LESSON NOTE ON CONTOURS- INTRODUCTION
The art of determining relative altitudes
of points on the surface of the earth of beneath the surface of earth is called
LEVELLING.
A contour is defined as an imaginary line
of constant elevation on the ground surface. It can also be defined as the line
of intersection of a level surface with the ground surface. For example, the
line of intersection of the water surface of a still lake or pond with the
surrounding ground represents a contour line.
Definition:
A line joining points of equal elevations
is called a contour line. It facilitates depiction of the relief of terrain in
a two dimensional plan or map.
Characteristics of contour:
The principal characteristics of contour
lines which help in plotting or reading a contour map are as follows:
1.
The
variation of vertical distance between any two contour lines is assumed to be
uniform.
2.
The
horizontal distance between any two contour lines indicates the amount of slope
and varies inversely on the amount of slope. Thus, contours are spaced equally
for uniform slope
3.
The
steepest slope of terrain at any point on a contour is represented along the
normal of the contour at that point. They are perpendicular to ridge and valley
lines where they cross such lines.
4.
Contours
do not pass through permanent structures such as buildings.
5.
Contours
of different elevations cannot cross each other (caves and overhanging cliffs
are the exceptions).
6.
Contours
of different elevations cannot unite to form one contour (vertical cliff is an
exception).
7.
Contour
lines cannot begin or end on the plan.
8.
A
contour line must close itself but need not be necessarily within the limits of
the map.
9.
A
closed contour line on a map represents either depression or hill . A set
of ring contours with higher values inside, depicts a hill whereas the lower
value inside, depicts a depression (without an outlet).
10.
Contours
deflect uphill at valley lines and downhill at ridge lines. Contour lines in
U-shape cross a ridge and in V-shape cross a valley at right angles. The
concavity in contour lines is towards higher ground in the case of ridge and
towards lower ground in the case of valley .
11.
Contours
do not have sharp turnings.
Contour Interval:
The vertical distance between any two
consecutive contours is known as a contour
interval. For example, if the various consecutive contours are
100m, 98m,96 m etc., then the contour interval is 2m. This interval depends
upon,
·
The
nature of the ground
·
The
scale of the map and
·
The
purpose of survey
Contour intervals for flat country are
generally small, eg. 0.25m, 0.5m, 0.75 m etc. For a steep slope in hilly
area is greater, eg. 5m, 10m, 15 m etc.
Again ,for a small-scale map, the interval
may be of 1m,2m,3m etc. and for large scale map,it may be of 0.25m,0.50m,0.75m
etc.
It should be remembered that the contour
interval for a particular map is constant.
Contouring:
The process of locating these contour
lines on the surface of the earth is known as contouring.
Methods of contouring:
The method of establishing / plotting
contours in a plan or map is known as contouring. It requires planimetric
position of the points and drawing of contours from elevations of the plotted
points. Contouring involves providing of vertical control for location of points
on the contours and horizontal control for planimetric plotting of points.
Thus, contouring depends upon the instruments used (to determine the horizontal
as well as vertical position of points). In general, the field methods of
contouring may be divided into two classes:
1.
Direct
methods
2.
Indirect
methods
Direct
Method:
In the direct method, the contour to be
plotted is actually traced on the ground. Points which happen to fall on a
desired contour are only surveyed, plotted and finally joined to obtain the
particular contour. This method is slow and tedious and thus used for large
scale maps, small contour interval and at high degree of precision.
Vertical
control :
In this method, a benchmark is required in
the project area. The level is set up on any commanding position and back sight
is taken on the bench mark. Let the back sight reading on the bench mark be
1.485 m. If the reduced level of the bench mark is 100 m, the height of
instrument would be 100 + 1.485 = 101.485 m.
To locate the contour of 100.5 m value,
the staff man is directed to occupy the position on the ground where the staff
reading is 101.485 -100.500 = 0.985 m. Mark all such positions on the ground
where the staff reading would be 0.985 m by inserting pegs. Similarly locate
the points where the staff reading would be 101.485 -101 = 0.485 m for 101m
contour.
The contour of 101.5 m cannot be set from
this setting of the instrument because the height of instrument for this
setting of the instrument is only 101.485 m. Therefore, locating contours of
higher value, the instrument has to be shifted to some other suitable position.
Establish a forward station on a firm
ground and take fore sight on it. This point acts as a point of known
elevation, for shifting the position of the instrument to another position,
from where the work proceeds in the similar manner till the entire area is
contoured.
Horizontal
control :
The horizontal control is generally
provided by method of plane table surveying or locating the positions of
points.
Indirect method of contouring:
In this method, the spot levels of
selected guide points are taken with a level and their levels are computed. The
horizontal positions of these points are measured or computed and the points
are plotted on the plan. The contours are then drawn by a process called
interpolation of contours from the levels of the guide points. The following
are the indirect methods are commonly used for locating contours.
1.
Squares
or Grid method
2.
Cross
section method
Square or
grid method:
In this method, the area to be surveyed is
divided into a grid or series of squares. The grid size may vary from 5 m x 5 m
to 25 m x 25 m depending upon the nature of the terrain, the contour interval
required and the scale of the map desired. Also, the grids may not be of the
same size throughout but may vary depending upon the requirement and field
conditions. The grid corners are marked on the ground and spot levels of these
comers are determined by leveling. The grid is plotted to the scale of the map
and the spot levels of the grid corners are entered. The contours of desired
values are then located by interpolation. Special care should be taken to give
the spot levels to the salient features of the ground such as hilltops, deepest
points of the depressions, and their measurements from respective corners of
the grids, for correct depiction of the features. The method is used for large
scale mapping and at average precision.
Cross
section method:
In these sections, a base line, centre
line or profile line is considered. Cross sections are taken
perpendicular to this line at regular intervals. After this points are
marked along the cross sections at regular intervals. A temporary bench
mark is set up near the site. Staff readings are taken along the base line
and the cross sections. The readings are entered in the level book the base
line and the cross sections should also be mentioned. The RL of each of
the points calculated. Then the base line and cross sections are plotted
to a suitable scale. Subsequently the RLs of the respective points are
noted on the map, after which the required contour line is drawn by
interpolation
This method is suitable for route survey,
when cross sections are taken transverse to the longitudinal section.
Method of interpolation of contours:
The process of locating the contours
proportionately between the plotted points is termed interpolation.
Interpolation may be done by:
1.
Arithmetical
calculation
2.
The
graphical method
By arithmetical calculation
Let A and B be two corners of the
squares. The RL of A is 98.75 m, and that of B 100.75 m. the
horizontal distance between A and B is 10m.
Horizontal distance between A and B = 10m
Vertical difference A and B =
100.75-98.75=2m
Let a contour of 99.00 m be
required. Then,
Difference of level between A and 99.00m
contour = 99.00-98.75=0.25m
Therefore, distance of 99.00 m contour
line form A= 10/2 *0.25=1.25m
This calculated distance is plotted to the
same scale in which the skeleton was plotted to obtain a point of RL of 99.00
m.
Similarly, the other points can be
located.
By
graphical method
On a sheet of tracing paper, a line AB is
drawn and divided into equal parts. AB is bisected at C and a
perpendicular is drawn at this point. A point O is selected on this
perpendicular line and then radial lines are drawn from O to the divisions on
AB. After this lines 1-1, 2-2, 3-3….are drawn parallel to AB. These
lines serve as guide lines. Boundary line and every fifth the line is
marked with a thick or red line.
Suppose we have to interpolate a 2m
contour between two points a and b of RLs 92.5 and 100.75m.
Let us consider the lowest radial line OB
to represent an RL of 90.00. So, every fifth line will represent 95,100,105,
etc. The tracing paper is moved over the plan until ‘a’ lies at 92.5 and
‘b’ at 100.25. Line ‘ab’ should be parallel to AB. Now the points 94, 96,
98,100 are picked through to obtain the positions of the required contours.
Method of interpolation of contours:
The process of locating the contours
proportionately between the plotted points is termed interpolation.
Interpolation may be done by:
1.
Arithmetical
calculation
2.
The
graphical method
By arithmetical calculation
Let A and B be two corners of the
squares. The RL of A is 98.75 m, and that of B 100.75 m. the horizontal
distance between A and B is 10m.
Horizontal distance between A and B = 10m
Vertical difference A and B =
100.75-98.75=2m
Let a contour of 99.00 m be
required. Then,
Difference of level between A and 99.00m
contour = 99.00-98.75=0.25m
Therefore, distance of 99.00 m contour
line form A= 10/2 *0.25=1.25m
This calculated distance is plotted to the
same scale in which the skeleton was plotted to obtain a point of RL of 99.00
m.
Similarly, the other points can be
located.
By graphical method
On a sheet of tracing paper, a line AB is
drawn and divided into equal parts. AB is bisected at C and a
perpendicular is drawn at this point. A point O is selected on this
perpendicular line and then radial lines are drawn from O to the divisions on
AB. After this lines 1-1, 2-2, 3-3….are drawn parallel to AB. These
lines serve as guide lines. Boundary line and every fifth the line is
marked with a thick or red line.
Suppose we have to interpolate a 2m
contour between two points a and b of RLs 92.5 and 100.75m.
Let us consider the lowest radial line OB
to represent an RL of 90.00. So, every fifth line will represent 95,100,105,
etc. The tracing paper is moved over the plan until ‘a’ lies at 92.5 and
‘b’ at 100.25. Line ‘ab’ should be parallel to AB. Now the points 94, 96,
98,100 are picked through to obtain the positions of the required contours.
