LESSON NOTE ON MEHODS OF TRAVERSING

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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.
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