Traversing - Compass Traversing, Surveying and Levelling | Surveying and Levelling Notes- Agricultural Engg - Agricultural Engineering PDF Download

TRAVERSING
As already stated in the last section, surveying which involves a series of connected lines is known as ‘traversing.’ The sides of the traverse are known as ‘traverse legs’.
In traversing, the lengths of the lines are measured by chain and the directions are fixed by compass or theodolite or by forming angles with chain and tape.
A traverse may be of two types – closed and open.

1. Closed traverse When a series of A connected lines forms a closed circuit, i.e. when the finishing point coincides with the starting point coincides with the starting point of a survey, it is called  a ‘closed traverse’. Here ABCDEA represents a closed traverse. Closed traverse is suitable for the survey of boundaries of ponds, forests estates, etc.
2. Open traverse When a sequence of connected lines extends along a general direction and does not return to the starting point, it is known as ‘open traverse’ or ‘unclosed traverse’. Here ABCDE represents an open traverse Open traverse is suitable for the survey of roads, rivers, coast lines, etc.

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.

CHECK ON CLOSED TRAVERSE
1. Check on angular measurements
(a) The sum of the measured interior angles should be equal to (2N – 4) x 90⁰ where N is the number of sides of the traverse.
(b) The sum of the measured exterior angles should be equal to (2N + 4) x 90⁰.
(c) The algebraic sum of the deflection angles should be equal to 360⁰.
Right-hand deflection is considered positive and left-hand deflection negative.
2. Check on linear measurement
(a) The lines should be measurement once each on two different days (along opposite directions). Both measurements should tally.
(b) Linear measurements  should also be taken by the stadia method. The measurements by chaining and by the stadia method should tally.

CHECK ON OPEN TRAVERSE
In open traverse, the measurements cannot be checked directly. But some field measurements can be taken to check the accuracy of the work. The methods are discussed below.
1. Taking cut-off lines Cut-off lines are taken between some intermediate stations of the open traverse. Suppose ABCDEF represents an open traverse. Let AD and DG be the cut-off lines. The lengths and magnetic bearings of the cut-off lines are measured accurately. After plotting the traverse, the distances and bearings are noted from the map. These distances and bearings should tally with the actual records from the field
2. Taking an auxiliary point Suppose ABCDEF is an open traverse. A permanent point P is selected on one side of it. The magnetic bearings of this point are taken from the traverse stations A,B,C,D, etc. If the survey is carried out accurately and so is the plotting, all the measured bearings of P when plotted should meet at the point P. The permanent point P is known as the ‘auxiliary point’