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Define Contour Line
A Contour line is an imaginary outline of the terrain obtained by joining its points of equal elevation. In our example of the cone, each circle is a contour line joining points of same level.
Define 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.
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There are three main differences between contour interval and horizontal equivalent as follows:
| 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 |
We at engineeringcivil.com are thankful to Mr Ramasesh Iyer for submitting this useful information to us.
The survey leader has to decide an appropriate contour interval for his project before start of survey work. The following factors govern the selection of contour interval for a project:
| S.No | Factor | Select High CI like 1m, 2m, 5m or more | Select Low CI like 0.5m, 0.25m, 0.1m or less |
| 1 | Nature of ground | If the ground has large variation in levels, for instance, hills and ponds | If the terrain is fairly level |
| 2 | Scale of the map | For small scale maps covering a wide area of varying terrain | For large scale maps
showing details of a small area |
| 3 | Extent of survey | For rough topographical map
meant for initial assessment only |
For preparation of detailed map for execution of work |
| 4 | Time and resources
available |
If less time and resources are
available |
If more time and resources are available |
Contours show distinct characteristic features of the terrain as follows:
i) All points on a contour line are of the same elevation.
ii) No two contour lines can meet or cross each other except in the rare case of an overhanging vertical cliff or wall
iii) Closely spaced contour lines indicate steep slope
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Contour maps are very useful since they provide valuable information about the terrain. Some of the uses are as follows:
i) The nature of the ground and its slope can be estimated
ii) Earth work can be estimated for civil engineering projects like road works, railway, canals, dams etc.
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Two methods of Contouring are:-
i) DIRECT METHOD
ii) INDIRECT METHOD
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Cross section method is most suitable for preparing contour maps for road works, rail works, canals etc.
Typically, this type of land has a very long strip but narrow width.
The steps involved are as follows:
i) The centre line of the strip of land is first marked
ii) Lines perpendicular to the longitudinal strip are marked dividing the strip into equal sections
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Squares or grid method is suitable for contouring of plains or gently sloping grounds.
The steps adopted are as follows:
i) Mark square grids on the land to be surveyed. The grid size would depend on the extent of survey.
Generally a 1m x 1m grid is selected for small works and a larger grid size for large works
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Tacheometric method is adopted for contouring of very steep hills.
The steps are as follows:
i) Set up the tacheometer at the top of the steep hill. Tacheometer is a theodolite fitted with stadia diaphragm. The stadia diaphragm has three horizontal parallel hairs instead of one as found in a conventional cross hair diaphragm.
ii) With the help of a tacheometer it is possible to determine the horizontal distance of the point from the telescope as well its vertical level.
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The Comparison of Direct and Indirect Contouring Methods is shown below in tabular form
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