METHODS OF TRAVERSING BY COMPASS

METHODS OF TRAVERSING

Traverse survey may be conducted by the following methods:

1). Chain traversing (by chain angles)

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 nor 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, as shown in Fig. 11.a and 11.b.

  The formation of chain angles is explained below.

First method- Suppose a chain angle is to be formed to fixed direction 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 chain angle. So, the chain angle is fixed by the tie line T₁T₂ (Fig. 11.a).

    

Second method- Sometimes the chain angle is fixed by chord. Suppose the angles between lines AB and AC is to be fixed. Taking A as the centre and a radius equal to 15 metre, 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 (Fig. 11.b).

Let Angle PAR = θ

Then Angle BAC = 2θ

Here, AP = AQ = 15 m

In triangle PAR,

Sinθ = PR/AP = 2PR/2AP = PQ/30

Θ = Sinˉ¹ PQ/30

  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, 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 attractions are applied. In this method, closing error may occur when the traverse is plotted. This error is adjusted graphically by using ‘Bowdithc’s rule’ (which is describe in future post).

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 tape, or employing the stadia method or by digital instrument. The magnetic bearing of starting leg is measured by theodolite. Then the magnetic bearings of other sides are calculated. The independent coordinates of all 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 stations in the clockwise or anticlockwise direction, and the circuit is finally closed. During traversing the sides of the traverse plotted according to any suitable scale. At the end of the work, any closing error which may occur is adjusted graphically.

(Next Post on “CHECK ON CLOSED AND OPEN TRAVERSE”.)