Traverse surveying is a widely used method of surveying in civil engineering. This article discusses the definition of traverse surveying along with its types, suitability, methods of traversing, errors and checks in traversing, and the plotting and computation of a traverse survey.
Definition
Traversing is a type of survey in which a number of connected survey lines form a framework. The lengths of the survey lines are measured using a chain or tape, and the directions are determined using an angle-measuring instrument. The framework thus formed serves as a basis for locating details within the surveyed area.
Objectives of Traverse Surveying
The main objective of traverse surveying is to establish a framework of connected survey lines for determining the relative positions of points on the ground. The specific objectives of traverse surveying are listed below:
- To establish horizontal control points for detailed surveying work.
- To determine the relative positions of points by measuring lengths and directions of survey lines.
- To provide a suitable framework for mapping roads, canals, railways, pipelines, and boundaries.
- To check and control the accuracy of survey measurements.
- To facilitate the plotting of plans and the preparation of maps.
Types of Traverse Surveying
Traverse surveying is classified into two main types based on the nature of the traverse:
- Closed Traverse: When the survey lines form a closed circuit and end at the starting point, the traverse is known as a closed traverse.
- Open Traverse: When the survey lines do not return to the starting point and end elsewhere, the traverse is called an open traverse.
Suitability of Traverse Surveying
The suitability of a traverse depends on the nature of the area being surveyed.
A closed traverse is suitable for surveying the boundaries of lakes, forests, estates, and other large enclosed areas. An open traverse is suitable for surveying long and narrow strips of land, such as roads, railways, canals, pipelines, and coastlines.
Methods of Traversing
Depending on the instruments used for measuring directions, traverse surveying can be carried out by several methods. The principal methods of traversing are listed below:
- Chain traversing
- Chain and compass traversing
- Transit traversing
- By fast needle method
- By direct measurement of angles
- Plane table traversing
Brief descriptions of these methods are given below.
Chain Traversing
Chain traversing is the simplest form of traversing, where all measurements are made using a chain or tape only. No angular measurements are taken. The directions of lines are fixed by linear or tie measurements, and the resulting angles are known as chain angles. This method is unsuitable for accurate work and is generally used only when angle-measuring instruments are not available.
Chain and Compass Traversing
In chain and compass traversing, the magnetic bearings of traverse lines are measured using a compass, and the lengths are measured by chain or tape. The magnetic meridian is established independently at each station. This method is also known as the loose needle or tree method.
Traversing by the Fast Needle Method
In this method, magnetic bearings of traverse lines are measured using a theodolite fitted with a compass. The magnetic meridian is established only at the first station, and all subsequent bearings are measured with reference to it. This improves accuracy compared to chain and compass traversing.
Bearings may be observed by the following methods:
- Direct method with transiting
- Direct method without transiting
- Back bearing method
Traversing by Direct Observation of Angles
In this method, the angles between traverse lines are directly measured using a theodolite. The bearings of the lines are then computed from the measured angles. The angles observed at the stations may be:
- Included angles
- Deflection angles
Traversing by Included Angles
An included angle at a station is the angle between two consecutive survey lines meeting at that station. These angles are measured clockwise from the back sight. The angles may be measured either directly or by repetition. Depending on the direction of progress, the angles measured may be interior or exterior angles.
If the direction of progress is counter-clockwise, the clockwise measured angle represents the interior angle. If the direction of progress is clockwise, the clockwise measured angle represents the exterior angle.
Traversing by Deflection Angles
A deflection angle is the angle between the prolongation of the preceding line and the succeeding survey line. It is designated as right (R) or left (L), depending on whether it is measured clockwise or anti-clockwise. This method is particularly suitable for surveys of roads, railways, and pipelines, where deflection angles are generally small.
Errors in Traversing
The errors involved in closed traversing are of two main types:
- Linear errors
- Angular errors
The most reliable check on linear measurements is to re-measure each survey line, preferably in the reverse direction and on different occasions. The following checks are applied to angular measurements:
- Traverse by Included Angles:
- The sum of the interior angles should be equal to (2N − 4) × 90°, where N is the number of sides.
- The sum of exterior angles should be equal to 360°.
- Traverse by Deflection Angles: The algebraic sum of deflection angles should be equal to 360°, taking right deflection as positive and left deflection as negative.
- Traverse by Direct Observation of Bearings: The fore bearing of the last line should differ from its back bearing by 180°.
Checks in Open Traverse
In an open traverse, no direct check on angular measurements is available. Therefore, indirect checks are applied. For example, bearings of a prominent point may be observed from successive stations. If the difference between corresponding bearings is 180°, the work may be considered satisfactory.
Plotting a Traverse Survey
Traverse surveys can be plotted by the following principal methods:
- Angles and Distance Method
- By protractor
- By tangent of the angle
- By chord of the angle
- Coordinate Method
Traverse Computations
In traverse computation, the latitude and departure of each traverse line are calculated. For a line AB of length l and reduced bearing θ:
Latitude (L) = ± l cos θ
Departure (D) = ± l sin θ
Before computation, whole circle bearings must be converted into reduced bearings. The signs of latitude and departure depend on the quadrant in which the line lies. These coordinates are also known as consecutive or dependent coordinates.
Conclusion
Traverse surveying is an important and widely used method of surveying in civil engineering. It is especially suitable for projects involving long and narrow strips of land, such as roads, railways, and canals. By providing a flexible and expandable framework, traverse surveying allows accurate determination of relative positions of points. When proper checks and corrections are applied, traverse surveying produces reliable results and forms a strong basis for detailed engineering surveys.