Contour lines - Survey & Levelling

Submitted by:
HARSHIT GUPTA
B.Arch 1st year
CONTOURING
Survey & Levelling
Submitted to:
Ar. Avitesh

Contouring
Contouring is the science of representing the
vertical dimension of the terrain on a two
dimensional map.

Let us assume that a right circular cone of base 5m diameter
and vertical height 5m is standing upright on its base. Let the
base be resting on a horizontal plane at zero level.

Let us now slice the cone at 1m height from the base
Until we reach to the top.

This will produce another circular outline corresponding to
the diameter of the cone at 1m elevation. Let us draw this
second circle on our contour map using the same scale.
And so on.

Our contour map for the conical object will now be ready. The circles on
the map are called contour lines.

Like the cone in our example, hills project upwards from ground level.
The contour map of a hilly terrain will be similar to that of the cone,
except that instead of perfect circles, the contour lines would be of
irregular shapes.

TERMS IN CONTOURING
• Contour Line:
A Contour line is an imaginary outline of the terrain obtained by
joining its points of equal elevation. In our example of cone, each
circle is a contour line joining points of same level.
• Contour Interval (CI):
Contour interval is the difference between the levels of consecutive
contour lines on a map. The contour interval is a constant in a given
map. In our example, the contour interval is 1m.

• Horizontal Equivalent (HE):
Horizontal equivalent is the horizontal distance between two
consecutive contour lines measured to the scale of the map.

Difference between CI and HE:
S. No Contour Interval Horizontal Equivalent
1 It is based on vertical levels Represents horizontal distance
2
No measurement or scaling is required
since the contour levels are indicated on
the contour lines
The distance must be measured on the
map and converted to actual distance by
multiplying with the scale of the map
3
In a given map the contour interval is a
constant
The horizontal equivalent varies with
slope. Closer distance indicates steep
slope and wider distance gentle slope

Types of Contour Lines:
• Isopleth:
Isopleth on a contour map connects the places having the same value
of some measurable quantity of geographical or meteorological
phenomena.
For Eg - The population density of an area can be
calculated by the quotient of population in the
area and surface area of a region

• Isohyet:
Isohyet indicates the points of equal rainfall of an area in the given
time.

• Isobar:
Isobars on contour maps indicates the point of equal or constant
atmospheric pressure for a given period.
This type of contour is used in the prediction of future weather
patterns isobars are commonly used in television weather reporting.

• Isobath:
It is a type of imaginary contour lines on a map of chart that connects
all the points having same depth below water surface like ocean, sea
and lake.

• Isohaline:
It is a type of contour lines on a map which connect the point of equal
salinity in the water body (majorly contour).

• Isotherm:
It is a type of contour lines on a map of chart with connects the point of
equal temperature of a given area. An isotherm at 0 degree is called
freezing level.
Further they are divided into two types.
• Isocheim: It is a line of equal mean winter temperature.
• Isothere: It is a line of equal means summer temperature.

• Isohel:
It is a type of contour lines on a map that connects the places of equal
duration of sunshine for a given period.

• Isohume:
It is a type of contour lines on a map that connects all the points of
equal relative humidity of a specific area.

• Isoneph:
It’s a line that connects all points of equal amount of cloud cover.

• Isopectic:
Isopectic is imaginary lines on a contour map or chart that connects all
the points at which the ice starts to form at the same time.

• Isotach:
It’s a line that connects all points of equal wind speed.

• Isogeotherm:
Isogeotherm on contour map connects all the points within the earth
having the same mean temperature.

Characterstics for Contour Lines:
1. Contour lines must close, not necessarily in the limits of the plan.
2. Widely spaced contour indicates flat surface.
3. Closely spaced contour indicates steep ground.
4. Equally spaced contour indicates uniform slope.
5. Irregular contours indicate uneven surface.

6. Approximately concentric closed contours with decreasing values
towards centre (Fig. 1) indicate a pond.
Figure 1

7. Approximately concentric closed contours with increasing value
towards centre indicate hills.
8. Contour lines with U-shape with convexity towards lower ground
indicate ridge (Fig. 2).
Figure 2

9. Contour lines with V-shaped with convexity towards higher ground
indicate valley (Fig.3).
Figure 3

10. Contour lines generally do not meet or intersect each other.
11. If contour lines are meeting in some portion, it shows existence of a
vertical cliff (Fig. 4).
Figure 4

• 12. If contour lines cross each other, it shows existence of
overhanging cliffs or a cave (Fig. 5).
Figure 5

Uses Of Contour Maps:
• A civil engineer studies the contours and finds out the nature of the
ground to identify.
• By drawing the section in the plan, it is possible to find out profile of
the ground along that line.
• The knowledge of contour characteristics helps in identifying the
natural feature of the area from the given map and in avoiding
mistakes in plotting contours correctly.

Methods of Contouring:
Contouring requires the planimetric position of the points whose
elevation have been determined by levelling.
Broadly , the method can be divided into the following two classes-
• Direct Method- By level and staff and By Hand level.
• Indirect Method- By Grid method, By Cross-Sectional method and By
Radial line method.

• Direct Method:
1. In the direct method of contouring, the reduced level of various
selected points on a contour line are obtained and their positions
are located. It is a very accurate method but it is slow and tedious.
And it can
2. This method is employed only for a small area where superior
accuracy is demanded.

1. By level and staff:
The method consists of locating a series of points on the ground having
the same elevation by level and levelling staff. Instrument is setup in
such a way that number of readings can be taken from the area to be
surveyed.

2. By Hand Level:
The principle used is the same as that used in the method using a level
and staff. Instead of the hand level, an abbey level may also be used.
However this method is very rapid and is preferred for certain works.

• Indirect Method:
Indirect methods are less expensive, less time consuming and less
tedious as compared with direct methods. These methods are
commonly employed in small scale survey of large areas.

1. By Grid Method:
• If the area is not large, it is divided into a grid or series of squares.
• The grid size may vary from 5m x 5m to 25m x 25m depending upon
the nature of the ground, the contour interval and the scale of the
map.
• The grid corner are marked on the ground and spot levels of these
corners are determined by normal method of levelling using a level.

2. By Cross-Section Method:
• In this method, suitable spaced cross- sections are projected on either
side of the centre line of the area. Several points are chosen at
reasonable distances on either sides.
• The observations are made in usual manner with a level.

3. By Radial Method:
• In this method, a number of radial lines are set out at known angular
interval at each station. The point are selected on line depend on the
nature of the ground surface.
• Instead of the level, a tachometer may be used.
• The observations are taken on the staff stations and elevations and
distances are then calculated. This method is convinent in hilly area.

Comparison between Indirect and Direct
Method:

Plotting of a Contour Map:
• Before plotting the contour map, suitable scale is selected
• E.g. 1cm=1m,1cm=2m,1cm=2.5m. Here 1cm = 2m selected
• A horizontal line is drawn as the centre line.
• The chainages are marked along the horizontal line according to the
scale.
• Ground levels are written on the level book.
• Contour Lines are then inked.

References:
• Surveying and levelling (Vol-1) – Kanetkar and Kulkarni (2011)
• Surveying (Vol-1) – Dr. B.C. Punamia

Contour lines - Survey & Levelling

In this document

More Related Content

What's hot

Similar to Contour lines - Survey & Levelling

More from Ar. Avitesh

Recently uploaded

Contour lines - Survey & Levelling