Contour lines on a topographic map represent points of equal elevation and can be used to interpret the three-dimensional shape of the land. There are direct and indirect methods for creating contour maps. The direct method traces contours on the ground while the indirect method interpolates contours between surveyed elevation points. When drawing contours, all adjacent elevation points must be considered to determine the correct path for the contour line to follow while avoiding violations of the rules governing contour characteristics. Contour maps have various uses including drawing cross sections, determining visibility between points, and planning infrastructure routes that account for terrain.

1. ContouringCOUNTOURING

2. Definition
A contour is an
imaginary line on the
ground that passes
through points having
the same elevation.

3. Contour Interval
• The vertical distance between
any two consecutive contours is
called contour interval
• The contour interval is kept
constant for contour plan.
• The horizontal distance
between two points on two
consecutive contour is known
as the horizontal equivalent

4. The choice of proper contour interval depends
upon
1. The nature of ground : For every
flat ground, a small interval is
necessary. If the ground is more
broken, greater contour interval
should be adopted
2. The scale of Map : Contour
interval should be inversely
proportional to the scale
3. The purpose and extent of the survey : For detailed design work or
for accurate earth work calculation small contour interval is to be
used.
4. Time and expense 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 field survey and in plotting the
map.

5. Characteristics of contour lines
1. Contour lines are continuous.
2. Contour lines are relatively parallel unless one of two
conditions exists.
3. A series of V-shape indicates a valley and the V’s point
to higher elevation.
4. A series U shape indicates a ridge. The U shapes will
point to lower elevation.
5. Evenly spaced lines indicate an area of uniform slope.

6. Contour Line Characteristics-cont.
6. A series of closed contours with increasing elevation
indicates a hill and a series of closed contours with
decreasing elevation indicates a depression.
7. Closed contours may be identified with a +, hill, or -,
depression.
8. Closed contours may include hachure marks. Hachures
are short lines perpendicular to the contour line. They
point to lower elevation.

7. Contour Line Characteristics-cont.
9. The distance between contour lines indicates the
steepness of the slope. The greater the distance between
two contours the less the slope. The opposite is also
true.
10. Contours are perpendicular to the maximum slope.
11. A different type of line should be used for contours of
major elevations. Common practice is to identify the
major elevations lines, or every fifth line, with a bolder,
wider, line.

8. 1. Contours are Continuous
• Some contour lines may close
within the map, but others will
not.
• In this case, they will start at a
boundary line and end at a
boundary line.
• Contours must either close or
extend from boundary to
boundary.

9. 1. Continuous Contours-cont.
• No
• Contour 1040 is very unlikely
Is the topo map correct?

10. 2. Contour lines are parallel
• Two exceptions:
1. They will meet at a vertical cliff
2. They will overlap at a cave or
overhang.
3. When contour lines overlap, the
lower elevation contour should be
dashed for the duration of the
overlap.

11. 3. Valleys and higher elevation
A series of V-shapes
indicates a valley and the
V’s point to higher
elevation.

12. 4. U shapes and ridge
A series of U shapes
indicates a ridge. The U
shapes will point to lower
elevation.

13. 5. Contour Spacing
Evenly spaced contours
indicate an area of uniform
slope.
Unevenly spaced
contours indicates an
area with variable slope.

14. 6. Hills and Depressions
A series of closed contours
with increasing elevation
indicates a hill.
Hills may be identified
with a “+” with the
elevations

15. 6. Hills and Depressions--cont.
A series of closed
contours with
decreasing elevation
indicates a depression.
Depressions may be
identified with a “-”.

16. 8. Hachures
Hachures are short lines which
are perpendicular to the
contour line.
Used to indicate a hill or a
depression.
Not used on modern maps.

17. 11. The dark colored contour lines represent every fifth
contour line to make it easier to read the map.

18. 9. Contour Spacing--cont.
Contours are
perpendicular to
maximum slope.
 Different types of lines
should be used for contours
of major elevations.
 Common practice is to
identify the major elevations
lines, or every fifth line, with
a bolder, wider, line.

19. Contour spacing - Intervals
Types of topo map Nature of terrain Recommended interval (feet)
Large Scale Flat 0.5 or 1
Rolling 1 or 2
Hilly 2 or 5
Intermediate scale Flat 1, 2 or 5
Rolling 2 or 5
Hilly 5 or 10
Small scale Flat 2, 5 or 10
Rolling 10 or 20
Hilly 20 or 50
Mountainous 50, 100 or 200
• Another decision that must be made is the contour interval.
• The “best” interval depends on the use of the data.

20. 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:
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. Direct method of contouring can be employed using Level and Staff as
follows:

22. Indirect Methods
In this method, points are located in the field, generally as corners of
well-shaped geometrical figures such as squares, rectangles, and spot
levels are determined. Elevations of desired contours are interpolated in
between spot levels and contour lines are drawn by joining points of
equal elevation.
Indirect methods are less expensive, less time consuming and less
tedious as compared to the direct method. These methods are
commonly employed in small scale surveys of large areas or during
mapping of irregular surface or steep slope. There are two different
ways usually employed for indirect method of contouring:
• Grid method and
• Radial line method

23. Grid Method
In this method, the area to be surveyed is divided into a grid or series of squares
(Figure 17.12). 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.

25. Radial Method
In this method, a number of radial lines are set out at known
angular interval at each station and points are marked at the
ground at convenient distance apart on the rays that are set.
Spot levels of these points are determined by leveling. The
points are plotted to the scale of the map and spot levels are
entered. The contours of desired values are then located by
interpolation. This method is convenient in hilly terrain with
level stations chosen at high points so as to command a
large area from each. Horizontal control may be obtained by
taping (Figure 17.13).

27. A Comparison between Direct and Indirect Methods of
Contouring
Sr.No Direct Method Indirect Method
1 Very accurate but slow and tedious
Not very accurate but quicker and less
tedious.
2 Expensive Reasonable cost
3
Appropriate for small projects requiring high
accuracy, e.g., layout of building, factory,
structural foundations, etc.
Suitable for large projects requiring
moderate to low accuracy, e.g., layout of
highway, railway, canal, etc.
4 More suitable for low undulating terrain. Suitable for hilly terrain.
5
Calculations need to be carried out in the
field
Calculation in the field is not mandatory.
6
After contouring, calculation cannot be
checked.
Calculations can be checked as and when
needed

28. Drawing Contour Lines
• Topographic maps are three dimensional.
• When drawing contour lines all possible paths must be investigated.
• A simple grid will be used to demonstrate this point.

29. Drawing Contour Lines-cont.
There is no right or wrong starting
point.
Pick a contour interval, start at
one edge and extend the contour
across the map.
This example starts with the 14
foot contour.
In this example even numbered contours will be used.

30. Grid Example--cont.
• A3 is at 13 ft elevation and A4 is at 15 ft elevation, a 14 ft contour
would be half way in between.
• The next step is to determine which diagonal and which grid line it
passes through.
• A3 and B3 are both at 13 ft,
therefore the 14 foot contour does
not pass between them.
• B4 is at 14 ft, therefore the 14 ft
contour interval would pass
through station B4.
• Before marking station B4 the
diagonals must be checked.

31. Grid Example--cont.
• The diagonals must be checked to
determine if the 14 ft contour
continues to station B4.
• B3 is at 13 ft, A4 is at 15 ft, a 14 ft
elevation is present between these
two points, therefore the contour line
can be extended to station B4.
• Each pair of grid points are
investigated and the contour is
extended until it is complete.

32. Grid Example--cont.
• To extend the contour the next
options must be checked.
• Station B3 is at 13 feet elevation and
station B4 is at 14 feet elevation.
• The contour extends from the
diagonal position to station B4.

33. Grid Example--Drawing Contour Lines
This contour is completed by
connecting the points with a
smooth line.

34. Grid Example--cont.
• The 12 foot interval is completed in
the same way.
• A2 is at 11 ft and A3 is at 13 ft,
therefore the 12 foot contour is half
way between A2 and A3.
• A2 and B2 are both at 11 ft, the
contour doesn’t go between these
two.
• A3 is 13 ft and B3 is 13 ft, it doesn’t go this way.
• B2 is 11 ft and B3 is 13 ft, a 12 ft elevation is half way between.
This 12 ft interval will pass through this point, as long as the
diagonals are ok.

35. Grid Example-cont.
• The next grid is more difficult
because a 12 foot contour line will
pass through both B2:C2 and
B3:C3.
• In addition to go either way would
violate the diagonals.
• The remaining points must be
investigated to determine the best
path.

36. Grid Example--cont.
• If the contour is extended through
B2:C2 and completed to D3, the
diagonal must also be violated at
D2:C3 to continue on to D3.
• There should be a better
interpretation.

37. Grid Example--cont.
• The contour should be
completed by passing between
B3:C3.
• This a better interpretation
because:
1. the contour is completed
without violating the diagonal
at C3:D4.
2. The 12 foot contour parallels
the 14 foot contour.

38. Grid Example--cont.
• The 10 foot interval is completed
in the same way.
• At this point there is a danger of
considering the map complete,
but you must always check for a
possible hill or depression on the
map.
• In this example there is 12 foot
contour around C2.

39. Grid Example--cont.
• Putting in the 12 foot contour
around C2 violates the diagonal
between C2 and B3.
• This is acceptable because it
should be clear that a valley exists
from A2 through B2 through C3
and then to at least C2.

40. Use of contour maps
1. Drawing of section
From given contour plan, the section along any given direction can
be drawn to know the general shape of the ground or to use it for
earth work calculation.
2. Determination of intervisibility between two points

41. 3. Tracing of contour gradients and location of route
contour plan is very useful in locating the route of
highway, railway, canal or any other communication line.
4. Measurement of drainage area
5. Calculation of reservoir capacity

42. End