Contour in Surveying:
A contour is a line that connects points of equal elevation on the surface of the earth.
Characteristics of Contours:
(a) Contour lines always form closed loops, although they may close either on the map itself or outside the map.
(b) Contour lines are always perpendicular to the direction of the steepest slope.
(c) The spacing of contour lines depends on the slope of the ground. Steep slopes require small spacing, while relatively mild slopes can have larger spacing.
(d) The horizontal distance between consecutive contours is called the horizontal equivalent. The horizontal equivalent is determined by the contour interval, the scale of the map, and the nature of the terrain. As the slope becomes steeper, the horizontal equivalent decreases.
(e) Equally spaced contour lines represent a uniform slope. Straight, parallel, and equally spaced contour lines indicate a plane surface.
(g) Irregular contours represent rough terrain.
(h) In depressions, contours increase in elevation from the inside to the outside, while in hills, contours increase from the outside to the inside.
(i) Contour lines intersect ridges at right angles.
(j) Contour lines intersect valleys at right angles.
(k) Two contour lines of altogether different elevations cannot cross each other, as it would imply two elevations for the same point, which is not possible. However, in special cases such as an overhanging cliff or a cave penetrating the hill inside, the contours may appear to cross each other.
Various Methods of Locating The Contours:
- Various Methods of Locating Contours Locating contours involves determining the positions of points on the ground with known horizontal and vertical coordinates.
- The method used for locating contours depends on the available instruments.
- Points of equal elevation can be determined using a leveling instrument.
- Once these points are determined, their horizontal positions can be established using a suitable horizontal control system.
- The type and size of the area being surveyed, as well as the required accuracy, determine the choice of horizontal control.
- For small areas, chain surveying and offsets from survey lines can provide approximate horizontal control.
- Plane tabling, where measurement and plotting are done simultaneously, is suitable for small areas.
- Compass surveying can be used for higher accuracy, and a theodolite can be employed for even greater precision.
All methods of locating contours can be classified as:
(a) Direct method of contouring: In this method, the contour to be plotted is physically traced on the ground by locating the points of that elevation. The horizontal positions of these points are then determined and plotted on the plan. The pegs representing different contours are coded to avoid confusion.
(b) Indirect method of contouring: In this method, spot levels are taken at selected points called guide points, and their elevations are determined. The horizontal positions of these guide points are determined, and the points are plotted on the plan. Contours are then drawn by interpolating the levels of the guide points. The indirect method is more convenient for contouring large areas compared to the direct method.
Direct Method of Contouring:
In the direct method of contouring, points on a contour are located directly by identifying the points of the desired elevation.
For example, if the contour of RL (Reduced Level) 198.00 is to be located and a staff reading of 4.550 is obtained at a specific area, then all the points in that area resulting in a staff reading of 4.550 (-202.550 – 198.00 = 4.550) will lie on the contour line of 198.00. Other points on the same contour can be located in a similar manner, and when joined, they form the contour line of 198.00.
Advantages: The direct method is more accurate than the indirect method, especially for small areas.
Disadvantages: This method is slower and more cumbersome, making it unsuitable for contouring large areas.
Indirect Methods of Contouring:
(a) Grid Method:
The grid method is employed when surveying relatively small areas with relatively flat terrain.
- Divide the survey area into a number of squares, with the size of each square determined based on the nature of the ground and the desired accuracy. Typically, the square size varies from 5m to 20m.
- Use a theodolite to establish grid lines that intersect at right angles to each other.
- Mark grid points at the intersections of the grid lines. In cases where a theodolite is unavailable, this process can be carried out using a tape measure and a cross-staff.
- Set up a leveling instrument at the center of the area to obtain the elevations of the grid points. Determine the instrument’s height by taking a backsight on a benchmark. Take intermediate sights on each grid point. If necessary, shift the instrument and determine a new instrument height for subsequent points. Repeat this process until the elevations of all grid points are determined.
- Based on the elevations of the grid points, locate the points on various contours through interpolation.
(b) Cross-Section Method:
- The cross-section method is typically employed to determine contours along fixed routes such as canals, railways, highways, and similar structures.
- Cross sections are established on the ground at right angles to the fixed line or center line of the route.
- The spacing of the cross sections is decided based on the nature of the ground and the desired contour spacing.
(c) Radial Lines Method:
- This method is particularly useful for contouring small hilly areas.
- Radial lines are drawn from a peak to cover the entire area.
- Grid points are taken on the radial lines, and their elevations are determined.
- Contour lines are drawn through interpolation.
(d) Controlling Point Method:
- In this method, elevations are determined for a few selected key or controlling points.
- Contour lines are then drawn through interpolation.
- This method is approximate but quick, and it gives good results for fairly uniform sloping ground.
Uses of Contour Maps:
- Assessment of terrain characteristics
- Selection of suitable project sites
- Determination of sections
- Inter-visibility between two points
- Location of a route
- Catchment area assessment
- Estimation of reservoir capacity