UNIT-1
THEODOLITE SURVEY
- It is a precise instrument for measuring angles in the horizontal and vertical planes.
- It is an accurate method or instrument used for measuring horizontal and vertical angles.
- The line of sight of the instrument can be rotated 180 degrees in a vertical plane about its horizontal axis.
TYPES OF THEODOLITE :
Theodolites may be thoroughly portrayed into two sorts
1) Transit theodolite
2) Non-travel theodolite
Key takeaway
It can use in prolonging survey lines and finding elevations etc. It is also used for levelling.
Following are part of theodolite
(1)Telescope
- Telescope in theodolite is used for focusing at which object piece and eyepiece at other.
- It rotates about a horizontal axis in the vertical plane.
(2)Vertical circle
- It is fitted to the telescope and more simultaneously.
- Graduation is there in each quadrant from 0 to 90 degree.
(3)Index frame
- It is also known as vernier frame or t frame.
- It consists of two parts vertical and horizontal.
- The horizontal arm is used to measure Vertical angle.
- Vertical arm cups to lock telescope at a certain level.
(4)Standards
- They are the frames which support telescope and it is rotated about a vertical axis.
- It is also called as A-frame.
(5)Upper plate
- It gives support to standards.
- It consists of two verinerwith magnifiers which are arranged diagonally.
- Upper clamping screw concerning tension screw which helps to fix with the lower plate.
(6)Lower plate
- It contains a scale on which 0 to 360 reading are graduate.
- It is also called a scale plate.
- It is attached to the outer spindle.
- It consists of a lower clamping screw.
- If the lower clamp is loosened and upper is tightened both plates can rotate together.
- if the procedure is reversed that is lower clamp is tightened and upper loosened and then the upper plate is movable and the lower plate is fixed.
7) Levelling head
- It contains two parallel triangular plates known as tri batch plates.
- The upper path that is upper tribrach used to level upper plates and telescope with help of levelling screw.
- Lower part i.e. lower tribrach plate and is attached through tripod.
8) Shifting head
- It contains two parallel plates which are right angle to each other with one of them is parallel to the trunnion axis.
9) Tripod:-
- It just stands for theodolite which mounted over it.
- The levelled position is required for a tripod to give correct observations.
- Steel shoes are there at the end of the tripod leg.
- These legs hold the ground strongly
10) Plumb Bob
- It is a cone-shaped weight attached or tie with long thread.
- These are used for entering of the theodolite.
11) Magnetic compass:-
- It is in circular compass box in the centre of the upper plate and used for selecting meridian.
Key takeaway
Uses:
- Used for measurement of horizontal and vertical angles
- Forming panels.
- Meteorology.
- Measuring and laying out angles and straight lines.
- Plumbing a column or building corner.
- Navigating.
- Laying outbuilding corners and lines.
- Aligning wood-frame walls.
Key takeaway
Hence theodolite has wide use in the survey. It is used to measure horizontal and vertical angles.
Theodolite has two types of adjustments- temporary and permanent.
The temporary adjustments are the following: (a) setting up and centring,(b) levelling, (c) focusing the eyepiece, and (d) focusing the objective.
Setting Up and Centring
The following procedure is adopted for this operation.
1. Remove the theodolite from its box carefully and fix it on a tripod kept over the station where the instrument is to be set up. The
tripod legs should be well apart and telescope should be at a convenient height for sighting.
2. Tie a plumb bob onto the hook provided at the base. If there is no shifting head in the instrument, centre it by adjusting the tripod legs and shifting the instrument as a whole to bring the plumb bob over station mark.
3. To centre the plumb bob, shift the tripod legs radially ad circumferentially. Moving any leg radially shifts the plumb bob in direction of the leg. This does not affect the level status of the instrument. Moving any leg circumferentially does not appreciably shift plumb. However, this movement tilts the instrument and affects the level of plate bubbles. By moving the legs the plumb bob is brought over the station mark at the same time ensuring that the instrument is approximately level. This saves a lot of time for the next operation of levelling.
4. If the instrument has a shifting head with a clamp, first centre the instrument using legs. Make the final adjustment by loosening the clamp and shifting the head to bring the plumb bob over station mark. The starting step should be to first bring the plumb bob very close to the mark and then make the final adjustment using the legs or shifting head.
Levelling
After setting up and centring instrument, levelling is done. Levelling has to be done at every station the instrument is set up. By levelling the instrument, it is ensured that as the instrument is swung about the vertical axis, the horizontal plate moves in a horizontal plane. The instrument may have a three screw or a four screw levelling head. The levelling operations differ slightly in these two cases as detailed in the following sections. Most instruments have only one bubble tube, but some instruments have two bubble tubes set at right angles over plates.
Three-screw levelling head
When the theodolite has a three-screw levelling head the following procedure is selected.
1. Swing the theodolite and bring plate bubble parallel to any two of the foot screws. Centre the bubble by rotating the foot screws. To do this, hold foot screws by the unit and range each hand and rotate both either inwards or outwards [see Fig. 6.3(a)]. Also, note that the bubble moves in the direction of movement of the left thumb during this operation.
Once bubble traverses (or comes to the central position from the graduation of the tube), swing instrument and bring the bubble over third foot screw. In this position, the bubble tube is at right angles to the earlier position. Centre the bubble by rotating third foot screw alone.
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Fig-1. Three-foot-screw levelling head
3. Bring a plate bubble to its previous position by swinging the instrument back. Check whether the bubble traverses. If it does not traverse, bring the bubble to the centre using the two-foot screws as before.
4. Repeat the procedure till bubble traverses in both these positions.
5. Swing the instrument through 180 and check whether bubble traverses. The bubble should traverse in all positions if the instrument has been properly adjusted. If two plate bubbles are provided [see Fig. 6.3(b)], the procedure is the same except that swinging the instrument through 90 is not required. When one plate level is kept parallel to a pair of foot screws, the other plate level is over the third foot screw (in a perpendicular direction). The third foot screw is adjusted alternately by the same process using foot screws over which they are parallel.
Four-screw levelling head
When the theodolite has a four-screw levelling head, the following procedure is adopted.
1. After setting up and centring the theodolite, bring the plate level parallel to any one pair of diagonally opposite foot screws. Operate these foot screws to centre bubble (Fig. 6.4).
2. Swing the instrument to bring plate level parallel to the other pair of foot screws. Centre the bubble.
3. Swing it back to the previous position. Check whether the bubble traverses. If it does not, centre it with foot screws to which the level is parallel.
4. Swing it back, check the position of the bubble, and repeat the procedure.
5. Once the bubble traverses in the two orthogonal positions, swing it through 180o. The bubble should traverse in this position
If two plate levels are provided, the procedure is the same. Bring one plate level parallel to a pair of opposite foot screws. The other pair will be parallel to the remaining pair of foot screws. There is no need to swing the instrument. Bring the bubble to central position alternately and check in the other positions.
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Fig-2.Four-foot-screw levelling head
a) Horizontal angle:-
- Horizontal angle is used to find bearing and direction in control survey.
- It is also used for locating detail when mapping.
b) Vertical angle:-
-It is used to find out the height of points and to calculate slope connections.
Measurement of horizontal angle
1) To measure the angle of ABC, the instrument is set to B.
2) Upper clamp is loosened and the lower clamp is fixed.
3) Telescope is turned and vernier A is a set O and Vernier B to 180°.
4) Now lower clamp is loosened and tightened scope is pointed to A and bisect ranging rod at A.
5) Now lower clamp is tightened and lower tangent screw is turned to perfectly bisecting ranging rod at A.
6) Now upper clamp is loosened and a telescope is turned clockwise to bisect the C which is tightened at the upper clamp.
7) Now, vernier reading A and B is noted.
8) Vernier A result in angle directly and vernier B result reading by subtracting initial reading from final reading.
Suppose it is required to measure the angles AOB, BOC and COD. Then to measure these angles by repetition method:
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Fig-3. reiteration-method
i) Set up the instrument over station point O and level it accurately.
ii) Direct the telescope towards point A which is known as the referring object. Bisect it accurately and check the reading of vernier as 0 or 360 degrees. Loosen the lower clamp and turn the telescope clockwise to sight point B exactly. Read the verniers again and The mean reading will give the value of angle AOB.
iii) Similarly bisect C & D successively, read both verniers at each bisection, find the value of the angle BOC and COD.
iv) Finally close the horizon by sighting towards the referring object (point A).
v) The vernier A should now read 360degrees. If not note down the error. This error occurs due to slip etc.
vi) If the error is small, it is equally distributed among several angles. If large the readings should be discarded and a new set of readings be taken.
Repetition method
It is defined as a method in which angle is measured in a clockwise direction for any number of times is known as the repetition method.
Usually six times three repetition with face left and three with face right.
Procedure
- Set up instrument over Q and levelled with accurately with a telescope in normal position.
- Set up Vernier A to 3600. Loosen the lower clamp direct the telescope to left hand ranging rod at P. exactly by using the upper clamp and lower tangent.
- Vernier reading should be the same.
- Lose upper clamp and turn the telescope in a clockwise direction and bisect the right hand ranging rod at R exactly by using the upper clamp and upper slow-motion screw.
- Read both Vernier. Let mean reading be 3002’ the object of reading both Vernier to obtain an approximate value of the angle.
- Leaving Vernier unchanged unclamp the lower plate and turn the telescope in the clockwise direction with the ranging rowith early witLoseer clamp and lower tangent scarf heading ofthe Vernier should be same as before.
- Loose the upper clamp and turn the telescope in clockwise direction and bisect ranging rod at R exactly the by using upper clamp and upper tangent screw.
- The Vernier now read twice the value of angle
- By the levelling Vernier clamp at 60040’ measure the angle third time.
- Read both the Verniers. Read the final angle.
Measurement of vertical angle
- The zeros of Vernier are set to 0 degrees to vertical circle.
- The telescope is den clamped.
- Plate bubble is brought to centre with foot screw and altitude bubble is brought to the centre by clip screw.
- Telescope in bisect to point A and measure the angle of elevation.
- Finally reading of vernier is noted and the elevation angle is also noted.
- Now, the face of the instrument is changed and point A is bisected again and reading are noted.
- The telescope is lowered slowly and B is bisected to measure the angle of depression.
vertical angle is an angle made by two connected lines in the vertical plane, which is, between a low point and two higher points. Since these angles are in the vertical plane, the lines that form them will usually be lines of sight
There are two methods of prolonging a given line such as AB
(1) Foresight method
(2) Back Sight Method
(1)Fore Sight Method. As shown in the fig.
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Fig-4 ForeSight Method
i) Set up the theodolite on A and level it accurately. Bisect the point b correctly. Establish a point C at the line beyond B approximately by looking over the top of the telescope and accurately by sighting through a telescope.
ii) Shift the instrument to B, take a foresight at C and establish a point D inline beyond C.
iii) Repeat the process until last point Z is reached.
(2) Back Sight Method. As shown in the fig.
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i) Set up the instrument at B and level that accurately.
ii) Take a backsight at A.
iii) Tighten the upper and lower clamps,
transit the telescope and establish a point C at the line beyond B.
iv) Shift the theodolite to C, backsight on B transit the telescope and establish a point D at line beyond C. Repeat the process until the last point ( Z) is established.
Now if the instrument is in adjustment, the points A, B, C, D and Z will be in one line, which is straight but if it is not in adjustment i.e. line of collimation is not perpendicular to the horizontal axis, then C’, D’ and Z’ will not be in a straight line.
Double reversing Method:
When the line is to be prolonged with high precision or when the instrument is an imperfect adjustment, the process of double sighting or double reversing, is used. Suppose the line AB is to be prolonged to a point Z.
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Procedure: As shown below:
i) Set up the theodolite at B and level accurately.
ii) With the face of the instrument left, backsight on A and clamp both upper and lower motions.
iii) Transit the telescope and setpoint C1 ahead in line.
iv) Loosen the lower clamp, revolve the telescope in the horizontal plane and backsight at A . Bisect A exactly by using the lower clamp and its tangent screw. Now the face of the instrument is right.
v) Transit the telescope and establish a point C2 at line beside the point C1.
vi) The exact position of true point C
must be mid-way between C1 and C2.
vii) Measure C1 C2 and establish a point C exactly mid-way, which lies on the true prolongation of AB.
viii) Shift the instrument to C, double sight on B, establish the point D1 and D2 and locate the true point D as before.
ix) Continue the process until the last point Z is established.
Theodolite used for locating the points on the line, prolonging the survey lines, establishing the grades, calculating the difference in elevation, setting out the curves etc.
Reference:
1 Madhu, N, Sathikumar, R and Satheesh Gobi, Advanced Surveying: Total Station, GIS and Remote Sensing, Pearson India, 2006.
2 Manoj, K. Arora and Badjatia, Geomatics Engineering, Nem Chand & Bros, 2011.
3 Bhavikatti, S.S., Surveying and Levelling, Vol. I and II, I.K. International, 2010
4 Chandra, A.M., Higher Surveying, Third Edition, New Age International (P) Limited, 2002.
5 Anji Reddy, M., Remote sensing and Geographical information system, B.S.
