.

Relative merits and demerits of direct and indirect methods of contouring:

2. It is used for small areas where great accuracy is desired.

3. It is not very useful when the around is hilly.

4. The calculation work of reducing the levels is comparatively more since the number of
points in command from one set -up of the level is very less.

Indirect method:
1. The method is not very accurate but is cheaper, quicker and less laborious.
2. It is used for large areas where great accuracy is not the main consideration.

3. Tacheometric method of contouring is mainly used for preparing, contour plans of hilly
area. The indirect method by cross -sections is used in route surveys such as a railway, a
canal etc.

4. Area in command from one set -up of the tacheometer is more, therefore, the calculation
work is less.