# Traverse Surveying

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TRAVERSING

To traverse: To move, travel or pass across a stretched land / area.

Traversing or Traverse Surveying: It is a type of Survey which comprises a series of straight lines, the length of each line and the angle between the lines are then measured.

It is carried out to establish the Control Networks. It is mostly used in Geodetic Surveying.

It involves placing the Survey Stations along a survey line or path of travel and then using the previously surveyed points as a base for observing and surveying the next point(s).

In Traversing, less reconnaissance and organization of work is needed.

In other systems, the survey is required to be performed along a rigid polygon; whereas in traversing the traverse can be of any form and can be changed to any shape and thus can accommodate different types of terrains.

In Traversing, only a few observations are needed to be taken at each station; whereas in other types of surveys a great deal of angular and linear observations are needed to be taken.

A Traverse comprises a series of established stations (of known position / coordinates and elevations) tied together with the angles and distances; all this set up is called Frame-work or Network.

Traversing progresses by measuring the distances and angles between the lines which form the boundary of a site or network.

Scale Error does not get added up in Traversing.

In Traversing, azimuth swing errors are reduced by increasing the distance between the stations.

Traversing is very accurate than Triangu-lateration (a combination of Triangulation and Trilateration Surveys).

Traversing is used in almost all forms of official, commercial and legal surveys.

In Traversing, the angles are usually measured by Theodolite / Total Station and distances are measured by the steel tapes, chains, Tacheometer, Total Stations and EDM instruments.

In Civil Engineering, Traversing is mostly used in determining the very long distances, land areas and earthwork volumes.

Types of Traverses: (i) Open Traverse (when the final point DOES NOT MEET the starting point; mostly both the said points are located very far away from each other and usually in the opposite direction).

(ii) Closed Traverse (when the final point MEETS the starting point).

Open Traverse Survey is used for Strip Surveying i.e. surveying along a long, narrow piece of land as in case of road (street, boulevard, highway and motorway), railway, runway, canal, river, pipeline, trench, coast-line, etc.

Closed Traverse Survey is used for Boundary Surveying e.g. surveying of boundary of a bridge, housing colony, residential area, school, college, university, hospital, factory/company/industrial land, park, lawn, play-ground, stadium, garden, farm, town, etc.

Plotting of Traverse: Two Methods: (i) Angle and Distance Method

(ii) Coordinate Method

(i) Angle and Distance Method: In this method, the distances between the stations are laid off according to the scale and included angles or bearings are plotted by protractor or by the methods of: (i) Tangent of the Angle (semi-inscribed circle in a traverse) or (ii) Chord of the Angle (circle completely circumscribing a traverse).

(ii) Coordinate Method: In this method, the survey stations are plotted by calculating their coordinates. This method is more accurate and practicable. The biggest advantage of this method is that the closing error can be eliminated by Balancing Method.

Theodolite Traversing: In Theodolite Traversing, the directions are measured in form of bearings, deflection angles, angles to the right, interior angles or azimuths, and distances are measured with the tape, chain, tacheometer and EDM. Bearing is measured with a Theodolite fitted with a compass. If the angles measured by Deflection Angle Method, the traverse is called Deflection Angle Traverse and vice-versa for Angles to the Right Traverse and so on.

Methods of Computing the Area in Traversing:

(i) Graphical Method (ii) Trigonometric Method

(i) Graphical Method: It is a simple method that is very useful for rough estimation of the area of traverse surveyed.

Graphical Method:

In this method, traverse is plotted on a graph paper according to a suitable scale and area of the individual triangles is calculated graphically (as you have studied in Middle Classes).

(ii) Trigonometric Method: Above method + use of trigonometric theorems / relationship:

Area of ABD = ½ a.d.Sina

Area of BCD = ½ b.c.Sinb

Area of the polygon (i.e. traverse) ABCD = Area of ABD + Area of BCD = ½ a.d.Sina + ½ b.c.Sinb

Balancing the Angles

it is necessary to have a closed traverse (you cannot calculate the area of an open traverse).

The interior angles of a closed traverse must total to (n - 2)(180°);

In surveying, the total angle should not vary from the actual correct value by more than the square root

If an error of 1’ is made, you can correct one angle by 1’.

If an error of 2’ is made, you can correct two angles by 1’ each.

you can correct each angle by 3’/12 = (3” x 60)/12 = 15”.

Closing Error and its Adjustment:

When a closed traverse is plotted on the paper (to a suitable scale) according to the field usually do not coincide (meet) each other due to the error in the measurement of angles and distances. This error is called Closing Error.

Error (usually Natural Error) in measuring the angles occur mainly due to Local Attraction.

Local Attraction: It is an error crept in and affects the normal performance of the magnetic devices while surveying, chiefly caused by the effect of earth’s magnetic field on the magnetic materials (materials which are susceptible to the magnetic field. Magnetic Materials: Diamagnetic, Ferromagnetic and Paramagnetic) of the angle measuring instruments.

Adjustment of Closing Error: (a) Graphical Method: The polygon ABCDEA’ represents an unequal closed traverse having a Closing Error equal to AA’ = e. The Closing Error is distributed linearly to all the sides of the said polygon in proportion to their lengths, by Graphical Method. Let AB’, B’C’, C’D’, D’E’ and E’A’ be the lengths of the (error bearing) polygon plotted on a paper according to the field measurements and to a suitable scale. Now the said sides of this polygon are to be corrected in order to eliminate the error e, so that the starting point and ending point must meet each other.

Adjustment of Closing Error by Graphical Method: The said lengths AB’, B’C’, C’D’, D’E’ and E’A’ are to be corrected to the lengths AB, BC, CD, DE and EA, corrected according to the same scale as that of the (error bearing) plotted polygon.

The ordinate A’a is equal to the closing error AA’ = e.

Join Aa in form of an inclined (straight) line making some angle with the horizontal line AA’ (the far points of these two lines are A’ and a and they are apart from each other by the distance e) and either drop perpendiculars from the points a, e, d, c and b or draw the parallel lines from these points onto the line AA’ (we draw the lines parallel to the line AA’ located on the polygon).

Now cut EE’ (of polygon) = E’e (on thin triangle), DD’ = D’d, CC’ = C’c and BB’ = B’b and join AE, ED, DC, CB and BA (to form a corrected polygon). The polygon so obtained would represent the Adjusted Traverse. This method of Adjusting the Closing Error is known as Check-in and Closing Error in Traversing by Plotting Method.

(b) Axis Method:

This method is similar to the Graphical Method; difference between the said two methods is that the correction is applied to the lengths only. General shape and direction of the lines of the original traverse (bearing the errors) remain unchanged (straight lines known as Axes are drawn passing through

Adjustment of Closing Error by Axis Method:

the first and last non-coinciding points and made to meet a side of the

traverse (say at point P) and then all error bearing points are joined by their axes to the said point P on the side of the traverse; finally the error bearing lengths are adjusted proportionately).

(c) Bowditch Method:

A popular method for balancing the errors is called the “Bowditch Method” or “Compass Rule” – two names for the same method (named after a renowned American surveyor, dated:1807).

In “Bowditch Method”, it is assumed that:

1) Angles and distances have the same error i.e. in the same proportion

2) Errors are accidental (not intentional)

And 3) (i) Errors in linear measurements are directly proportional to the square root of length

_

i.e. ELin and (ii) Errors in angular measurements are inversely proportional to the square root of length

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i.e. EAng 1 /

l is the length of a line under consideration.