Traverse Surveying: Definition, Types, Methods, and Inspections
Traverse surveying is a popular surveying technique. This article defines traverse surveying, as well as its classification, errors in traversing, checks, the completed method of traversing, and traverse survey plotting.
Definition
Traversing is a type of survey in which a network of connected survey lines forms the framework and the survey lines' directions and lengths are measured using an angle measuring instrument and tape or chain, respectively.
Surveying Types
Traverse surveying is classified into two types. They are as follows:
- Closed traverse: A closed traverse occurs when the lines form a circuit that terminates at the starting point.
- An open traverse occurs when the lines in a circuit terminate somewhere other than the starting point.
Suitability
The closed traverse is useful for locating the boundaries of lakes, woods, and other bodies of water, as well as for surveying large areas. The open traverse is ideal for surveying a long, narrow strip of land, such as a canal or coastline road.
Methods of Travel
There are several methods of traversing, depending on the instruments used to determine the relative directions of the traverse lines. The following are the primary methods:
- traversing the chain
- traversing the chain and compass
- Transit traversing a)by fast needle method b)by angle measurement between the lines
- Traversing a plane table
These traverse surveying methods are described briefly below.
Traversing the Chain
Chain traversing refers to the method of doing all of the work with chain and tape. There is no use of angles, and the directions of the lines are entirely determined by linear measurements. Chain angles are angles that are defined by linear or tie measurements. The method is unsuitable for precise work and is typically employed when an angle measuring instrument, such as a compass, sextant, or theodolite, is available.
Chain and Compass Traversing
Chain and compass traversing involves measuring the magnetic bearings of survey lines with a compass and the lengths of the lines with either a chain or a tape. The magnetic meridian's direction is determined independently at each traverse station. The technique is also known as the tree method or the loose needle method.
Fast Needle Method of Crossing
The fast needle method refers to measuring the magnetic bearings of traverse lines with a theodolite equipped with a compass. Instead of establishing the magnetic meridian at each station, the magnetic bearings of the lines are measured with a reference so that the magnetic meridian is established at the first station. There are three ways to observe line bearings using the fast needle method.
- Back bearing method,
- Direct method with transiting,
- Direct method without transiting.
Traversing by Direct Angle Observation
The angles between the lines are directly measured with a theodolite in this method, and the magnetic bearing of other lines can be calculated. Angles measured at various stations could be either
- Included Angles and
- Deflection Angles
Included Angle Traversal
Because the progress in Fig(a) is counter-clockwise, the angles measured clockwise are the interior angles. Because the direction of progress in Fig(a) is clockwise, the angles measured clockwise are the exterior angles.
Deflection Angles Traverse
A deflection angle is the angle formed by a survey line with the extension of the preceding line. It is labelled as right (R) or left (L) depending on whether it is measured clockwise or anti-clockwise from the previous line's extension. This type of traversing is better suited for surveying roads, railways, pipelines, and other structures with small deflection angles.
Traversing Errors
There are two types of errors in closed traversing:
- Linear Error and
- Angular Error
The most satisfactory method of double-checking linear measurements is to chain each survey line a second time, preferably in the opposite direction, on different dates and by different parties. The following are angular work checks:
Traversing through included angles:
- The sum of the measured interior angles should equal (2N-4), where N is the number of traverse sides.
- If the exterior angles are measured, the total should be (2N=4)p/2.
Traversing by deflection angles: The algebraic sum of the deflection angles should equal 360°, with the right hand and deflection angles being positive and the left hand and deflection angles being negative.
Direct bearing observation: The force bearing of the last line should be equal to its back bearing 180° measured from the starting station.
Open Traverse Checks
There are no direct checks for angular measurement available. As a result, indirect checks can be performed. As shown in Fig(a), in addition to observing the bearing of AB at station A, the bearing of AD can also be measured if possible. Likewise, at D, the bearing of DA can be measured and checked. The work may be accepted as correct if the two bearings differ by 180°.
Another method, shown in Fig (b), provides a check when work is plotted and consists of reading the bearing to any prominent point P from each consecutive station. Check-in plotting entails laying off the lines AP, BP, CP, and so on and noting whether the lines pass through a single point.
Constructing a Traverse Survey
There are two main types of traverse surveys:
There are three types of angles and distance methods.
- Using a protractor
- Using the angle's tangent
- By the angle's chord.
Coordinated approach.
Computing in Traverse Space
In the figure, L= + l cosq and D= + l sinq are the latitude and departure of the line AB of length l and reduced bearing q, respectively.
Reduce the bearing in the quadrant system before calculating the latitude and departure of the traverse lines. The reduced bearing of the line will determine the latitude and departure signs. The table below provides latitude and departure indicators.
Table-1
W.C.B | R.B and quadrant | Sign of | |
Latitude | Departure | ||
0° to 90° | NqE; I | + | + |
90° to 180° | SqE;II | - | + |
180° to 270° | SqW;III | - | - |
270° to 360° | NqW;IV | + | - |
As a result, the latitude and departure coordinates of any point with respect to the point before it are equal to the latitude and departure of the line connecting those two points. These coordinates are also referred to as dependent coordinates or consecutive coordinates.
Surveying: Three Different Types of Traversing and Their Purposes
Table of Contents
1. Initialization
A network of control points is established during traversing in surveying.
The method of surveying known as traversing involves joining several survey lines that make up a framework.
Another name for it is a controlled method of surveying.
The following variables are typically measured during traversing:
1. How long each survey line is.
2. The arc formed by the bearings of the survey lines or the angle between them.
Typically, a theodolite or other angle measuring tool is used to measure the direction of the lines, and a piece of tape is used to measure their length.
The network of straight lines that connects the survey stations is known as a traverse.
In this method of surveying, the first established station's coordinates and the bearing of the first line are determined, and the formula is then used to calculate the coordinates of all succeeding stations.
Xb = Xa + L * Cos θ
Yb = Ya + L * Cos θ
Where,
L = Length of the traverse leg
Θ = Bearing of the line ab
2. Traversing's Purposes in Surveying
The following is a list of the primary goals of traversing:
1. To establish the reference points for other surveying techniques, such as chain surveying, plane table surveying, etc.
2. Creating a network or framework of traverse legs and stations while ensuring visibility of all ground features to enable accurate instrument location.
3. To correct boundary alignments and other features when greater accuracy is needed, such as canals, rivers, roads, etc.
4. To determine the coordinates of boundary pillars, such as forest boundary pillars and international boundary pillars, so that they can be saved in numerical terms for use as a reference in the future.
5. To make topographic maps of the area that will be surveyed for a variety of reasons.
6. To determine the viability of the project and to establish the contours of the ground while taking appropriate intervals to provide an exact approximation of the nature of the ground.
7. To indicate where various details that might affect the upcoming project are located.
3. Surveying Traverse Types
The following types of traverses are possible:
Closing Traverse
The traverse is referred to as a closed traverse if the survey lines form a closed traverse at a station, or if the traverse begins and ends at the same survey station with known coordinates.
Typically, a closed traverse is used to locate boundaries or to survey large areas.
Open Traverse 2.
A traverse is referred to as an open traverse if it begins and ends at locations other than the starting point of known coordinates.
For preliminary and construction surveys for highways, roads, pipelines, transmission lines, etc., an open traverse is typically used.
Link Traverse 3.
A link traverse is a type of traverse in which two open traverses are connected at their ends to create a closed loop or closed traverse.
The coordinates at the endpoints, which were identified by the prior survey, are typically used to define the closing line.
Balance The Traverse, No. 4
Due to the various errors that could occur during the field measurement, it becomes necessary to balance the traverse during computation.
Therefore, adjusting such a traverse can be done using a variety of techniques. As follows:
1. Bowditch Technique
2. Transit Approach
3. Visual Method
4. Axis Technique
These methods of adjustment are predicated on the notion that while angular measurements are inversely proportional to the length of the traverse leg, errors in linear measurements are directly proportional to the length of the traverse leg.
The Bowditch method is most frequently used to balance a traverse when both the angular and linear measurements are accurate. The total error in latitude and departure is distributed according to the side length in this method of balancing.
The Bowditch's rule correction formula is as follows:
Correction of Latitude or Departure = (Total Error in Latitude or Departure* Leg Length)/Transverse Perimeter
Continue reading: Bowditch and Transit Method with Numerics
5. Angular error and bearing adjustment
By comparing the two bearings of the last line as seen from the first and last stations of the traverse, it is possible to determine the error in a closed traverse caused by the bearing.
If N sides make up the closed traverse, then
Error (e)/N for the first line's correction
2*(e/N) is the second line's correction.
Similarly,
N*(e/N) = e as a correction for the last line
According to geometry, the sum of the interior angles for a closed traverse must be equal to (2n - 4)* 90.
The number of traverse stations is shown here by the letter n. The error in the sum of the angles can be distributed evenly among all of the traverse's angles if the angles are measured with the same level of accuracy.
6. Inconsistency and linear mis-closure
Two methods of taping, namely forward taping and backward taping, are used to measure the traverse's side lengths.
The discrepancy is the difference between the values obtained from forward and backward taping.
Precision is defined as the reciprocal of the two measurements divided by the calculated discrepancy.
Both the discrepancy and the precision for each leg of the traverse should fall within the permitted ranges.
Mathematically,
Forward Length - Backward Length equals discrepancy.
Linear precision is equal to 1/(mean length/difference).
7. Surveying Traversing Techniques
The following is a list of the traversing techniques that are most frequently used:
1. Crossing chains (Traversing by means of chaining using chains and tapes).
2. Traversing a chain and compass (Traversing by means of chaining for the linear measurements and angular measurements by means of the compass).
3. Traversing a plane table (Traversing by means of a plane table)
Traversing of the Transit type (Traversing by the measurement of angles between the lines or by use of fast needle).
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