CONTOURING
A contour is an imaginary line on
the ground joining the points of equal elevation. It is a line in which the
surface of the ground is intersected by a level surface. A contour is a line on
the map representing a contour. A topographic map represents a clear picture of
the surface of the ground. If a map is to a big scale, it shows where the
ground is nearly level, where it is sloping, where the slopes are steep and
where they are gradual. If the map is to a small scale, it shows the flat
country, the hills and valleys, the lakes and the water courses and the other
topographic features. Every fifth contour is darkened as the index contour.
CONTOUR INTREVAL
The vertical distance between any
two consecutive contours is called the contour interval. The contour interval
is kept constant for a contour plan or the topographic map, otherwise the
general appearance of the map will be misleading. The horizontal distance
between two points on two consecutive contours is known as horizontal
equivalent and depends upon the steepness of the ground. The choice of the
proper contour interval depends upon the following considerations:
·
The
nature of the ground
·
The
scale of the map
·
The
purpose and the extent of the survey
·
Time
and expense of field and the office work
CHARACTREISTICS OF CONTOUR
The characteristic features of the
contour which are used while plotting and reading a contour map or the
topographic map are summed up as follows:
ü Two contour lines of different
elevations cannot cross each other. They can cross each other only in the case
of overhanging cliff..
ü Two contour lines of different
elevations cannot unite to form a single. If they do, it is only in the case of
vertical cliff.
ü Closely spaced contour lines
represent a steep slope. Broadly spaced contour lines represent a gentle slope.
Equally spaced contour lines represent a uniform slope. A series of straight,
parallel and equally space contours represent a plane surface.
ü A contour line cannot split into two
or more contour lines.
ü A series of closed contour lines
with higher value of contour i.e. with contour having higher value of elevation
inside represent a hill where as a series of closed contour lines with lower
value of contour inside represent a pond or the depressed land.
ü A contour line must close upon
itself, though not necessary within the limits of the map.
ü Contour lines cross a watershed or
the ridge line at right angles. They form curves of U- shaped round it with the
concave side of the curve towards the higher ground.
ü Contour lines cross a valley line at
right angles. They form sharp curves of V-shaped across it with the convex side
of the curve towards the higher ground.
ü
The same contour appears on the either side of a ridge or valley, for
the highest horizontal plane that intersects the ridge line must cut it on the
both the sides. The same is true of the lower horizontal plane that cuts a
valley.
METHODS OF LOCATING CONTOURS
The location of a point in
topographic survey involves both horizontal as well as vertical control. The methods
of locating contours, therefore, depend upon the instruments used.
a) The direct method
b) The indirect method
In the direct method, the contour to
be plotted is actually traced on the ground. Only those points are surveyed
which needs to be plotted. After having surveyed those points, they are plotted
and the contours are drawn through them. The method is slow and tedious and is used
for the small areas where great accuracy is required.
In the indirect method, some
suitable guide points are selected and surveyed; the guide points need not
necessarily be on the contours. These guide points, having been plotted, serve
as basis for the interpolation of contours. This is the method most commonly
used in engineering surveys.
INTERPOLATION OF CONTOURS
Interpolation of the contour is the
process of spacing the contours proportionately between the plotted ground
points established by indirect methods. The method of interpolation is based on
the assumption that the slope of the ground between the two points, which are surveyed,
is uniform. There are different methods of interpolation of contours. They are
as follows:
(i) By estimation
(ii) By arithmetic calculations
(iii) By graphical method
§ By Estimation
This method is extremely rough and
is used for small scale work only. The position of the contour points between
the guide points are located by estimation.
§ By Arithmetic Calculations
The method, though accurate, is time
consuming. The position of contour points between the guide points are located
by arithmetic calculation.
§ By Graphical Method
In the graphical method, the
interpolation is done with the help of a tracing paper or a tracing cloth.
