CE 104
Practical Surveying
CONTOURING
What is contouring in Surveying?
Contouring in surveying is the determination of elevation of
various points on the ground and fixing these points of same
horizontal positions in the contour map.
• To exercise vertical control leveling work is carried out and
simultaneously to exercise horizontal control chain survey or
compass survey or plane table survey is to be carried out.
• If the theodolite is used, both horizontal and vertical controls
can be achieved from the same instrument. Based on the
instruments used one can classify the contouring in different
groups.
Characteristics of Contours
1. Contour lines are closed, however they may be close on the
map itself or outside the map depending upon the topography.
2. The spacing between contour lines depends upon the slope of
the ground.
3. In steep slopes, the spacing is small, for gentle slopes the
spacing is large.
4. If the contour lines are equally spaced, they indicate uniform
slope.
5. If the contour lines are parallel, straight or equally placed, they
represent plane surface.
6. In a series of contour lines on the plan or map indicates either a
hill or depression.
7. In case of the hill, the values of the elevation go on increasing
towards the center whereas in case of depression, the values go
on decreasing towards the center
Uses of Contours
1. Selection of suitable site
2. Earth work
3. Area of drainage basin
4. Inter-visibility between two points
Methods of Contour Surveying
1. Direct method
2. Indirect method
Direct method of Contouring
• In this method, the different contour lines are first assumed (say,
100 m, 101 m, etc.) and points on each contour line are located on
the ground by a level. These points are fixed on the ground and
their positions on the maps are plotted by plane tabling or by any
other method. This operation is sometimes called tracing out
contours
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:
i. Method of squares
ii. Method of cross-section
iii. Radial line method
• For general topographical work, the contour interval may be
decided from the following rule:
Contour interval =
=
Problem: Find a suitable contour interval on a map of scale
1: 40,000
Solution:
i) SI units:
On a scale of 1: 40,000
40,000 m = 1 m = 100 cm
or, 40 km = 100 cm
So, 1 km = 100/40 = 2.5 cm
Therefore, contour interval = 25/2.5 = 10 m
ii) FPS units:
On a scale of 1: 40,000
40,000 ft = 1 ft = 12 inch
So, 1 ft = 12/40000 inch
So, 5280 ft or 1 mile = (12 * 5280)/40000 =1.58 inch
Therefore, contour interval = 50/1.58 = 31.58 ft
Direct Method
•This method is 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 area and where great accuracy is required
Procedure:
1. To start with, a temporary B.M is established near the area to
be surveyed with reference to a permanent B.M by fly leveling.
2. The level is then set up in such a position so that the maximum
number of points can be commanded from the instrument
station.
3. The height of instrument is determined by taking a back sight on
the B.M. and adding it to the R.L. of bench mark.
4. The staff reading required to fix points on the various contours
is determined by subtracting the R.L. of each of the contours
from the height of instrument.
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
positions of the contour points are located suitably
either simultaneous with levelling or afterwards. A
theodolite , 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.
Example:
If the height of instrument is 82.48m., then the staff readings
required to locate 82, 81 and 80m contours are 0.48, 1.48 and
2.48m respectively.
The staff is held on an approximate position then moved up and
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 give 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.
Indirect Method
In this method the points located and surveyed are not
necessarily on the contour lines but the spot levels are
taken along the series of lines laid out over the area .The
spot levels of the several representative points representing
hills, depressions, 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.
SQUARE METHOD
