Traverse Surveying - Objective, Method and Procedure

A traverse survey is a type of control survey that involves the establishment of a series of points that are linked together by lines to form a framework. The series of straight lines that connect the successive points are called traverse lines. 

The ends that defined each traverse line are called traverse stations or traverse points. The framework formed by connected survey lines of known length and direction is called a traverse.

Image Courtesy: Braincart

In the figure-1 below, A,B, C and D are traverse stations. AB, BC, and CD are traverse lines. 

Fig.1. Traverse Survey

In traversing, the surveyor moves from one point to another by simultaneously measuring bearings and distances by "dead reckoning". Dead reckoning is the process of calculating the current position of some moving object by using a previously determined position. Employed when the construction work is long and narrow ( tunnel or motorway construction).

Scope and Objective of Traverse Survey

A Traverse survey is conducted to establish horizontal control in land areas, especially in areas where the line of sight (LOS) is short due to heavily built-up areas, where the survey methods triangulation and trilateration are not applicable.

The main objective of the traverse survey are:

1. To locate or establish boundaries
2. To achieve horizontal control for topographic surveys
3. To locate and prepare construction layouts for highways, railways, and other private and public works
4. To conduct ground control surveys for photogrammetric surveys

Types of Traverse

The two types of traverse encountered while conducting surveys are:

1. Open Traverse
2. Closed Traverse

1. Open Traverse

Open traverse is a traverse that starts at a point of known position and terminates at a point of unknown position.  An open traverse is suitable for surveying along a narrow strip of land. For example, it is used for surveying roads, railways, canals, rivers, coastlines, pipelines, etc. 

An open traverse can run from a few hundred meters to kilometers. Figure (b) below shows an open traverse ABCDEF.

Fig.2. Types of Traverse - Open and Closed Traverse

The consistency of angles and distance measured cannot be checked in an open traverse. So, in order to minimize the errors, the distances can be measured twice, angles turned by repetition, etc.

2. Closed Traverse

Closed traverse originates at a point of known position and closes on another point of known horizontal position. A closed traverse can be a closed link traverse ( where the position of A and D is known) or a closed loop traverse ( where the traverse starts and ends at A, whose position is known). A closed traverse is suitable for locating the boundaries of lakes, houses, lawns, and gardens and for large areas like towns, residential campus etc. 

The figure-3 shows a closed traverse ABCDE.

Fig.3. Types of Closed Traverse

Closed traverse lets have a computation check that allows the detection of systematic errors in both distance and direction.

How are Traverse Lines Measured?

The traverse lines are either determined by:

1. Direct measurement: Tapes, EDM
2. Indirect measurement: Tachometric Methods
3. Angular measurement: Theodolite

Whenever there is a change in direction of the traverse, an angular measurement is taken. The traverse survey is performed by a traverse party and traverse equipment. 

Traverse Party: Traverse party consists of an instrument operator, a head tape man, and a rare tape man.

Traverse Equipment: The equipment used for the traverse survey includes: theodolites, compass,  tapes, chains, tachometer, hand level, leveling staff, ranging pole, plumb bobs, EDM and reflector, stakes and hubs, tacks, marking crayons, points, walkie talkies, & hammer, etc. The instrument used for traversing is dependent on the method of traversing employed

Different Methods of  Traversing in Surveying

Traverse surveying is a control survey that involves a number of survey lines forming a framework. The survey lines and directions are measured using distance-measuring devices and angle-measuring devices.

Usually, it is recommended to perform traversing using a theodolite and a tape to measure the angles and distances respectively. However, any combination of linear and angular measuring devices can be used.

The different methods of traversing are mentioned below:

1. Chain Traversing
2. Free or Loose Needle Method of Traversing
3. Fast Needle Method

1. Chain Traversing

Here, the traverse lines are measured using chain and tape alone. This is a very crude method and cannot be completely relied on.

Also Read: What is Chain Surveying?

2. Chain and Compass Surveying - Free or Loose Needle Method

In this method, the magnetic bearings of the survey lines are measured using compass and the traverse lines are measured using, a tape or chain. The direction of the magnetic meridian of each traverse line is determined independently. 

Also Read: What is Compass Surveying? Principle & Bearings

3. Theodolite Surveying 

Traversing conducted using theodolite can be conducted using the following methods:

3.1. Included Angle Method

3.2. Deflection Angle Method

3.3. Fast Needle Method

3.1. Included Angle Method

• Included angle method of traversing makes use of theodolite to determine the included angles.
• Hence, the method is suitable only for closed traversing.

The procedure can be explained by considering a closed traverse ABCD. The included angles that need to be determined are x, y, z, and w.

Initially, the theodolite is set-up at point A. The North direction is set using the compass in the theodolite.

•  After setting the instrument at A, fore-bearing to line AB is determined (Fab) and AD (Fad) is also determined. Hence, the included angle, x = Fab-Fad.
• Now, set the instrument at B, and find the North direction. At B, determine the fore-bearing of line BC (Fbc) and back-bearing of line BA (Fba). Then the included angle, y = Fba-Fbc.
• Now, set the instrument at C, and find the North direction. At C, find the fore-bearing of line CB

Traverse Using Included Angle Method

(Fcb), and the fore-bearing of CD (Fcd). Hence, the included angle, y = 360 - ( Fcd-Fcb).

• Now, set the instrument at D, take the fore-bearing of line DA (Fda), and the back bearing of line DC (Fdc). Hence, the included angle, l = Fdc-Fda.
•  The linear measurement AB,BC,CD, and DA are measured either using a tape or a chain.

Here, after determining the included angles, starting from A, it must end at A itself. To check whether we get a closed traverse, the following formula is used: 

Sum of the interior angles = 180 x (n-2);

Here, 'n' is the number of traverse lines involved in the traverse. In the above examples, n = 4; Therefore, (180 x (n-2)) = (180 x (4-2)) = 360 degrees. The sum of the included angles measured using theodolite must be equal to the 360 degrees as calculated above. If there is variation, the correction is provided. It is mainly performed using Bowditch Rule.

Fast needle method in traversing gives the direct measurement of included angle. The work can be checked during its progress and the errors can be detected and rectified immediately. Hence, the field work become less cumbersome and the computations are simple compared to other method of measuring angles.

3.2. Fast Needle Method

In this method, the magnetic meridian is established only at the starting station. This method is used to measure the magnetic bearings and length of traverse lines.

In this method, the instrument is set at A, and the magnetic meridian of line AB  (Fab) is only determined at first. We will lock this angle in the theodolite (by tightening the upper clamp of the theodolite). Now take the instrument and place it on station B, loosen the upper clamp and sight C. Now after sighting, tighten the upper clamp and angle measured, and place the instrument at D. Now loosen the upper clamp, sight A. Now, if the procedure is done correctly, the final angle shown on the scale will be 360 degrees. 

If it does not close by 360 degrees, there exist closing error, that need checking.

3.3. Deflection Angle Traversing

• Mostly employed for open traversing
• Used for location survey of railways, pipelines, highways, etc.. 

Traverse by Deflection Angle Method

The above open traverse is ABCD.

• Place instrument at A and find magnetic bearing of that line AB. 
• Now set instrument at B, sight A and set the instrument reading to zero. Transit the telescope, (now the instrument is set along the direction of line AB). Now measure the deflection angle by sighting to C.
• Now place the instrument at C, sight B and set the instrument to zero reading. Now transit the telescope and sight D. This gives the deflection angle at C.

Based on whether the angle is measured clockwise or anti-clockwise, positive and negative signs are given.

What is the Procedure to Perform Traverse Survey?

The steps involved in Traverse Survey are:

1. Reconnaissance
2. Selection of Traverse Stations
3. Linear and Angular Measurements

Step 1: Reconnaissance

Reconnaissance is defined as a preliminary field inspection of the entire area that needs to be surveyed. This involves:

1. The surveyor goes to the field and checks the entire area.
2. He decides the best plan of working.
3. He checks the intervisibility of the traverse stations
4. He decides the method of traversing to be adopted
5. Based on the method chosen, the instruments and accessories are selected accordingly.

Step 2: Selection of Traverse Stations

The basic principle followed in surveying is " working from whole to part "and it is adopted.

1. A minimum number of traverse stations should be selected.
2. Take the length of the traverse line as long as possible to reduce the time and centering effect of stations.
3. Try to select stations on a level and firm ground
4. After selecting the stations, mark them using pegs.

Step 3: Linear and Angular Measurements

The distances between the stations are measured using a tape or chain or the Tacheometric method or EDM instruments. The angular measurements are done using a compass or theodolite.

Read More On:  Difference between Triangulation & Trilateration in Surveying