Contour interval, index contour, plotting, uses

Contour interval

Contour interval (CI) is the constant vertical distance between two consecutive contours, for example 1 m or 5 m. It is kept the same over one map.

Index contour

Index contour is every fifth contour (for example, when CI = 2 m, every 5th line, giving 10 m, 20 m, 30 m ...), drawn as a thicker line and labelled with its RL so that the map can be read quickly. The other contours are called intermediate (minor) contours.

Procedure for contour plotting (applying the surveying principle)

The surveying principle of working from the whole to the part is followed: a strong framework of control is fixed first, and the details and contours are added within it.

1. Control survey: A traverse or triangulation framework is run and plotted accurately. Benchmarks are fixed by levelling.
2. Field survey of contour points: The ground points are located (and their RLs found) from the control stations by the direct or the indirect method (grid, cross-section or tacheometry).
3. Plotting the points: The control stations are plotted first by coordinates; the survey points are then plotted from them to scale. The RL of each point is written beside it.
4. Interpolation: Between each pair of neighbouring points, the contours of the required values are located by proportion, assuming a uniform slope between the points.
5. Drawing the contours: Points of equal RL are joined by smooth curves. Every fifth contour is drawn thick as an index contour and labelled. Contours must not cross each other.
6. Checking: The map is checked against the ground (shape of hills, valleys, ridge lines), and the title, scale, CI and north line are added.

Uses of contours

1. To find the elevation of any point and the slope between two points from the map (without going to the field).
2. To draw the longitudinal section and cross-sections of a road, railway, canal or pipeline on paper.
3. To fix the best alignment of a road, railway or canal with a given gradient (contour gradient).
4. To find the intervisibility of two points (to decide whether a signal station can be seen from another).
5. To calculate the capacity of a reservoir, the catchment area and the volume of earthwork (from the areas enclosed by contours).
6. To select suitable sites for a dam, building, bridge, airport or water tank, and to mark the drainage lines (watershed).
7. To find the quantity of cutting and filling for levelling a site, and to plan irrigation and drainage layouts.
8. To study the general topography (hills, valleys, ridges) of a large area, and for military purposes and town planning.