MODERN SURVEYING - 20CE32P (Session – 2) 2021-22

DEPARTMENT OF COLLEGAITE AND TECHNICAL EDUCATION

STUDY MATERIAL

CONTOURING

SESSION 2

MODERN SURVEYING – 20CE32P

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MODERN SURVEYING - 20CE32P (Session – 2) 2021-22

METHODS OF PLOTTING CONTOURS

The location of a point in topographic survey involves both horizontal as well as vertical control.
The methods of locating contours, therefore depends upon the instruments used. In general the
field method may be divided into two classes.
1. Direct Method: The contour to be plotted is actually traced on the ground. Only these
points are surveyed which happen to be plotted. After having surveyed those points, they
are plotted and contours are drawn through them. The method is slow and tedious and is
used for small areas and where great accuracy is required.
2. Indirect Method: In this method some suitable guide points are surveyed, the guide
points need not to be on the contours. These guide points, having been plotted serve as
basis for the interpolation of contours. This method is most commonly used in
engineering survey.

DIRECT METHOD
As stated earlier in the direct method each contour is located by determining the positions of
series of points through which the contour passes. The operation is also sometimes called tracing
out of contours.
The field work is two-fold:
Vertical control: Location of points on the contour.
Horizontal control: Survey of those points.
1. Vertical Control: The points on the contours are traced either with the help of a level
and staff or with the help of a hand level. In the former case, the level is set at a point to
command as much area as is possible and is levelled. The staff is kept on the BM and the
height of the instrument is determined, If the BM is not nearby. Fly leveling may be
performed to establish a temporary benchmark (TBM) in that area. Having known the
height of the instrument, the staff reading is calculated so that the bottom of the staff is at
an elevation equal to the value of the contour.
For example, if the height of the instrument is 101.80 meters, the staff reading to get a
point on the contour of 100.00 meters will be 1.80 meters. Taking one con- tour at a time
(say 100.0 m contour), the staff man is directed to keep the staff on the points on contour
so that reading of 1.80 m is obtained every time. Thus, in Figure the dots represent the
points determined by this method explained above.

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MODERN SURVEYING - 20CE32P (Session – 2) 2021-22

2. Horizontal Control: After having located the points on various contours, they are to be
surveyed with a suitable control system. The system to be adopted depends mainly on the
type and extent of areas. For small area chain surveying may be used and the points may
be located by offsets from the survey lines. In a work of larger nature, a traverse may be
used. The traverse may be a theodolite or compass or plane table traversing. In this
method two survey parties generally work simultaneously, one for locating the points on
the contours and the other surveying those points. However if the work is of a small
nature, the points may be located first and then surveyed by the same party. In figure the
points shown by dots have been surveyed with respect to points A & B which may be tied
by a traverse shown by chain dotted lines.

RADIAL METHOD
If the area to be contoured is not very extensive, it is more convenient to range out radial lines
from a common centre by theodolite of compass, in direction of greatest utility. Their relative
positions being fixed by measuring the angles between them or by chain survey. For checking the
levels, the bench marks are established at the centre and near the ends of the radial lines, and the
contour points located on these lines, working either inward or outward.
The positions of the pegs making contour points are found either at the same time or afterwards
by measuring their distances along the radial line. They are then plotted on a plan, and the
contours are drawn by joining all the corresponding points.

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MODERN SURVEYING - 20CE32P (Session – 2) 2021-22

For contouring small hilly areas, radial lines are run from the peak to cover the area. The guide
points are taken on the radial lines and their elevations are determined. The contour lines are
drawn by interpolation as shown in figure.

INDIRECT METHOD
In this method some guide points are selected along a system of straight lines and their elevations
are found. The points are then plotted and contours are then drawn by interpolation. These guide
points are not, except by coincidence points on the contour to be located. While interpolating it is
assumed that the slope between any two adjacent guide points is uniform.
The following are some of the indirect methods of locating the ground points.
A. Grid Method.
B. Cross Section Method.
C. Tacheometric Method
D. Radial Method.

1. Grid Method
The method is used when the area to be surveyed is small and the ground is not very much
undulating. The area to be surveyed is divided into a number of squares. The size of the square
may vary from 5 to 20 m depending upon the nature of the contour and contour interval. The
elevations of the corners of the square are then determined by means of a level and a staff. The
contour lines may then be drawn by interpolation. It is not necessary that the squares may be of
the same size. Sometimes rectangles are also used in place of squares. When there are
appreciable breaks in the surface between corners, guide points in addition to those at corners
may also be used.

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MODERN SURVEYING - 20CE32P (Session – 2) 2021-22

The squares should be as long as practicable, yet small enough to conform to the in equalities of
the ground and to the accuracy required. The method is also known spot leveling.

2. Cross Section Method

This method is used for determination of contours along a fixed route such as canal, road &
railways. Cross sections are located on the ground at right angles to the center line.
The spacing of cross section depends on nature of ground, the contour interval and purpose of
contouring. The space should be kept small in ravines & spur, where contour changes its
direction abruptly.
The elevation of guide points on a given cross section is determined using a leveling instrument.
Staff readings should also be taken at points on breaks in ground surface and recorded with
respective distances from fixed line or central line.

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MODERN SURVEYING - 20CE32P (Session – 2) 2021-22

3. Tacheometric Method
In the case of hilly terrain the tacheometric method may be used with advantages. A tacheometer
is a theodolite fitted with stadia diaphragm so that staff readings against all the three hairs may be
taken. The staff intercept ‘S’ is then obtained by taking the difference between the readings
against the top and bottom wires. The line of sight can make any inclination with the horizontal
thus increasing the range of instrument observation. The horizontal distances need not be
measured since the tacheometer provides both horizontal as well as vertical control. Thus if ‘ is
the inclination of line of sight with horizontal, the horizontal distance ‘D’ between the instrument
and the staff and the vertical difference in elevation ‘V’ between the instrument axes and the
point in which the line of sight against the central wire intersects the staff are given by:

D = KS Cos θ + C Cos θ

Sin 2θ
V = KS + C Sin θ
2

Where, K = Multiplying Constant

C = Additive Constant

The tacheometer may be set on a point from which greater control can be obtained. Radial lines
can then be set making different angles with either the magnetic meridian or with the first radial
line. On each radial line, readings may be taken on leveling staff kept at different points. The
points must be so chosen that approximate vertical difference in elevation between two
consecutive points is less than the contour interval. Thus, on the same radial line, the horizontal
equivalent will be smaller for those two points and the vertical difference in elevation of which is
greater and vice versa. To survey an area connected by series of hillocks, a tacheometric traverse
may be run, the tacheometric traverse stations being chosen at some commanding positions.

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MODERN SURVEYING - 20CE32P (Session – 2) 2021-22

At each traverse stations, several radial lines may be run in various directions as required, the
horizontal control being entirely obtained by the tacheometer. The traverse, the radial lines and
the points can then be plotted. The elevation of each point is calculated by tacheometric formulae
and entered, and the contours can interpolated as usual.

FACTORS AFFECTING CONTOUR INTERVAL

The vertical distance between any two consecutive contours is called contour interval. The
contour interval is kept constant for a contour map; otherwise in 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
proper contour interval depends upon the following considerations.

1. The nature of the ground

The contour interval depends upon whether the country is flat or highly undulated. A
contour interval chosen for a flat ground will be highly unsuitable for undulating ground.
For every flat ground, a small interval is necessary. If the ground is more broken, greater
contour interval should be adopted; otherwise the contours will come too close to each
other.

2. The scale of the map

The contour interval should be inversely proportional to the scale. If the scale is small, the
contour interval should be large. If the scale is large, the contour interval should be small.
The following table suggests some suitable values of contour interval.
SCALE OF MAP TYPE OF GROUND CONTOUR INTERVALS
(METERS)
LARGE FLAT 0.20 TO 0.50
(1cm = 10m or less) ROLLING 0.50 TO 1.00
HILLY 1.00, 1.50 TO 2.00
INTERMEDIATE FLAT 0.50, 1.00 OR 1.50
(1cm = 10m or less) ROLLING 1.00, 1.50 OR 2.00
HILLY 2.00, 2.50 OR 3.00
SMALL FLAT 1.00, 2.00 OR 3.00
(1cm = 10m or less) ROLLING 2.00 TO 5.00
HILLY 5.00 TO 10.00
MOUNTAINIOUS 10.00, 25.00 OR 50.00

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MODERN SURVEYING - 20CE32P (Session – 2) 2021-22

3. The purpose and extent of the survey

The contour interval largely depends upon the purpose and the extent of the survey. For
example, if the survey is intended for detailed design work or for accurate earth work
calculations, small contour interval is to be used. The extent of survey in such cases will
generally be small. In the case of location surveys for lines of communication and for
reservoir and drainage areas, where the extent of survey is large, a large contour interval is to
be used.

4. Time and expenses of field and office work

If the time available is less, greater contour interval should be used. If the contour interval is
small, greater time will be taken in the field survey in reduction and in plotting of map.
Considering all these aspects the contour interval for a particular contour plan is selected.
This contour interval is kept constant in that plan otherwise it will mislead the general
appearance of the ground.

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MODERN SURVEYING - 20CE32P (Session – 2) 2021-22

REFERENCES

1. Surveying Volume I by B.C. Punmia, Ashok Kumar Jain & Arun Kumar Jain.16th Edition,
Laxmi publications.
2. Surveying and Levelling by S.S. Bhavikatti, I.K. International Publishing House Pvt. Ltd.
3. Surveying and Levelling by T.P. Kanetkar & S.V. Kulkarni, I.K. Pune Vidyarthi Griha
Prakashan

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