In surveying, a traverse is a series of connected lines whose lengths and directions are measured to establish the relative positions of points. The process of surveying to obtain such measurements is known as traversing. Chains are commonly used for measuring lengths, while compasses or theodolites are used to determine the directions of traverse lines. This article explores the two main types of traverse and the four methods used in traversing.
Types of Traverse
Traverses can be categorized into two types: Open Traverse and Closed Traverse.
1. Open Traverse
An open traverse starts at one point and ends at another, creating a sequence of lines without forming a closed loop. It is also referred to as an unclosed traverse. This type of traverse is particularly suitable for surveying linear features such as roads, coastal lines, and rivers.
For example, when mapping a roadway, the survey starts at one endpoint and proceeds along the route to the other endpoint. While open traverses are practical for such projects, they do not inherently provide checks for errors due to their unclosed nature.
2. Closed Traverse
A closed traverse forms a closed circuit, where the starting and terminating points coincide. This type of traverse is useful for surveys that require clear boundary delineations, such as ponds, sports grounds, or forests. Since the circuit is closed, any error in measurements becomes apparent during computation, making it easier to apply corrections.
For instance, when surveying a forest boundary, the closed traverse ensures that the measured area is accurate and complete.
Methods of Traversing
Traversing can be performed using various methods, each relying on specific surveying instruments and techniques. The four primary methods of traversing are:
- Chain Traversing
- Compass Traversing
- Theodolite Traversing
- Plane Table Traversing
1. Chain Traversing
Chain traversing involves measuring only linear distances. Instruments such as chains or tapes are used to determine the lengths of traverse lines. Angles between adjacent sides are calculated using the concept of chain angles, which involves establishing a third side with tie stations or using a chord of known length.
This method is suitable for areas like ponds or small plots where triangulation is challenging. Although chain traversing is relatively simple, it is less precise than other methods due to its reliance on linear measurements alone.
2. Compass Traversing
In compass traversing, both linear and angular measurements are taken. Chains or tapes are used for measuring distances, while a prismatic compass is used for determining angles. Fore bearings and back bearings of traverse lines are recorded, and corrections are applied for any local magnetic attraction.
During plotting, any closing error is identified, and the Bowditch rule is used to adjust the measurements. This method is commonly employed for surveys requiring both distance and direction accuracy.
3. Theodolite Traversing
Theodolite traversing is a highly accurate method involving both linear and angular measurements. Chains or stadia methods are used to measure distances, while a theodolite is used to determine precise angles. The magnetic bearing of the first traverse line is measured, and from it, the magnetic bearings of the remaining lines are calculated.
This method is preferred for projects requiring high precision, such as large construction layouts or detailed boundary surveys.
4. Plane Table Traversing
Plane table traversing involves simultaneous measurement and plotting of the traverse. A plane table is set up at each traverse station, and the sides of the traverse are drawn on paper to scale in a clockwise or anti-clockwise direction. This method allows for immediate visualization of the survey and quick detection of errors.
Closing errors, if present, are adjusted graphically. Plane table traversing is particularly useful for fieldwork where direct plotting is advantageous, such as topographical surveys.
Conclusion
Traversing is a fundamental surveying technique used to establish points and boundaries with accuracy. Whether employing an open or closed traverse depends on the project’s requirements, while the choice of traversing method depends on the level of precision needed and the available instruments. Proper error correction and adjustment ensure reliable results, underscoring the importance of traversing in various fields of engineering and land surveying.