Traverse Surveying - Objective, Method and Procedure

A traverse survey is a type of control survey which 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.

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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 move from one point to other by simultaneously measuring bearings and distances by "dead reckoning". Dead reckoning is the process of calculating 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

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

The main objective of traverse survey are:

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

Read On: Difference between Triangulation & Trilateration in Surveying

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, coast line, pipeline, etc. 

An open traverse can run from few hundred meters to kilometers. The figure (b) below shows open traverse ABCDEF.

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

The consistence of angles and distance measured cannot be checked in 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 close 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, 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 to have computation check that allow the detection of systematic errors in both distance and direction.

How are Traverse Lines Measured?

The traverse lines is either determined by:

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

Whenever there is 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 instrument operator, a head tape man and rare tape man.

Traverse Equipment: The equipment used for traverse survey includes: theodolites, compass,  tapes, chains, tacheometry, 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

A traverse survey can be performed in several ways based on the method and the instrument used:

1. Chain Traversing
2. Compass Traversing
3. Interior angle traverse
4. Theodolite Traversing
1. Deflection angle traverse
2. Angle to the right traverse
3. Azimuth traverse
5. Stadia Traversing
6. Plane table and alidade Traversing

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 preliminary field inspection of the entire area that need to be surveyed. This involves:

1. The surveyor go to the field and check 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 Tacheometric method or EDM instruments. The angular measurements are done using a compass or theodolite.

Also Read: Types of Surveying Based on Methods Adopted