What is contour and its applications?

In construction, the correct study of land is very important to build a property on top of it. There are many ways to do it- one such method is called ‘mapping.’ It helps one understand the exact topography of a piece of land by mapping its lines and shape. Since a barren land will always have grooves and elevations, the line mapping of such elevations is called contouring in the construction world. Read the article to learn more about what contour is.

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See also: What is the CBR test, and where is it used in road construction?

Contour: Definition

The term contour in construction means locating all the elevations in a horizontal land by connecting them with an imaginary line in a map. It is a fully detailed survey conducted to understand the grooves and ridges of any existing land and understand their differences from other sections of the land. The process of tracing these contour lines is called ‘contouring.’

When builders survey land, they are needed to draw a topographic map showing these contour lines. This map with a clear illustration of contour lines in a valley, hill or ridge is called a ‘contour map.’ The contour maps provide the builders with the correct idea of the elevation of the land, how steep it is and the shape of the slopes. The lines that are traced in a contour map which joins all the elevated points are called ‘contour lines.’

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Contour: Important terms

• Contour interval– It is the continuous perpendicular distance between any two ongoing contour lines in a contour map. The distance is always constant between any two contours. 

• Horizontal interval– The parallel distance between two contour lines that run side by side is a horizontal interval. There could be a difference in distances between contours as they depend on the alternate height and steepness between any slopes and elevations on the land.

• Contour gradient– It is defined as a line that holds a constant disposition to the horizontal elevation. The instrument named ‘Clinometer’ is used to study the contour gradient.

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Contour: Methods used to conduct contour survey

There are mainly two methods a builder must follow to conduct a proper contour survey:

Direct method of conducting a contour survey

This method requires one to pinpoint all the perpendicular and straight controls of the main points that lay on the chosen contour lines. One must use a vertical levelling instrument to conduct the survey in a direct method. Once a top fly shot of the selected area is conducted from a place nearer to the selected benchmark, a level can be set on the required position. Then the necessary staff reading for the selected contour line is calculated.

The person handling the staff reading instrument is then made to move to and fro in the area until accurate readings are noted down. The surveyor then sets the horizontal elevated control of the surveyed point by using his set of instruments. Then again, the staff reading man is made to find another point with the same readings, and they repeat this process. This way they note down many contour lines with points on them.

For the direct method, the plane table survey is best suited, which is a faster method of noting down the field monitoring and plotting together. This method does take up a lot of time before it is completed, but the results are always accurate. One must only use this method when the plotting area is smaller in size.

Indirect method of conducting a contour survey

Unlike the direct method, the staff survey reading is not done on the contour lines. The reading is conducted on the spot levels of a group of contour lines laid all over the selected area. Along with the contour lines, all the visible slopes, ridges, and depressions along the area are also surveyed. After surveying, their locations are plotted on the contour plan, and then drawn down using interpolation. This process can also be called spot-level contouring. This method is best suited for areas that have hills and depressions. One can further use three different methods to find the correct reading point:

• Square method of selecting points: In this method, the points are read by dividing the area into small squares, and then the grid points are marked down. The size of the squares can be anywhere from 5 m to 30 m. All the squares must be of the same dimension. Then levelling is done to note down all the grid points. After this, draw the squares down.

• Cross-section method of selecting points: At regular distances, cross-section points are noted down at this method. Then using levelling, the lessened level of all the points is noted down. Then these established points are jotted down in the drawing papers. Reduced levels of these points and found contour lines are also marked. The spacing distance in this method ranges from 20 m to 100 m. This method works best when charting down contour lines for narrow roads, railway tracks, and canals.

• Radial line method for points: This method requires one to note down various radial lines from a particular point in the selected area. Note down the direction of every contour line. Contour lines for only chosen points are jotted down, and levels of the same lines are marked. A tachometer is used to conduct this process. Further methods like – 1. Arithmetic 2. Estimation, and 3. Graphic methods are used. This process is good for dense hillside areas.

Contour: Applications

• Before builders start building projects on barren land, a contour study helps determine the status of the land and its depressions or elevations.

• When laying a railway line, or building a long road, contouring helps determine the depth of the land for filing purposes.

• Contouring can also be used to study flood levels in an area. They also help locate deep lakes, wells and dams.

• In hilly areas, it lets a person know about the exact elevation of the hills.

• The colouring method can only be conducted when the contouring of an area is done.

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