Theodolite in Surveying | Types of Theodolite | Parts of Theodolite | Uses of Theodolite

  

THEODOLITE

THEODOLITE - An instrument used for measuring horizontal and vertical angles accurately is known as theodolite.

 

Uses of Theodolite

·       Measurement of Horizontal and vertical angles.

·       Setting out lines and angles

·       Optical distance measurement

·       Plumbing tall building

·       Setting out of Railway curves

·       Locating the position of piers for Bridge etc.

·       Geographical position fixing from observation of sun and stars.

·       Alignment control in tunnel construction.

 

Types of Theodolite

Transit theodolites are categorised into 3 types:

1. Vernier theodolite

2. Optical Reading Theodolite

3. Digital Theodolite/Electronic Theodolite

 

Basically Transit Theodolite are those in which the telescope can revolve through a complete revolution about its Horizontal axis in vertical plane.

 

Components Of Transit Theodolite

Transit theodolite consists of the following parts:

1. Levelling Head

2. Lower Plate or Scale Plate

3. Upper Plate or Vernier Plate

4. The standard or A Frame

5. T-Frame or Index Bar.

6. Plate Levels

7. Telescope

 

Levelling Head - Levelling Head consists of upper tribrach and lower Tribrach. Upper tribrach has three arms, each arm carries a levelling screw for levelling the equipment. Lower tribrach has got a circular hole through which a plumb bob may be suspended for centering.

Three distinct functions of levelling head are:

I) To Support The Main Part Of The Instrument

Ii) To Attach The Theodolite To The Tripod

Iii) To Provide A Means For Levelling The Theodolite

 

Lower Plate (Scale Plate): Lower Plate which is attached to outer spindle, carries a horizontal graduated circle, it is graduated from 0-360. Each degree is further divided into 10 minutes or 20 minutes. Scale plate can be clamped to any position by a clamping screw and a corresponding slow motion screw. When the lower plate is tightened, the lower plate is fixed to the upper tribrach of the levelling head. The size of the Theodolite is determined by the size of the diameter of this lower plate.

 

Upper plate or Vernier Plate: Upper plate is attached to Inner spindle axis. Two verniers are screwed to the upper plats. It carries an upper clamp screw and tangent screw. On clamping the upper clamp and unclamping the lower clamp, the instrument may be rotated on its outer spindle without any relative motion between the two plates. On the other hand if lower clamp screw is tightened and upper clamp screw is unclamped, the instrument may be rotated about its inner spindle with a relative motion between the vernier and graduated scale of the lower plate. This property is utilised for measuring angles.

 

Plate Levels: Upper plates carries two plate levels placed at right angles to each other. One of the plate bubble is kept parallel to the trunion axis. Plate levels can be centred with the help of foot screws.

 

Telescope: Telescope is supported on the pivots of the trunion axis which affords its movement in the vertical plane.

 

IMPORTANT DEFINITIONS

Line of Collimation: the line which passes through the Intersection of the cross hairs of the eye piece and optical centre of the objective and its continuation is called as line of collimation. This is also known as line of sight.

Transiting: The process of turning the telescope in a vertical plane through 180 deg. about its horizontal axis is known as transiting.

Swing: the continuous motion of the telescope about the vertical axis in the horizontal plane is called swing. The swing may be in either direction i.e. Right swing or left swing.

Face left observation: When the vertical circle is on the left of the telescope at the time of observation, the observations are called face left observation.

Face right observation: When the vertical circle is on the right of the telescope at the time of observation.

 

TEMPORARY ADJUSTMENT OF THEODOLITE

1) Setting up the Theodolite over the station

2) Levelling up the theodolite

3) Elimination of the parallax

1. Setting up : Operation of setting up a theodolite includes:

a) centering the theodolite over the ground mark

b) approximate levelling with the help of tripod legs.

 

2. Levelling up of theodolite: The operation of making the vertical axis truly vertical is known as levelling of Theodolite.

i) Turn the horizontal plate until the longitudinal axis of the plate level is approximately parallel to a line joining any two levelling screws.

ii) Bring the bubble to the centre of its run by turning both foot screws simultaneously in opposite directions either inwards or outwards. The movement of the left thumb indicates the direction of movement of bubble.

iii) Turn the instrument through 1800 in azimuth.

iv) Note the position of the bubble. If it occupies a different position, move it by means of the same two foot screws to the approx. mean of the two positions.

v) Turn the theodolite through 90 in azimuth so that the plate level becomes perpendicular to the previous position.

vi) With the help of third-floor screw, move the bubble to the approx. mean position already indicated.

vii) Repeat the process until the bubble, retains the same position for every setting of the instrument.

 

3. Elimination of Parallax: Elimination of parallax may be done by focusing the eyepiece for the distinct vision of crosshairs and focusing the objective to bring the image of the object in the plane of crosshairs.

 

How To Measure Horizontal Angle Using Theodolite

Procedure : to measure a Horizontal Angle ABC between BA & BC the following procedure is followed.

1.Set up, Centre and level the theodolite over the ground point B.

2. Loosen the upper plate, set the vernier to read zero and clamp the upper clamp.

3. Loosen the lower plate and swing the telescope until the left point A is sighted. Tighten the lower clamp. Accurate bisection of the arrow held on the Point A is done by using the lower tangent screw. Read both the vernier and take the mean of the reading.

4. Unclamp the upper plate and swing the telescope in clockwise direction until point C is brought in the field of view. Tighten the upper clamp and bisect the mark of C accurately, using the upper clamp tangent screw.

5. Read both the verniers and take the mean of readings. The difference of the means of the reading to C to A is the required angle ABC.

6. Change the face of the instrument and repeat the show procedure, the measure of the angle is again obtained by taking the difference of the means of the readings to C&A on face right.

7. The mean of the two measures of the angle ABC on two faces is the required value of the angle ABC.

 

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