2. CONTOUR
• An imaginary line on the ground surface joining the points of equal
elevation is known as contour.
• Contour line is a line on the map representing a contour.
5. CONTOUR MAP
• A map showing contour lines is known as Contour map.
• A contour map gives an idea of the altitudes of the surface features as well
as their relative positions in plan serves the purpose of both, a plan and a
section.
8. CONTOURING
• The process of tracing contour lines on the surface of the earth is called
Contouring.
CONTOUR INTERVAL
The constant vertical distance between two consecutive contours is called the
contour interval.
9. HORIZONTAL EQUIVALENT
•The horizontal distance between any two adjacent contours is called as
horizontal equivalent.
• The contour interval is constant between the consecutive contours while the
horizontal equivalent is variable and depends upon the slope of the ground.
10. Purpose of contouring
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. For getting information about the ground whether it is flat, undulating or
mountainous.
4. To find the capacity of a reservoir and volume of earthwork especially in a
mountainous region.
5. To locate the physical features of the ground such as a pond depression, hill,
steep or small slopes.
11. FACTORS ON WHICH CONTOUR INTERVALS DEPENDS
1. The Nature of the Ground
2. The Purpose and extent of the survey
3. The Scale of the Map.
4. Time and Expense of Field and Office work.
12. VALUES FOR CONTOUR INTERVAL
Purpose of survey Scale Interval (m)
1 Building sites 1cm = 10 m or less 0.2 - 0.5
2 Town planning
schemes, reservoirs,
etc.
1cm = 50 m to 100 m 0.5 – 2.0
3 Location surveys 1 cm = 50 m to 200 m 2.0 – 3.0
13. CHARACTERISTICS OF CONTOURS
1. All points in a contour line have the same elevation.
2. Flat ground is indicated by widely separated contours and steep slope
where they run close together.
3. A uniform slope is indicated when the contour lines are uniformly
spaced
4. A plane surface when they are straight, parallel and equally spaced.
16. CHARACTERISTICS OF CONTOURS
5. A series of closed contour lines on the map represent a hill , if
the higher values are inside
6. A series of closed contour lines on the map indicate a
depression if the higher values are outside
21. 7. Contour line cross ridge or valley line at right angles.
If the higher values are inside the bend or loop in the contour, it
indicates a ridge.
If the higher values are outside the bend, it represents a Valley
25. 8. Contours cannot end anywhere but close on themselves either
within or outside the limits of the map.
9. Contour lines cannot merge or cross one another on map
except in the case of an overhanging cliff.
10. Contour lines never run into one another except in the case of a
vertical cliff.
32. Methods of contouring
There are mainly two methods of locating contours:-
1. Direct Method
2. Indirect Method.
33. 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.
DIRECT METHOD OF
CONTOURING
50
48
46
B.M
34. Procedure:
1. First ensure that appropriate B.M is available near the site. If B.M not
available locate it by fly levelling.
2. Set up level in such a position that it covers a large part of area to be
surveyed.
3. Take Back Sight on B.M
Then determine Height of instrument
H. I = B.S + RL of B.M
4. Determine the staff reading required to fix points on the various contours by
subtracting the R.L. of each of the contours from the height of instrument.
35. 5. The new height of instrument and the required staff readings are then
calculated
6. The process is repeated till all the contours are located.
7. The positions of the contour points are located. A theodolite, a compass or a
plane table traversing is usually adopted for locating these points.
8. The points are then plotted on the plan and the contours drawn by joining
the corresponding points by dotted curved lines.
36. Example:
If H.I = 82.48m, then staff readings required to locate 82, 81 and 80m contours then,
H.I – RL required = staff reading
82.48 – 82 = 0.48 m
82.48 – 81 = 1.48 m
82.48 – 80 = 2.48 m
The staff is held on an approximate position of point and 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.
37. This method is suitable for small
areas, where a single point in the
centre can command the whole area.
Radial lines are laid out from the
common centre by theodolite or
compass and their positions are fixed
up by horizontal angles and
bearings.
Direct Method By Radial Lines :
70
65
60
55
Fig. RADIAL LINES
METHOD OF CONTOURING
38. Direct Method By Radial Lines (contd.):
1. Locate a B.M near the centre of the area by fly levelling from an available B.M
from a nearby site.
2. Layout radial lines from centre point conveniently to cover the area suitably.
3. Locate points on these radial lines having the required elevation after
calculating staff reading required for that elevation.
4. Once a point is located, mark it suitably with arrow or other marks with a tag
indicating the elevation of the contour.
5. Repeat the process with all radial lines and for all desired contours.
6. Then locate the points on horizontal plane by a chain or tape by measuring
distances from the centre point already marked.
39. • Direct 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.
•Two teams are required.
•A levelling team to mark the points with required elevations and Second team to
locate these points with chain or tape and a compass or theodolite.
Direct Method Advantages and disadvantages
40.
41. 2. Indirect Method
Instead of locating points of required elevation a general survey is conducted to
obtain the vertical and horizontal distances of the points.
When a large number of such points are available the points of required elevations are
obtained by interpolation.
Indirect methods
1. By square method(spot levels)
2. By cross sections
3. By Tacheometry
42. BY SQUARE METHOD(SPOT LEVELLING)
This method employed in case of areas which are not very expensive.
1. Divide the area into squares of reasonable dimensions. 5m - 20m squares
are generally employed.
2. Take levels at the corners of squares only. This is known as spot levelling.
3. Depending on the undulations of ground, levels may be taken at the
intermediate points on the sides of a square.
4. Once the levels are available determine the contours of required elevation
by interpolation.
45. 2. Indirect Method: By cross sections
In this method cross sections run transverse to the centre line of road, railway or
canal etc.
70.6 69.1 68.8 69.1 70.8
70.8 70.2 69.1 70.4 70.5
71.2 70.8 66.3 70.6 70.8
71.6 71.2 70.6 72.4 71.7
RD 580
RD 560
RD 540
RD 520
71
71
70
70
69
68
67
71
71
70
69 69
Fig. X-Section Method
46. 2 Indirect Method: (By Cross sections)
1. Take sections at right angles to the centre of the road. They need not be at right
angles and can be inclined to the centre line if required.
2. The distance between sections depends upon the terrain. They should not be
less than 20 m in hilly terrain, but can be 50 to 100m in plane country.
3. Take levels at a number of points along the section line.
4. Locate the points by chain or tape.
5. Obtain the contours of required elevation by interpolation.
6. Join such points to get the contour lines
47. 2. Indirect Method: By Tacheometric
method
• Tacheometry is the method of finding the distances and elevations
simultaneously.
• This method is suitable in case of hilly terrain, where measuring
distances with chain or tape is time consuming.
• Tacheometry involves a tacheometer or theodolite fitted with stadia
wire, one above and other below the central wire.
Fig. Stadia Wires at Diaphragm
48. • The horizontal distance between the instrument and staff station may be
determined by multiplying the difference of the staff readings of the upper
and lower stadia wires with the stadia constant of the instrument, which is
usually 100.
2. Indirect Method: By Tacheometric method
49. 1. Set up the instrument at suitable place having a commanding view of the
area. Carry out the temporary adjustments as required.
2. Hold the staff on a benchmark of known elevation to get the elevation of
the line of sight.
3. Then hold the staff in positions marked either on the vertices of squares.
4. At each position of the staff, read the vertical angle from the vertical
circle and the stadia intercept
5. To survey small area keep theodolite at central position from where
radial lines can be laid out.
2. Indirect Method: By Tacheometric method
50. Interpolation of contours
Contours of desired elevation is obtained by interpolation
Methods:
1. By estimation
2. By arithmetical calculation
3. By graphical methods
51. Interpolation of contours
1. By Estimation:
The position of the contour points between ground - points are estimated
roughly and the points are joined by smooth curves to obtain the required
contour lines. This is a rough method and is suitable for small scale maps.
2. By arithmetical calculation:
The required points are obtained by linear interpolation from the elevations of
guide points. This is very tedious but accurate method and is used for small
areas where accurate results are necessary. The contours are interpolated as
under:
52. Interpolation of contours
Suppose A and B are two points at a distance of 30 m
and the reduced level of A and B are 25.45m and 27.54m
respectively .
By arithmetical calculation:
A (25.45) B (27.54)
30 m
30 m
53. Interpolation of contours
Suppose A and B are two points at a distance of 30 m and the reduced
level of A and B are 25.45m and 27.54m respectively .
Taking the contour interval as 1m, 26 and 27 m contours may be interpolated in
between A and B. Difference of level between A and B = 2.09m.
Difference of level between RL of A and 26m = 0.55 m
Difference of level between RL of A and 27m = 1.55 m
Horizontal distance between A and 26 m contour
= 30 x 0.55/2.09 = 7.89m from A
Horizontal distance between A and 27 m contour
= 30 x 1.55/2.09 = 22.25m from A
These distances are then plotted to scale on the map.
By arithmetical calculation:
54. Interpolation of contours
Interpolation is done with the help of tracing paper or a tracing cloth.
Compared to arithmetical method this method is simple and accurate.
By Graphical method:
56. 1. As shown in the fig. suppose the contour interval is 5m, then on
a piece of tracing cloth, a number of parallel lines spaced at 0.5
m (usually 1/10th of the contour interval) are drawn. Every
tenth line being made thick.
2. Suppose it is required to interpolate contours between two
points A and B of elevation 51.5m and 62.5m respectively.
3. If the bottom line represents an elevation of 50m. Then the
successive thick lines will represent 55m, 60m and 65m, etc.
Graphical method:
57. By Graphical method:
4. 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.
5. The intersection of the thick lines 1 and 2 representing elevations of 55m
and 60 m.
6. Then 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.
58. Drawing the contour lines
Contour lines are drawn as fine and smooth free hand curved lines. Sometimes
they are represented by broken lines .They are inked in either in black or brown
colour. A drawing pen gives a better line than a writing pen and French curves
should be used as much as possible. Every fifth contour is made thicker than
the rest.
The elevation of contours must be written in a uniform manner, either on the
higher side or in a gap left in the line .When the contour lines are very long, their
elevations are written at two or three places along the contour. In the case of
small scale maps, it is sufficient to figure every fifth contour.
59. CONTOUR GRADIENT
• An imaginary line on the surface of the earth having a constant
inclination with the horizontal is called contour gradient.
• The inclination of a contour gradient is generally given either as rising
gradient or falling gradient, and is expressed as ratio of the vertical
height to a specified horizontal distance.
• If the inclination of a contour gradient is 1 in 50, it means that for
every 50 m horizontal distance, there is a rise (or fall) of 1 m.
60.
61. CONTOUR GRADIENT ON MAP
• With the help of contour plan, it is easy to trace a contour gradient of
desired inclination on a paper, and even transfer it later to the ground
62. LOCATION OF CONTOUR GRADIENT
• Suppose it is required to locate the centre line of a road in a hilly
area with a ruling gradient of 1 in 20.
• Let the starting point A be on a 94.00m contour line.
• Since the contour interval is 2m and gradient is 1 in 20, the
horizontal distance between A and the point on the next contour
(96.00m) is 2 x 20 = 40m
• With the centre at A and radius equal to 40m (taken on the same
scale) an arc is drawn cutting the contour line of 96.00 at the
point B.
• Taking B as centre and with the same radius, another arc is
drawn to get the next point C. The other points are located in a
similar manner.
63. Uses of contour map
1. A contour map gives information regarding the features of the ground,
whether it is flat, undulating or mountainous.
2. From a contour map , sections may be easily drawn in any direction
3. Inter-visibility between two ground points plotted on map can be ascertained
4. It enables an engineer to approximately select the most economical or suitable
site for an engineering project such as a road, a railway, a canal or a pipe line
etc.
5. A route of a given grade can be traced on the map.
6. Catchment area and capacity of a reservoir may be determined from the
contour map.
7. Contour map may be used to determine the quantities of earth work.
