Contouring
Contours are a primary method for depicting elevations on a map, providing a visual representation of both absolute elevations and variations in the terrain’s topography.
a) Contour Interval: The vertical distance between two consecutive contours. This interval is constant for a map.
b) Horizontal Equivalent: The horizontal distance between two consecutive contours. This is not constant and varies depending on the land surface’s slope.
Methods of Contouring
There are two primary methods for contouring:
- Direct Method
In the direct method, contours are traced directly in the field. Here’s how it works:
Step 1: Points of equal elevation are first determined using a level.
Step 2: These points are then marked in the field, typically using pegs.
Step 3: The location of the pegs is recorded using a plane table or other suitable surveying methods.
Both steps can be done simultaneously, allowing for real-time contour plotting in the field.
- Indirect Method
In the indirect method, reduced levels of different points are measured at regular intervals. The procedure involves:
Step 1: Reduced levels (RL) of different points are recorded.
Step 2: The locations of these points are then plotted on a map to scale.
Step 3: Suitable interpolation methods are used to determine the contour points.
Step 4: These points are joined to form continuous contour lines.
There are two main techniques for plotting contours using the indirect method:
a. Using Squares: The area to be contoured is divided into squares. The RLs of the corner points are recorded, and contours are drawn accordingly.
b. Using Cross-sections: Cross-sections are taken at regular intervals perpendicular to a given alignment. These are often used in route surveys. The RLs of points along the cross-sections are recorded and used to draw the contour lines.