Types of traverse

Traverse Surveying is a popular method of surveying. This article includes the definition of traverse surveying along with its classification, errors in traversing, checks, the completed method of traversing and plotting of traverse survey.

Definition

Traversing is that type of survey in which a number of connected survey lines form the framework and the directions and lengths of the survey lines are measured with the help of an angle measuring instrument and tape or chain respectively.

Types of Surveying

There are two types of traverse surveying. They are:

1. Closed traverse: When the lines form a circuit which ends at the starting point, it is known as a closed traverse. 
2. Open traverse: When the lines form a circuit ends elsewhere except starting point, it is said to be an open traverse.

Suitability

The closed traverse is suitable for locating the boundaries of lakes, woods, etc and for a survey of large areas. the open traverse is suitable for surveying a long narrow strip of land as required for a road of the canal or the coastline.

Methods of Traversing

There are several methods of traversing, depending on the instruments used in determining the relative directions of the traverse lines. The following are the principal methods:

      Chain traversing

      Chain and compass traversing

      Transit type traversing a)By fast needle method b)By measurement of angles between the lines

      Plane table traversing

 Brief descriptions of these traverse surveying methods are given below.

Chain Traversing

The method in which the whole work is done with chain and tape is called chain traversing. No angle measurement is used and the directions of the lines are fixed entirely by linear measurements Angles fixed by linear or tie measurements are known as chain angles. The method is unsuitable for accurate work and is generally used if an angle measuring instruments such as a compass, sextant or theodolite is available.

Chain and Compass Traversing

In chain and compass traversing, the magnetic bearings of the survey lines are measured by a compass and the lengths of the lines are measured either with a chain or with a tape. The direction of the magnetic meridian is established at each traverse station independently. The method is also known as a tree or loose needle method.

Traversing by Fast Needle Method

The method in which the magnetic bearings of traverse lines are measured by a theodolite fitted with s compass is called traversing by fast needle method. The direction of the magnetic meridian is not established at each station but instead, the magnetic bearings of the lines are measured with a reference so that direction of the magnetic meridian established at the first station. There are three methods of observing the bearings of lines by fast needle method.

1. Direct method with transiting,
2. Direct method without transiting,
3. Back bearing method.

Traversing By Direct Observation Of Angles

In this method, the angles between the lines are directly measured by a theodolite and the magnetic bearing of other lines can be calculated in this method. The angles measured at different stations may be either

1. Included Angles and
2. Deflection Angles

Traversing by Included Angle

An included angle at a station is either of the two angles formed n\by two survey lines meeting there and these angles should be measured clockwise. The method consists simply in measuring each angle directly from a back sight on the preceding station. The angled may also be measured by repetition. The angles measured from the back station may be interior or exterior depending on the direction of progress.

In Fig(a) the direction of progress is counter-clockwise and so the angles measured clockwise are the interior angle. In Fig(a) the direction of progress is clockwise and so the angles measured clockwise are the exterior angle.

Traverse by Deflection Angles

A deflection angle is an angle in which a survey line makes with the prolongation of the preceding line. It is designated as right (R) or left (L) as it is measured clockwise or anti-clockwise from the prolongation of the previous line. This type of traversing is more suitable for the survey of roads, railways, pipe-lines, etc where the survey lines make small deflection angles.

Errors in Traversing

The errors involved in closed traversing are two kinds:

1. Linear Error and
2. Angular Error

The most satisfactory method of checking the linear measurements consists in chaining each survey line a second time, preferably in the reverse direction on different dates and by different parties. The following are checks for the angular work:

A.     Travers by included angles:

o    The sum of measured interior angles should be equal to (2N-4), where N=number of sides of the traverse.

o    If the exterior angles are measured, their sum should be equal to (2N=4)p/2

2. Travers by deflection angles: The algebraic sum of the deflection angles should be equal to 360°, taking the right hand and deflection angles as a positive and left-hand angle as negative.
3. Traversing by direct observation of bearings: The force bearing of the last line should be equal to its back bearing ±180° measured from the initial station.

Checks in Open Traverse

No direct checks of angular measurement are available. So indirect checks can be made. As illustrated in Fig(a) the addition to the observation of bearing of AB at station A, bearing of AD can also be measured., if possible. Similarly, at D, the bearing of DA can be measured and check applied. If the two bearings differ by 180°, the work may be accepted as correct.

Another method, which furnishes a check when work is plotted is shown as in Fig (b) and consists of reading the bearing to any prominent point P from each of the consecutive stations. The check-in plotting consists in laying off the lines AP, BP, CP, etc and noting whether the lines pass through one point.

Plotting a Traverse Survey

There are two principal methods of traverse survey:

1. Angles and distance method: This method is of three types.
   1. By protractor
   2. By the tangent of the angle
   3. By the chord of the angle.
2. Co-ordinate method.

Traverse Computations

In the figure, the latitude and the departure of the line AB of length l and reduced bearing q are given by

L= + l cosq and D=+ l sinq

To calculate the latitude and departure of the traverse lines, it is first essential to reduce the bearing in the quadrant system. The signs of latitude and departures will depend upon the reduced bearing of the line. The following table gives signs of latitudes and departures.

Table-1

Thus, latitude and departure co-ordinate of any point with reference to the preceding point are equal to the latitude and departure of the line joining the preceding point to the point under consideration. Such coordinates are also known as consecutive co-ordinates or dependent coordinates.