Theodolite traverse – Field work

What is theodolite? Theodolite parts, survey, Traversing

In this article, we show all information about theodolites like parts of the theodolite, survey of theodolite, and classification of theodolite, theodolite traversing, and the process of theodolite traversing.

What is theodolite?

An instrument used for measuring horizontal and vertical angle accurately is known as a theodolite.

It is used for prolongation of survey lines, finding the difference in elevation and setting out engineering works requiring higher precision, i.e. ranging the highway and railway curves, aligning the tunnels, etc.

What is theodolite

Classification of theodolites

       Transit theodolite

       non-transit theodolite

Transit theodolite

A theodolite is called a transit theodolite if is telescope can be transited, for example it can revolve through a complete revolution about its horizontal axis in a vertical plane.

non-transit theodolite

In non-transit theodolite its telescope cannot be transited, i,e, it cannot revolve through a complete revolution about its horizontal axis in a vertical plane.

Theodolite survey

The length of traverse legs measures directly on the ground by changing or taping according to the accuracy aimed at, and the nature of the ground.

The traverse angles, the angles, the angles between consecutive traverse legs are measured with a theodolite by setting up the instrument at each station in turn.

If the co-ordinated of one station and true bearing of the traverse leg connected to it is known, the coordinates of the next and all succeeding traverse station may be calculated with the following-

X-co-ordinate of B=X co-ordinate of A+l sin@

Y co-ordinate of B= Y co-ordinate of A+l cos@

Where l is the length of traverse leg AB and @ is the reduced bearing of the leg AB.

Theodolite surveying

Theodolite traverse

A traverse is a series of connected lines whose lengths and directions measures in the field. In a theodolite traverse, to directions measured with a theodolite.

A theodolite traverse in commonly used for providing a horizontal control system to determine the relative positions of the various points on the surface of the earth.

It especially uses for providing control for site surveys in urban areas where the triangulation is not feasible.

The equipment required for conducting a theodolite traverse will include a theodolite, a steel tape, two ranging poles, stakes, tacks, plumb bobs, chain pins, tripods, crayons, makers, an ax and a hammer.

The traverse may be an open traverse or a closed traverse. A closed traverse commonly uses in control surveys, construction surveys, property surveys, and topographic surveys.

Theodolite traversing

In this type of traversing, traverse legs measure by direct chaining on the ground a the traverse angles at every traverse station measures accurately with a theodolite.

The basic procedure for theodolite traversing is the same as that in any other method of traversing. First reconnaissance has to conducted with a sketch drawn the terrain using the approximate location of traverse station then the important details are to pick up, the intervisibility of station to check.

Theodolite traversing required station marking tools such as pegs. arrows, etc., a theodolite with its stand and steel tape.

Measure with a theodolite

The method of repetition used to measure traverse angles to a finer degree of accuracy than that achievable with the least count of the vernier fitted on the theodolite. In this method, an angle measure there or four times by keeping the vernier clamp when sighting at the back station. While swinging from froward station to back station, the upper plate is let loose and made free to rotate. Thus an angle reading mechanically adds as many times as the number of repetitions. The difference of the firest and the last reading five the integrate traverse angle and the average traverse angle then obtained by dividing the integrated angle by the number of repetitions.

Fieldwork during a theodolite traversing

·         Reconnaissance

·         Selection and marking of stations

·         Measurement fo traverse legs

·         Measurement of traverse angles

·         Booking of filed notes

·         Computation.

Reconnaissance

Reconnaissance is the preliminary inspection of the area to survey to have some idea of the terrain and the principal features of the ground. In reconnaissance, the surveyor thoroughly examines the ground and then decides upon the best possible arrangement of triangles.

In reconnaissance, the surveyor obtains the required information and data about the shape and extent of the area to survey. During reconnaissance, the surveyor generally makes an index sketch to show the principal features, such as buildings, roads, Dallas, boundaries. The positions of the stations and survey lines also mark. The direction in with the chain lines are to measure is mark by arrowheads.

Selection and Marking of Stations

Every traverse station selected keeping in view that consecutive stations are intervisible without much clearance. The traverse legs, as far as possible, kept of the same length to have a systematic error in angular measurements.

The closing error in angular measurement is, therefore divide equally to all traverse angles assuming all angles of equal weights.

As far as possible traverse stations mark on pakka points, i.e. distance stones, culverts, road crossing, etc. A precise description of each station should enter in teh field book giving the exact distance of the marks on easily recognizable points close by.

Description of traverse stations neatly written in the filed book enables the plane tables to find them at a later date.

Measurement of Traverse Legs

Distances between traverse stations measures directly by chaining which is a more reliable method except in rough ground. Each distance must measure independently by a 30-meter chain. Both chains testes regularly against standard chains.

The undermentioned points kept in mind while obtaining a distance-

·         Measurement starts and closes at the centers of the station marks.

·         Arrows put into the ground vertically.

·         Chains always laid straight and pull out their full-lengths.

·         Measurements are taken from the point where the chain pin enters the grounds.

·         In rocky grounds or cemented surface, the full chain length is marking by chalk or paint.

Measurement of Traverse Angles

The measurement of traverse angles may make by one of the following methods-

       Repetition method

       Reiteration method

       Practical method