4.

Contouring (8 marks)
4.1 Introduction

An Imaginary line on the ground surface joining the points of equal elevation is known as
contour.
This line on the map represents a contour and is called Contour line.
A map showing Contour Lines is known as Contour Map.

Contouring
The process of tracing contour lines on the surface of the earth is called Contouring.

Purposes of Contouring
Contour survey is carried out at the starting of any engineering project such as a road, a
railway, a canal, a dam, a building etc.

1. For preparing contour maps in order to select the most economical or suitable site.
2. To locate the alignment of a canal so that it should follow a ridge line.
3. To mark the alignment of roads and railways so that the quantity of earthwork both
in cutting and filling should be minimum.
4. For getting information about the ground whether it is flat, undulating or
mountainous.
5. To locate the physical features of the ground such as a pond depression, hill, steep or
small slopes.
Contour Interval
The vertical distance between any two consecutive contours, is called contour interval. It is
kept the same on a map to depict correct topography of the terrain.

 Contour interval is kept large up to 2m for projects such as highways and railways
whereas it is kept as small as 0.5m for earthwork, building sides and Dams etc.
 Contour intervals for flat ground are generally small, e.g 0.25m, 0.5m, 0.75m etc.
 Contour interval for a steep slope in a hilly area is generally greater i.e 5m, 10m etc.
 It should be recommended that the contour interval for a particular map is constant.
Factors for deciding contour interval
a) The nature of the ground (i.e. whether flat or sleep)
b) The scale of the map
c) The purpose of the survey
d) Availability of time and funds
For general topographical maps,
25
𝑐𝑜𝑛𝑡𝑜𝑢𝑟 𝑖𝑛𝑡𝑒𝑟𝑣𝑎𝑙 = 𝑚
𝑛𝑜 𝑜𝑓 𝑐𝑒𝑛𝑡𝑖𝑚𝑒𝑡𝑟𝑒𝑠 𝑝𝑒𝑟 𝑘𝑖𝑙𝑜𝑚𝑒𝑡𝑟𝑒
Example: find a suitable contour interval on a map drawn to a scale of 1:50000
On a scale, 1 cm = 50000cm
Or, 1cm = 0.5 km
Number of cm per km = 1/0.5 = 2
Now contour interval = 25/2 = 12.5 m.
Horizontal Equivalent

The least-horizontal distance between two

consecutive contours, is called horizontal
equivalent. It is different for different contours
and is dependent on the slope of the ground
surface. It is comparatively less in hills than in
plains.
Characteristics of Contours
1. Two contours of different elevations do not cross each other except in the case of an
overhanging cliff.
2. Contours of different elevations do not unite to form one contour except in the case of
a vertical cliff.
3. Contours drawn closer depict a steep slope and, if drawn far apart, represent a gentle
slope.
4. Contours equally spaced depict a uniform slope. When contours are parallel,
equidistant and straight, these represent an inclined plane surface.
5. Contour at any point is perpendicular to the line of the steepest slope at the point.
6. A contour line must close itself but need not be necessarily within the limits of the map
itself.
7. Contours do not have sharp turnings.
8. A set of ring contours with higher values inside, depict a hill whereas a set of ring
contours with lower values inside depict a pond or a depression without an outlet.

9. When contours cross a ridge or V-shaped valley, they form sharp V-shapes across them.
Contours represent a ridge line, if the concavity of higher value contour lies towards
the next lower value contour and on the other hand these represent a valley if the
concavity of the lower value contour, lies towards the higher value contour.

10. The same contour must appear on both the sides of a ridge or a valley.
METHODS OF CONTOURING
There are mainly two methods of locating contours:-
(1)Direct Method and (2) Indirect Method.
A) Direct Method:

In this method, the contours to be located are directly traced out in the field by locating and
marking a number of points on each contour. These points are then surveyed and plotted on
plan and the contours drawn through them. The method is slow, tedious and is applicable for
small areas and where great accuracy is required.
The whole field work may be divided into two steps :

(a) The location of the points on the contours i.e., vertical control.
(b) Plotting of the points on the plane table section i.e., horizontal control.
Vertical control

 In this method BM is required in the project area. The level is set up in any
commanding position and back sight is taken on the BM.
 Let the BS reading on the BM be 1.385 m. if the RL of the BM is 1300m, the HI would
be 1300+1.385 = 1301.385m.
 To locate the contour of 1300.5m value, the staffman is directed to occupy the
position on the ground where the staff reading is 1301.385 – 1300.5 = 0.885 m.
 Mark all such positions on the ground where the staff reading would be 0.885 m by
inserting pegs. Similarly locate the points where the staff reading would be 1301.385
– 1301= 0.385 m for 1301 m contour. The contour of 1301.5 m cannot be set from this
setting of the instrument because the HI for this setting of the instrument is only
1301.385m. 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

In case of small areas, the locations of the points on each

contour are surveyed on the plane table section by radiation
method. If the area is large and all the contours cannot be
plotted from one setting, the plane table is shifted to another
commanding station B to which a long radial line AB is drawn.
On reaching the station B, the planetable is set by ‘Back Ray
Method’ and the measured distance between the two stations
is plotted on the scale of the planetable section. The points on
different contours are surveyed by radial line method and the
process continued till all the contours are plotted.
B) Indirect Method:

 In this method some suitable guide points are selected and their elevations are found,
theses points may form well shaped geometrical figures.
 The location of such points are plotted by plane tabling and contours are drawn by
interpolation, theses guide points do not fall except by coincidence on the contours
to be located.
 While interpolating it is considered that the slope between any two adjacent guide
points is uniform.
 Several representative points represent hills, depressions, ridge, valley lines and the
changes in the slope all over the area to be contour are also observed. Other guide
points are then plotted on the plan and the contours drawn by interpolation.
 This method is commonly employed in all kinds of surveys as this is cheaper, quicker
and less tedious as compared to direct method.
Depending upon the field indirect method of contouring has divided into three categories.
A) By Squares or Grid Method

 In this method the whole area is divided into number of squares or Grid the side of
which may vary from 5m to 30m depending upon the nature of the ground and the
contour interval.
 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 contours of desired values are then interpolated.

Suitability: This method is suitable in low undulations without any vegetative covers.
B) By Cross Section

 In this method cross sections perpendicular to the centre line of the area are set out.
 The spacing of the cross-section depends upon the contour interval, scale of plan and
the characteristic of ground.
 The common value is 10 to 20m in hilly country and 20-30m in flat country.
 The centre line and cross sections are plotted along with important features on the
desired scale and their R.Ls are entered.
 The contours are then interpolated with respect to these R.Ls.
 The levels of the points along the section lines are plotted on the plan and the contours
are then interpolated as usual.

Suitability: This method is suitable for preparing a contour plan of a road, railway or canal
alignment.
C) By Tacheometric

 The tacheometer is used for both the vertical as well as horizontal measurements.
 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.

Suitability: This method is suitable for contouring the area of long strips with
mountaneous/undulations where direct chaining is difficult.
 S.N. Direct Method Indirect Method
1 Very accurate but slow and tedious Not very accurate but quicker & less tedious
2 Expensive Reasonable Cost
3 Appropriate for small projects requiring Suitable for large projects requiring
high accuracy, e.g. layout of building moderate to low accuracy, e.g; layout of
factory structural foundation, etc. highway, railway, canal etc.
4 More suitable for low undulating terrain Suitable for hilly terrain
5 Calculation need to carried out in the field Calculation in the field in not mandatory
6 After contouring calculation cannot be Calculations can be checked as and when
checked needed

Interpolation of Contours
The process of spacing the contours proportionally between the plotted ground-points is
termed as interpolation of contours. This becomes necessary in the case of indirect
contouring as the spot levels are taken in this method. While interpolation of contours the
ground between any two points is assumed to be uniformly sloping
There are three main methods of interpolation
A) By Arithmetical Calculation
In this method, positions of contours between two known points, are located by making
accurate calculations. Hence, the method, though very
accurate is time consuming and laborious. It is generally
adopted when higher accuracy is demanded for a limited
area.

Suppose A, B, C and D are four plotted points at 2 cm apart

and their ground reduced levels are 57.5, 63.2, 68.2, 59.2 m
respectively. It is required to draw contours at 2 m vertical
interval.
(a) Interpolation along AD.
The total difference in elevation between A and D is 59.2 − 57.5 = 1.7 m.
The difference of level between A and 58 m contour is 58.0 − 57.5 = 0.5 m.
Hence, the distance of the 58 m contour from A = 0.5 /1.7 × 2 cm = 5.9 mm.
(b) Interpolation along AB.
The total difference in elevation between points A and B is 63.2 − 57.5 = 5.7 m.
The difference of level between point A and the 58 m contour is 58.0 − 57.5 = 0.5 m.
Hence, the distance of the 58 m contour from the point A along AB is 0.5 /5.7 × 2 = 1.8 mm.
Similarly the distances of the 60 m and 62 m contours from the point A can be calculated.
These will be 2.5/ 5.7 × 2 = 8.8 mm and 4.5 /5.7 × 2 = 15.8 mm respectively.
(c) Interpolation along BC.
The total difference in elevation between points B and C is 68.2 − 63.2 = 5 m.
The difference of level between point B and the 64 m contour is 64.0 − 63.2 = 0.8 m.
Hence, the distance of the 64 m contour from the point B along BC is 0.8/ 5.0 × 2 = 3.2 mm.

Similarly the distances of the 66 m and 68 m contours from the point C will be 2.8/ 5.0 × 2 =
11.2 mm and 4.8/ 5.0 × 2 = 19.2 mm respectively.
(d) Interpolation along CD.
The total difference is elevation between points D and C is 68.2 – 59.2 = 9.0 m.
The difference of level between point D and the 60 m contour is 60.0 − 59.2 = 0.8 m.
Hence, the distance of the 60 m contour from the point D along DC is 0.8 9 × 2 = 1.8 mm.
Similarly, the distances of 62 m, 64 m, 66 m, and 68 m, contours from the point D can be
calculated. These will be 2.8 /9 × 2 = 6.2 mm 4.8 /9 × 2 = 10.7 mm, 6.8 /9 × 2 = 15.1 mm and
8.8/ 9 × 2 = 19.6 mm respectively.
The points having equal elevations are joined and the required contours of 58 m, 60 m, 62 m,
64 m, 66 m and 68 m may be drawn.
To achieve better accuracy, interpolation along the diagonals AC and BD may also be done.
B) By Graphical method
Graphical method of interpolation is simpler as compared to arithmetical method and also
the results obtained are accurate.
Out of several graphical methods the most common is as given below.

 As shown in figure below suppose the contour interval is 5m then on a piece of tracing
cloth or tracing paper a number of parallel lines spaced at 0.5m (usually1/10th of the
contour interval) are drawn every tenth line being made thick. Suppose it is required to
interpolate contours between two points A and B of elevation 51.5m and 62.5m
respectively.
 If the bottom line represents an elevation of 50m then the successive thick lines will
represent 55m, 60m and 65m etc.
 Place the tracing cloth so that the point A is on
the third line from the bottom now move the
tracing cloth until B is on the fifth line above
the 60m thick line.
 The interpolation of the thick lines 1 and 2
representing elevations of 55m and 60m and
the line AB give the position of the points on
the 55m and 60m contours respectively and
are pricked through on the plan with a pin.
C) By Estimation

The points on the required contour are located by eye judgment or estimation between points
whose elevation are already known. This is a rough method and is suitable for small scale
maps, the position of the contour points between ground points are estimated roughly and
the contours are then drawn through these points. Accuracy of work depends upon the skill
and experience of surveyor
Uses of contour maps :

1. To study the general character of the tract of the country without visiting the ground.
With the knowledge of the characteristics of the contours, it is easier to visualize
whether the country is flat, undulating or mountainous.
2. To decide the most economical and suitable sites for engineering works such as canals,
sewers, reservoirs, roads, railways etc.
3. To determine the catchment area of the drainage basin and hence the capacity of the
proposed reservoir.
4. To compute the earth work required for filling or cutting along the linear alignment of
projects such as canals, roads, etc.
5. To ascertain the intervisibility of points.
6. To trace a contour gradient for the road alignment.
7. To draw longitudinal sections and cross-sections to ascertain the nature of the ground.
8. To calculate the water capacities of reservoirs.
9. To decide the best positions of the guns, the line of March and camping grounds by the
army commanders during wars.