Theodolite Traversing - lecture notes PDF

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Theodolite Traversing In this method of traverse, theodolite is commonly used for providing horizontal control systems. Measurement of theodolite traverse lengths Depending on accuracy required, • Chaining. • Taping. • Tacheometry or, • EDM equipment. For greater accuracy, lengths are measured in both directions and average value is taken. Measurement of theodolite traverse angles • Interior angles, • Deflection angles, • Direct angles • Azimuth angles or • Magnetic bearings. Theodolite traversing methods Any one of the following methods may be used for fixing direction in theodolite traversing: 1. Loose needle method In this method, a theodolite fitted with a magnetic compass is used to determine the bearings of the lines. From bearings included angles are determined and checked for theoretical sum of interior angles. This method is rarely used as it is prone to local attraction. 2. Fast needle method In this method, the magnetic bearing of only the first line is measured. The magnetic bearing of all other lines are determined indirectly. This method is more accurate than the loose needle method and is generally preferred. There are three methods of traversing by fast needle method: a) Direct method with transiting • • • • • • • •

Instrument is set up at the starting station A and the rearing is set to zero. The telescope is brought in magnetic meridian using lower plate clamp. Unclamp the upper plate and the telescope is directed towards B and bearing of the line AB is taken. Instrument is shifted to B with both the clamp in tightened position. Set up the instrument at B, Unclamp lower clamp, direct the telescope to A. Plunge the telescope, the telescope is in the line of sight of AB. Release upper clamp, telescope is directed towards C. Take the reading which is the bearing of line BC. Shift the instrument to the next station and repeat the above steps

b) Direct method without transiting • Follow the steps described in the first paragraph of the direct method with transiting. • Shift the instrument to B, without plunging the telescope, direct the telescope to sight C take reading. • Add 180º if the reading measured is less 180º and subtract 180º if the measured reading is more than 180º c) Back bearing method • Follow the steps described in the first paragraph of the direct method with transiting. • Shift the instrument to station B. • Calculate the back bearing of the AB • Set the vernier to the back bearing of AB, tighten upper clamp, using lower clamp bisect A. • Release upper clamp, rotate telescope in clockwise direction and bisect C, the reading is fore bearing of BC. Comparison Direct method with transiting is quite easy, but direct method without transiting gives the best results even when the instrument is not in perfect adjustment. Back bearing method is seldom used. 3. Included angle method • Suitable for closed traverse. • Set up instrument at A, take a fore bearingof AB • Set vernier A zero and bisect the last station, say E. • Tighten lower clamp and loose upper clamp; turn the telescope in clockwise direction to sight B. measure included angle at A. • Measure the length of the closed loop and record it. • Check the measured angles to theoretical sum of the included angles. • Angles measured are interior angles if traverse is run in anticlockwise direction and theoretical sum is equal to (2n -4) x 90º • Angles measured are exterior angles if traverse is run in clockwise direction and theoretical sum is equal to (2n+4) x 90º.

4. Direct angle method Similar to included angle method ad used for open traverse or closed traverse. Rotation of telescope for measuring direct angle at each station should be clockwise. 5. Deflection angle method • This method is suitable for open traverse where only a few details are located as the traverse is run. • At station A, take fore bearing of the line AB. • Shift instrument to B, Set vernier A zero and bisect A • Tighten lower clamp, transit the telescope and loose the upper clamp, turn the telescope either left or right to sight the next station C. • Record the angle with deflection direction. • Similarly measure deflection angles at other traverse stations. 6. Azimuth Method (Fig 3) The angle between a line and the meridian measured in clockwise direction, usually from the north of the meridian is called azimuth of the line. Azimuths may be true, magnetic or assumed, depending upon the reference meridian adopted. Azimuth and bearing are similar and azimuth is mostly used in geodetic and astronomical surveying, whereas the term bearing is commonly used in plane surveying. In this method, at each traverse station, the back azimuth of the preceding line and the azimuth of the forward line are measured using a transit. This method is used extensively for topographic and other surveys.

Theodolite traversing phases • Reconnaissance: Preliminary field inspection of the entire area • Selection of traverse stations: number of stations should be minimum, stations should be intervisible, and selected on firm and level ground etc. • Marking of stations: should be permanent in nature as far as possible can be used in future if required. • Measuring linear and angular measurements. • Locating details: As per field conditions, adopting any of the methods of principle of surveying, locate details. • Plotting and adjusting closing error....