This method is accurate but time consuming. The position contour points between the guide points are located by estimation. This method is extremely rough and is used for small scale work only. This method is based on the assumption that the slope of ground between two points is uniform. It is the process of spacing the contours between the plotted points. Instruments is places at fixed positions and reading is taken in a radial line at different angles and then RL of different points are found out. In this method contour point is located by stadia hair reading and the RL of the points are found. This method is suitable for hilly terrain. Spacing of the cross section depends upon the terrain of the land. This method is most suitable for railway route survey. In this method, cross-section’s are run transverse to the center line of a road, railway or canal etc. The RL of each square corner are determined then required contour line is interpolated. The size of squares may vary from 5-20m depending upon the contour and contour interval. The area to be surveyed is divided into is number of squares. This method is suitable if the are is small and ground is not very much undulating. In small area chain survey may be used but in large area traverse may be required.
When the required points on the ground are located then such points should be suitably controlled by the control points or lines. When the staff(ground) point is located by required staff reading it is pegged and number of such types of points are joined to form the contour line. If 100m RL is to be plotted then first staff is placed on BM and then HI is to be found out, then staff point is searched on ground for the required RL of 100m as : In this method work has mainly two folds. Only the required points are surveyed then plotted. Indirect method, the contour to be plotted is acutually traced on the ground. In general the field methods of conturing may be divided into two classes: The method of establishing/ plotting contours is a plan or map is known as conturing. They form sharp curves of V-shape across it with convex side of the curve towards the higher ground. Contour cross a valley line at right angles. Thery form curves of U-shape around it with concave side of the curve towards the higher ground. Contour cross a water shead or ridge line at right angles. A closed contour line with one or more higher one inside represents a hill similarly contour line with lower value inside indicates a depression or pond. A single contour line cannot split into two contour line. A contour line must close upon itself through not necessarily within the limits of the map.į. If the contour line are spaced closer and non-uniform then it shows steep and undulated ground.Į. If the contour lines are parallel and distance apart then it shows that the ground is gentle slope.ĭ. Contour overlap or unit in a single line in case of vertical cliff.Ĭ. Two contour lines donot cross each other except in the case of overhanging cliff.ī. The horizontal distance between two points on two consecutive contour lines for a given slope is called the horizontal equivalent. So, if the available time and cost is limited larger contour interval is used. If the contour interval is small greater time and cost is required in survey and map plotting. But in case of location surveys where the desired accuracy is less, higher contour interval should be used. Surveying for detailed design work or for earthwork calculations need high accuracy so small contour interval is used. if the scale of map is large, the contour interval is considered to be small and vice versa.Īccuracy needed in survey work also decides the contours interval. The contour interval normally varies inversely to the scale of the map i.e. For flat ground, a small contour interval is choosen, whereas for undulating ground, higher contour interval is chosen. The contour interval depends upon the nature of the ground. The choice of a suitable contour interval in a map depends upon four principle considerations. This vertical distance between any two contour lines in a map is called the Contour Interval ( CI) of the map.
The difference in elevation between successive contour lines on a given map is fixed.
Figure: Elevation of nearby contour line. A contour line marked by a heavier line weight to distinguish it from intermediate contour lines is called index contour line.Īn index contour line shows the elevation of nearby contour line.
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