A contour line is a curve along which the function has constant line joins points of equal elevation above a given level such as mean sea level
Or
Is an engineering line on the ground joining the points of equal elevation above the assumed datum. It is the plan projection of the plane passing through the points of equal height on the surface of the earth.
2. Characteristics of contours.
Contours showdistinct characteristics features of the terrain as follows;
i) Two contours of different elevations do not cross each other except the case of overhanging cliff.
ii) Contour of different elevation does not unite to form one contour except in the case of vertical cliff.
iii) Contours drawn closer depict at steep slope and if drawn far apart represent gentle slope.
vi) Contours equally spaced depict a uniform slope. When contour are parallel, equidistance and straight, these represent an inclined plane surface.
v) Contour at any point is perpendicular to the line of the steepest slope at the point.
vi) A contour lines must close it but need to be necessary within the limit of map itself.
vii) A set of ring contours with higher values inside depict a hill whereas a set of ring contours with lower value inside a pond or a depression without another.
viii) When contours cross a ridge or V-shaped valley, they form v-shape across them.
ix) The same contour must appear on both sides of the ridge or valley.
x) Contours do not have sharp turning.
3. Uses of Contours
a) To study general character of the tract of the country without visiting the ground.
b) To decide the most economical and suitable sites for engineering works such as reservoirs, roads, railways etc.
c) To determine the catchment’s area of the drainage basin and hence the capacity of proposed reservoir.
d) To ascertain the intervisibility of points.
e) To trace contour gradient for roads alignment
f) To draw longitudinal sections and cross sectional to ascertain the nature of the ground.
g) To calculate the water capacity of the reservoir.
h) To compute the earth’s work required for filling or cutting along the linear alignment of projects such as canals, roads etc.
i) To decide the first positions of the guns, the line of March and camping ground by the army commanders during wars.
3. Methods of Contouring
Process of locating contour may be executed in various ways according to the instrument used. The following are common method used;
Ø Direct method:
Direct method No. 1
In this method contours to be plotted are actually traced out in the field by locating and making a number of points on each. These points are surveyed on a plane table section and appropriate contours are drawn through them. This method is comparative slow and generally not adopted on larger surveys unless superior accuracy is demanded. It is suitable for contouring of small areas where better accuracy is required. It includes two steps:-
a) The location of points on the contours (vertical control)
b) Plotting of the points on the plane section (horizontal control)
Direct method No. 2
In this method contours are also actually traced by locating number of points on each. For the vertical control clinometers is used instead of a level. The distances are measured by direct chaining from the plane table station to the points.
Direct method No. 3
Photogram metric plotting machine a pair of vertical is aerial. The method of contouring with the help of aerial photography is the latest technique in surveying. In this method a pair of vertical aerial. The method pair of vertical aerial photographs overlapping 60% is exposed keeping the camera axis vertical. These dispositive or negative of these exposures are mounted on the picture carries of photogrammetric machine such as wild A7, A8, B8, act;
Ø Indirect method (Contouring by spot level)
In this method sufficient numbers of points are given spot levels. The location of such points can be conveniently plated on a plane table section as these generally corners of wall, shaped geometrical figer i.e. requires rectangular, triangular etc. The spot level of important features which represent hill tops, ridge lines, bed of streams and lowest point of the depression are also taken to depict their correct features while drawing contour lines.
This method is cheaper, quicker and less tedious compared with direct method of contouring.
This method of contouring can be employed in three different ways below:-
a. By square method
In this method the entire area is divided into a number of square the side of which may vary from 5 to 25m depending upon the nature of the ground, the contour interval and the side of the plan.
b. By cross section method
In this method cross section perpendicular to the centre area are set out. The spacing of the cross section depend upon contour interval scale of the plan and the characteristics of the ground
- This method is suitable in low undulation without any vegetative covers.
- In general spacing of cross-section of 20m in hill country and 10mm in flat are adopted.
- This method also is suitable for preparing of contouring plan of a road, railways and alignment.
c. By tachometric method
In this method a number of radial lines at known angular interval are drawn on the ground and the position of the point at equal distance are marked. Salient points of a ground are also locating in the field by observing the vertical angles and the staff reading of the bottom, middle and the wires.
- This method is suitable for contouring the area of long trips with mountainous/undulations where direct chaining is difficult.
d. Interpolation of contours
Interpolation of contour is the process of drawing contours proportion between the plotted ground points or in between plotted contours. It may be done by one for the following methods:-
i) By estimation method
The positions of contour points between ground points are estimated and the contours are then drawn through. This method is rough and the accuracy depends upon the skills and experiences of the surveyor. It is usually used for method of contouring.
ii) By Arithmetical calculations
In this method positions of contours between known points are located by making accurate calculations. Hence the method though the accurate is time consuming and laborious. It generally adopted when higher accuracy is demanded for a laminated area
i) Graphical method of interpolation
In this method location of contour are graphically with the help of a tracing paper or tracing cloth. Graphical method may be done by drawing Radiating lines or by drawing parallel lines.
4. Method of representation a relief on a map are the following;
i) Relief represented by spot heights
Whenever ground is flat, there is hardly any difference in elevation of different points in the area e.g the plains of West Bengal.
i) Relief represented by altitude tints or layers
Whenever ground is steep, the relief may be represented by layers of different heights and coloured with different colors. One layers usually contains several contours. The tints generally range from green to red, brown blue and white representing natural colors.
ii) Relief represented by shading
In case the area consists of moderately low hills, the relief can be indicated by varying intensity of shading with a by color.
There are three systems of hill shading:
Ø Shade formed by vertical light
Ø Shade formed by horizontal light from a definite direction
Ø Shade formed by combined vertical and horizontal light
Ø With vertical light, all flat ground and crest of ridges appear.
iii) Relief represented by hachuring.
This method of representing a relief is by lines in the direction of water and ground surface. There are of two types which are vertical hachure and horizontal hachure
Measurement of catchment area and capacities of reservoir using trapezoidal and prizomoidal.
a) Measurement of catchments area.
Catchment area is the tract of a land which contributes water which flows over the surface of the earth into stream at any point.
To obtain the maximum water level of reservoir, the extent of the area to be submerged, will be the area enclosed by the contour of the maximum water level.
Area sub = a/Ax100%
Where a = area of water body of reservoir
A = area of its catchment basin
Area sub is area to be submerged under water
b) Measurement of capacities of reservoirs.
The capacity of reservoir may be calculated by knowing the maximum water level for the dam and contour interval. Two methods may be used to compute the capacity of reservoir
a. Trapezoidal method
V = h [(A1 + An) + A2 + A3……………………….+ An – 1]
2
Where h is the vertical contour interval
A1, A2, A3…………………….An are area enclosed between successful contours and V is calculated volume (reservoir capacity)
b. Prizomoidal method
V = h/3[A1 + An + 4(A2 + A4+…………) + 2 (A3 +A5 +………..)]
Where h is the vertical contour interval
A1, A2, A3…………………….An are area enclosed between successful contours and V is calculated volume (reservoir capacity).
10) Conclusion
According to our discussion we can conclude that contour is among of very important topic in survey especially for engineering works. Hence help to decide the most economical and suitable site for engineering works such as canals, sewers, reservoirs, roads, railways etc. Also can be used to determine earth work required for filling or cutting along linear alignment of the project and tracing of contour gradient for the road alignment.
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