Traverse in Surveying- Types and Methods

Table of Contents

In surveying and geography, a traverse is a series of connected straight lines, each joining two ground stations. These stations, known as traverse stations or traverse points, are carefully measured and marked locations on the terrain. The straight lines between two consecutive stations are called traverse legs. Traverses are fundamental in establishing control networks and are used for various purposes in surveying, mapping, and construction projects.

Types of Traverse

There are two type of traverse as open traverse and closed traverse.

1.Closed Traverse

A closed traverse is a surveying method where a series of connected measurements forms a complete circuit, either by returning to the original starting point or by connecting two known points. This method provides a high degree of accuracy and built-in error checking, as the known coordinates at the beginning and end allow surveyors to calculate and distribute any accumulated errors. Closed traverses are fundamental in establishing control networks, property boundary surveys, and various engineering projects where precise spatial relationships are crucial.

It can be categorized into two subtypes:

Types of Closed Traverse

i. Loop Traverse:

Originates and terminates at the same station, completing a circuit.
Example: As shown in Figure below the traverse starts at station A and follows the route through stations B, C, D, E, and so on, eventually returning to station A.
The closed circuit formed by the traverse legs is known as the traverse circuit.

Starting and Ending Point

Example of Loop Traverse

ii. Connecting Traverse:

Begins at one known station and ends at a different known station.
Example: As illustrated in Figure below, the traverse starts at known station A, passes through stations B, C, D, E, and so on, finally closing on another known station G.

Traverse Diagram

Figure of Traverse Diagram

Key advantages of closed traverses:

Error detection: The known starting and ending points allow surveyors to calculate and distribute errors.
Increased accuracy: The ability to adjust and balance the traverse improves overall precision.
Redundancy: Multiple measurements provide a check against gross errors.

Applications of closed traverses:

Establishing control networks for large-scale mapping projects
Boundary surveys for property demarcation
Construction site layouts
Mining surveys

Measurement techniques:

Angles are typically measured using a theodolite or total station.
Distances are measured using electronic distance measurement (EDM) devices, tapes, or chains.

It’s worth noting that modern surveying often employs GPS technology in conjunction with traditional methods to enhance accuracy and efficiency in closed traverse surveys.

2.Open Traverse

An open traverse is a surveying method where the survey line neither returns to its starting point nor closes on a known station. This type of traverse extends in a generally consistent direction through a series of connected lines.

An open traverse is characterized by:

Starting at a known point but ending at an unknown location
A series of connected survey lines extending in the same general direction
Lack of closure on a predetermined point

As illustrated in Figure below, the traverse begins at station A and proceeds through stations B, C, D, and finally terminates at E, where E’s position is not predetermined.

Key characteristics of open traverses:

Lack of closure: Unlike closed traverses, there’s no built-in check for accuracy.
Accumulation of errors: Errors in measurement can compound along the traverse.
Limited error distribution: Without closure, it’s challenging to distribute and adjust errors.
Suitable for certain applications: Despite limitations, open traverses are useful in specific scenarios.

Applications of open traverses:

Preliminary surveys for road or pipeline routes
Exploration surveys in remote areas
River or coastline surveys
Rapid surveys where time is a critical factor

Measurement techniques:

Angles are typically measured using a theodolite or total station
Distances are measured using electronic distance measurement (EDM) devices, tapes, or chains
GPS technology can be used to enhance accuracy, especially for longer traverses

Advantages of open traverses:

Speed: Can be faster to execute than closed traverses in certain situations
Simplicity: Requires less complex calculations compared to closed traverses
Flexibility: Useful in situations where closing the traverse is impractical or impossible

Open traverses were more common in early surveying when technology and time constraints often made closing traverses impractical. Modern surveying techniques and equipment have reduced reliance on open traverses for many applications, but they remain useful in specific scenarios.

In summary, while open traverses have limitations in terms of accuracy and error checking, they remain a valuable tool in the surveyor’s toolkit, particularly for preliminary or exploratory surveys where speed and flexibility are prioritized over absolute precision

Classification of Traverses Based on Instruments Used

Traverses can be classified into several categories based on the surveying instruments employed. Each method offers unique advantages and is suitable for different scenarios. The main classifications are:

Chain Traversing
Compass Traversing
Plane Table Traversing
Theodolite Traversing
Tacheometric Traversing
Electronic Distance Measurement (EDM) Traversing
Global Navigation Satellite System (GNSS) Traversing

1.Chain Traversing (Chain Angles Method)

Chain traversing, also known as the chain angles method, is one of the simplest and oldest forms of traverse surveying. In this method, the entire survey is conducted using only a chain or tape, without any angle-measuring instruments.

Key features:

Uses only linear measurements
Angles are computed indirectly from distance measurements
Suitable for small-scale surveys in open, relatively flat terrain
Generally less accurate than other methods

Limitations:

Accuracy decreases for larger angles
Not suitable for rough or heavily obstructed terrain
Time-consuming for large surveys
Doesn’t follow the principle of “working from whole to part

2.Compass Traversing

Compass traversing is a method in which angular measurements are made using a surveying compass. This technique is widely used for moderate-scale surveys and offers a good balance between speed and accuracy.

Key features:

Uses a magnetic compass for angle measurements
Distances measured with tape or EDM
Suitable for moderate-scale surveys
Affected by local magnetic variations and nearby metal objects

Procedure:

In compass traversing, the traverse angle between two consecutive legs is computed by observing the magnetic bearings of the sides. The process typically involves the following steps:

Set up the compass at each traverse station.
Measure the magnetic bearing of each traverse line.
Measure the distance between stations using a tape or EDM.
Calculate the included angle between adjacent lines using the difference in bearings.

Advantages:

Faster than chain traversing
Provides a reasonable level of accuracy for many applications
Relatively simple equipment and procedures
Useful for preliminary surveys and route reconnaissance

Limitations:

Accuracy is lower than theodolite or total station methods
Subject to errors due to local magnetic anomalies
Can be affected by nearby metal objects or electrical currents
Requires regular calibration and checks for magnetic declination

Applications:

Topographic surveys of moderate-sized areas
Boundary surveys of rural properties
Preliminary surveys for road or pipeline routes
Geological and forest surveys

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3.Plane Table Traversing

Plane table traversing is a unique method of surveying where the map is drawn directly in the field as measurements are taken. In this technique, angular measurements between traverse sides are plotted graphically on a plane table with the help of an alidade.

Key features:

Combines field measurements and mapping in one step
Uses a plane table, alidade, and drawing sheet
Provides immediate visual representation of the survey
Suitable for topographic surveys and mapping of moderate-sized areas

Equipment:

Plane table: A flat surface mounted on a tripod
Alidade: A sighting device with a straightedge for drawing lines
Drawing sheet: Mounted on the plane table
Plumb bob: For centering the table over survey points
Compass: For orientation
Measuring tape or stadia rod: For distance measurements

Procedure:

Set up the plane table at the first station, ensuring it’s level and properly oriented.
Place the drawing sheet on the table and mark the station point.
Use the alidade to sight to other stations or objects of interest.
Draw lines along the alidade’s straightedge to represent sighted directions.
Measure distances to the sighted points using a tape or stadia methods.
Scale the distances on the drawn lines to plot the points.
Move to the next station and repeat the process, using previously plotted points for orientation.

Methods of plane table traversing:

a) Radiation method: Suitable for small areas visible from a single station

b) Intersection method: Uses two known positions to locate new points

c) Resection method: Determines the position of the plane table using known points

d) Traversing method: Used for larger areas, similar to other traversing techniques

Advantages:

Provides an immediate visual check of the survey’s progress
Eliminates the need for field notes and office plotting
Errors can be detected and corrected in the field
Ideal for filling in details in topographic surveys

Limitations:

Accuracy is limited by the scale of the drawing and instrument precision
Weather-dependent (wind, rain can affect the process)
Difficult to use in heavily forested or built-up areas
Not suitable for high-precision surveys

Applications:

Topographic mapping of small to moderate areas
Updating existing maps with new features
Preliminary surveys for engineering projects
Geological and archaeological surveys

Accuracy considerations: The accuracy of plane table traversing depends on several factors:

Skill of the surveyor
Precision of the alidade and other instruments
Scale of the drawing
Weather conditions
Careful leveling and orientation of the plane table

While plane table traversing is less precise than modern electronic methods, it remains valuable in certain situations due to its simplicity and the immediate production of a graphical result. It’s particularly useful in areas where electronic equipment might be impractical or unavailable.

4.Theodolite Traversing

Theodolite traversing is a high-precision surveying method in which angular measurements between traverse sides are made with a theodolite. This technique offers significantly improved accuracy over compass and plane table methods, making it suitable for a wide range of professional surveying applications.

Key features:

Uses a theodolite for precise angle measurements
Can measure both horizontal and vertical angles
Distances typically measured with EDM or tapes
Suitable for high-accuracy surveys and large-scale projects

Equipment:

Theodolite: A precision instrument for measuring angles
EDM (Electronic Distance Measurement) device or surveying tape
Tripod and tribrach for stable instrument setup
Targets or reflectors for sighting
Field book for recording measurements

Procedure:

Set up the theodolite at a traverse station, ensuring it’s level and properly centered.
Orient the theodolite to a known reference direction (often true or magnetic north).
Measure the horizontal angle to the next traverse station.
If required, measure the vertical angle for elevation differences.
Measure the distance to the next station using EDM or tape.
Move to the next station and repeat the process.
Close the traverse by returning to the starting point or a known control point.

Advantages:

High precision in angle measurements
Capable of measuring both horizontal and vertical angles
Suitable for large-scale and high-accuracy surveys
Forms a strong geometric network when properly executed

Limitations:

More time-consuming than simpler methods
Requires skilled operators for best results
More expensive equipment compared to simpler methods
Necessitates clear lines of sight between stations

Applications:

Establishing precise control networks for large projects
High-accuracy boundary and cadastral surveys
Engineering surveys for construction and infrastructure projects
Precise topographic mapping

Accuracy considerations:

The accuracy of theodolite traversing depends on several factors:

Precision of the theodolite (optical or digital)
Skill of the operator
Environmental conditions
Quality of the distance measurements

5. Tacheometric traversing

Tacheometric traversing is a surveying method in which direct measurements of traverse sides by chaining is dispensed with and these are obtained by making observations with a tacheometer. This technique allows for rapid data collection of both horizontal distances and elevations, making it particularly useful in certain surveying scenarios.

Key features:

Uses a tacheometer for distance and elevation measurements
Eliminates the need for direct chaining of distances
Allows for rapid data collection in varied terrain
Suitable for surveys in hilly or difficult-to-access areas

Equipment:

Tacheometer: A specialized theodolite with stadia lines in its reticle
Stadia rod: A graduated rod for sighting
Tripod for instrument setup
Field book for recording measurements

Procedure:

Set up the tacheometer at a traverse station.
Sight the stadia rod held vertically at the next traverse point.
Read the upper, middle, and lower stadia hair readings on the rod.
Measure the vertical angle to the rod.
Calculate the horizontal distance and elevation difference using tacheometric formulas.
Move to the next station and repeat the process.

Tacheometric formulas:

Horizontal distance = K × (Upper reading – Lower reading) × cos²(vertical angle)
Vertical distance = K × (Upper reading – Lower reading) × sin(vertical angle) × cos(vertical angle) Where K is the multiplying constant of the tacheometer (usually 100)

Advantages:

Faster than conventional chaining methods, especially in rough terrain
Provides both horizontal distances and elevations in one observation
Useful in areas where direct distance measurement is difficult
Reduces field time compared to separate distance and leveling operations

Limitations:

Less accurate than precise EDM measurements
Accuracy decreases with distance and steep slopes
Requires clear sight lines between stations
Affected by atmospheric refraction over long distances

Applications:

Topographic surveys in hilly or mountainous terrain
Preliminary surveys for road or railway alignments
Hydrographic surveys of river cross-sections
Rapid surveys where moderate accuracy is acceptable

Accuracy considerations: The accuracy of tacheometric traversing depends on:

Precision of the tacheometer
Skill of the operator
Length of sight lines
Atmospheric conditions
Verticality of the stadia rod

While traditional optical tacheometers are still used, modern total stations often incorporate electronic tacheometry features, allowing for more precise and automated data collection.

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