MEHODS OF TRAVERSING
Traverse survey may be conducted by the following methods :
1. Chain traversing (by chain angle)
2. Compass traversing (by free needle)
3. Theodolite traversing (by fast needle) and
4. Plane table traversing (by plane table)
1.Chain traversing Chain traversing is mainly conducted when it is not possible to adopt triangulation. In this method, the angles between adjacent sides are fixed by chain angles. The entire survey is conducted by chain and tape only and no angular measurements are taken. When it is not possible to form triangles, as, for example, in a pond, chain traversing is conducted,
The formation of chain angles is
(a) First Method Suppose a chain angle is to be formed to fix the directions of sides AB and AD. Tie stations T1 and T2 are fixed on lines AB and AD. The distances AT1, AT2 and T1T2 are measured. Then the angle T1AT2 is said to be the chain angle. So, the chain angle is fixed by the tie line T1T2.
(b) Second Method Sometimes the chain angle is fixed by chord. Suppose the angle between the lines AB and AC is to be fixed. Taking A as the centre and a radius equal to one tape length (15 m), an arc intersecting the lines AB and AC at points P and Q, respectively, is drawn. The chord PQ is measured and bisected at R.
The angle θ can be calculated from the above equation, and the chain angle BAC can be determined accordingly.
2. Compass traversing In this method, the fore and back bearings of the traverse legs are measured by prismatic compass and the sides of the traverse by chain or tape. Then the observed bearings are verified and necessary corrections for local attraction are applied. In this method, closing error may occur when the traverse is plotted. This error is adjusted graphically by using ‘Bowditch’s rule’ (which is described later on).
3. Theodolite traversing In such traversing, the horizontal angles between the traverse legs are measured by theodolite. The lengths of the legs are measured by chain or by employing the stadia method. The magnetic bearing of the starting leg is measured by theodolite. Then the magnetic bearings of the other sides are calculated. The independent coordinates of all the traverse stations are then found out. This method is very accurate.
4. Plane table traversing In this method, a plane table is set at every traverse station in the clockwise or anticlockwise direction, and the circuit is finally closed. During traversing, the sides of the traverse are plotted according to any suitable scale. At the end of the work, any closing error which may occur is adjusted graphically.