Traversing in surveying is used to establish a network of control points. Traversing is the type of surveying connecting a number of survey lines that form a framework. It is also commonly referred to as a controlled method of surveying. Generally, traversing involves the measurement of the following parameters:
1. The length of each survey line.
2. The angle between the successive survey lines or the bearings of the lines.
The measurement of the directions of the lines is usually done by means of an angle measuring device such as theodolite and the length is measured with the help of tape. Traverse may be defined as the series of straight lines that connect the survey stations. In this method of surveying, the co-ordinates of the first established station, as well as the bearing of the first line, are determined, then, the co-ordinates of all the successive stations are computed using the formula:
Xb = Xa + L * Cos θ
Yb = Ya + L * Cos θ
L = Length of the traverse leg
Θ = Bearing of the line ab
Objectives of Traversing in Surveying
The main objectives of traversing may be listed as follows:
1. To establish the control points for other methods of surveying such as plane table surveying, chain surveying etc.
2. To form a network or framework of traverse legs and traverse stations and ensuring the visibility of all the features on the ground so that it can be duly located from the instrument.
3. To fix the alignments of canals, rivers, roads etc and boundaries when better accuracy is required.
4. To ascertain the coordinates of the boundary pillars such as forest boundary pillars, international boundary pillars etc. so that they can be stored in numerical terms for the use as reference in the future.
5. To prepare the topographic maps of the area to be surveyed for various purposes.
6. To establish the contours of the ground taking suitable intervals to provide an exact approximation of the nature of the ground as well as determining the feasibility of the project.
7. To mark the position of various details that can have effects on the forthcoming project.
Types of Traversing in Surveying
The traverse may be categorized into the following types:
1. Closed Traverse
If the traverse formed by the survey lines closes at a station i.e. if the survey lines form a polygon or starts and finishes at the same survey station of the known co-ordinates then the traverse is known as closed traverse. Closed traversing is generally used for locating the boundaries or for surveying of large areas.
2. Open Traverse
If a traverse starts and finishes at the points other than the starting point of known coordinates, then such traverse is known as an open traverse. An open traversing is generally used for preliminary and construction surveys for highways, roads, pipelines, transmission lines, etc.
3. Link Traverse
A link traverse is the type of traverse where an open type traverse is linked at the ends of an existing traverse so as to form a closed loop or closed traverse. The closing line is generally defined by the coordinates at the endpoints which have been determined by the previous survey.
Balancing The Traverse
During the computation of the traverse, it becomes necessary to balance the traverse because of the different errors that may persist during the field measurement. Thus, there are a number of methods that can be used for adjusting such traverse. They are:
1. Bowditch Method
2. Transit Method
3. Graphical Method
4. Axis Method
These methods of adjustment are based on the assumptions that the errors in the linear measurements are directly proportional to the length of the traverse leg whereas the angular measurements are inversely proportional to the length of the traverse leg. Most commonly the Bowditch method is used to balance a traverse where both the angular and linear measurements are of equal precision. In this method of balancing, the total error in latitude and in the departure is distributed in proportion to the length of the sides. The correction formula of the Bowditch’s rule is as follows:
Correction of Latitude or Departure = (Total error in Latitude or Departure* Length of the leg)/ Perimeter of the traverse
Adjustment of Angular Error and Bearing
The error in a closed traverse on account of the bearing may be determined by the comparison of the two bearings of the last line as observed from the first and the last station of the traverse. If the closed traverse has N number of sides then,
Correction of the first line = Error (e)/ N
Correction for the second line = 2*(e/N)
Correction for the last line = N*(e/N) = e
In case of a closed traverse, according to the geometry, the sum of the interior angles must be equal to (2n – 4)* 90. Here, n indicates the number of traverse stations. In case, the angles are measured with the same degree of precision, the error in the sum of the angles can be equally distributed among each of the angles of the traverse.
Discrepancy and Linear Mis-closure
In order to measure the length of the sides of the traverse, two ways taping namely forward taping and backward taping is done. The difference in the values obtained by the forward and the backward taping is known as the discrepancy. The reciprocal of the two measurements divided by the obtained discrepancy is known as precision. The discrepancy, as well as the precision for each of the leg of the traverse, should be within the prescribed permissible limits. Mathematically,
Discrepancy = Forward Length – Backward Length
Linear Precision = 1/ (Mean Length / Discrepancy)
Methods of Traversing in Surveying
The most commonly adopted methods of traversing can be listed as follows:
1. Chain Traversing (Traversing by means of chaining using chains and tapes).
2. Chain and Compass Traversing (Traversing by means of chaining for the linear measurements and angular measurements by means of the compass).
3. Plane table Traversing (Traversing by means of a plane table)
4. Transit type Traversing (Traversing by the measurement of angles between the lines or by use of fast needle).
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