Transit Theodolite | Instruments | Surveying

After reading this article you will learn about:- 1. Description of a Transit Vernier Theodolite 2. Terms Used While Manipulating a Transit Theodolite 3. Fundamental Lines of a Transit Theodolite 4. Adjustments of a Theodolite.

Description of a Transit Vernier Theodolite:

[Figs. 9.1. a&b]:

A transit vernier theodolite essentially consists of the following parts:

1. Levelling Head:

This supports the main working parts of the instrument and screws on to a tripod. It comprises of two parts:

(i) Tribrach and trivet stage fitted with levelling screws, and

(ii) Centre shifting arrangement for centering the instrument quickly and accurately.

2. Lower Circular Plate:

This carries the circular scale graduated from 0° to 360° to degrees and half degrees or degrees and third of a degree, and a tapered spindle which works in the outer conical bearing. To the spindle is fitted a well ribbed bracket carrying on opposite side the slow motion and clamping screws for upper plate and lower plate.

3. Upper Plate:

The centre of the vertical spindle of the lower plate is bored to form a bearing for another vertical spindle which carries the upper circular horizontal plate. The upper plate can be rotated relative to the lower plate about this spindle as axis. It carries two verniers marked A and B, which are used for taking readings accurately upto 20″ on the lower graduated circle. This plate also carries a level tube and two vertical standards for supporting telescope, vertical circle and detachable compass.

The compass may be:

(i) The circular box compass,

(ii) The trough compass or

(iii) The tubular compass.

4. Telescope:

The telescope of theodolite may be:

(i) External focussing, and

(ii) Internal focussing.

The first type is used in older type of theodolites, while the later is used in modern instruments. It is mounted near its centre on a horizontal axis at right angles to the main longitudinal axis of the telescope.

5. Vertical Circle:

The vertical circle is rigidly fixed to the horizontal axis of the telescope and moves with it. It is silvered and is usually divided into four quadrants. The graduations in each quadrant are numbered from 0° to 90° in opposite directions from the two zeros placed at ends of the horizontal diameter of the vertical circle so that the line joining the zeros is parallel to the line of collimation of the telescope when it is horizontal.

The sub-divisions on the vertical circle are similar to those of horizontal circle. A clamp and fine motion tangent screws are provided for the vertical circle.

6. T-Frame or Index Bar:

It is T-shaped and is centered on the horizontal axis on the telescope in front of the vertical circle. The two verniers C and D are provided on it at the ends of the horizontal arms called the index arm. A vertical leg, known as clipping arm is provided with a fork and two clipping screws at its lower extremity. The index and clipping arms together are known as T-frame. At the top of this frame is attached a bubble tube which is called the altitude bubble tube.

7. Plumb-Bob:

A plumb-bob is suspended from the hook fitted to the bottom of the vertical axis for centering the instrument exactly over a station-point.

8. Tripod Stand:

The theodolite is supported on a tripod when in use.

Terms Used While Manipulating a Transit Theodolite:

1. Centering:

It means setting the theodolite exactly over an instrument-station so that its vertical axis lies immediately above the station mark. It can be done by means of plumb-bob suspended from a small hook attached to the vertical axis of the theodolite.

The centre shifting arrangement if provided with the instrument helps in easy and rapid performance of the centering.

2. Transiting:

It is also known as plunging or reversing. It is the process of turning the telescope about its horizontal axis thought 180° in the vertical plane thus bringing it upside down and making it point exactly in opposite direction.

3. Swinging the Telescope:

It means turning the telescope about its vertical axis in the horizontal plane. A swing is called right or left according as the telescope is rotated clockwise or counter clockwise.

4. Face Left:

If the vertical circle of the instrument is on the left of the observer while taking a reading, the position is called the face left and the observation taken on the horizontal or the vertical circle in this position, is known as the face left observation.

5. Face Right:

If the vertical circle of the instrument is on the right of the observer while taking a reading, the position is called the face light, and the observation taken on the horizontal or the vertical circle in this position, is known as the face right observation.

6. Changing Face:

It is the operation of bringing the vertical circle to the right of the observer, if originally it is to the left, and vice- versa. It is done in two steps: firstly revolve the telescope through 180° in a vertical plane and then rotate it through 180° in the horizontal plane i.e. first transit the telescope and then swing it through 180°.

7. Line of Collimation. (Fig. 7.9):

It is also known as the line of sight. It is the imaginary line joining the intersection of the crosshairs of the diagram to the optical centre of the object-glass and in its continuation.

8. Axis of the Telescope, (Fig. 7.10):

It is also an imaginary line joining the optical centre of object-glass to centre of the eye-piece.

9. Axis of the Level Tube:

It is also called the bubble line. It is a straight line tangential to the longitudinal curve of the level tube at the centre of the tube. It is horizontal when the bubble is central.

10. Vertical Axis:

It is the axis about which the telescope can be rotated in the horizontal plane.

11. Horizontal Axis:

It is also called the trunion axis or the transverse axis. It is the axis about which the telescope can be revolved in the vertical plane.

Fundamental Lines of a Transit Theodolite:

These are:

(i) The vertical axis.

(ii) The axis of the plate levels.

(iii) The axis of telescope.

(iv) The line of collimation.

(v) The horizontal axis.

(vi) The axis of the altitude bubble.

Adjustments of a Theodolite:

The adjustments of a theodolite are of two kinds:

1. Permanent Adjustments.

2. Temporary Adjustments.

1. Permanent Adjustments:

The permanent adjustments are made to establish the fixed relationships between the fundamental lines of the instrument, and once made, they last for a long time. They are essential for the accuracy of observations.

The permanent adjustments in case of transit theodolite are:

(i) Adjustment of the Horizontal Plate Levels:

The axis of the plate- levels must be perpendicular to the vertical axis.

(ii) Collimation Adjustment:

The line of collimation should coincide with the axis of the telescope and the axis of the objective, slide, and should be at right angles to the horizontal axis.

(iii) Horizontal Axis Adjustment:

The horizontal axis must be perpendicular to the vertical axis.

(iv) Adjustment of the Telescope Level or the Altitude level:

The axis of the telescope level or the altitude level must he parallel to the line of collimation.

(v) Vertical Circle Index Adjustment:

The vertical circle vernier must read zero when the line of collimation is horizontal.

2. Temporary Adjustments:

The temporary adjustments are made at each set up of the instrument before starting taking observations with the instrument. There are three temporary adjustments of a theodolite.

(i) Centering.

(ii) Levelling.

(iii) Focussing.

(i) Centering:

Centering means bringing the vertical axis of the theodolite immediately over a station mark. The station mark should be represented by a well-defined point such as end of a nail driven on the top of a peg or the intersection points of a cross marked at the surface below the instrument etc.

To do this:

(a) Attach the string of the plumb-bob to the hook under the vertical axis of the instrument.

(b) Place the instrument over the station by spreading the legs well apart so that the telescope is at a convenient height, the plumb-bob is approximately over the station mark, and the levelling head is approximately levelled.

(c) Lift the instrument bodily without disturbing the relative positions of the legs and move it until the plumb-bob hangs within 1 cm horizontally of station mark.

(d) Unclamp the centre-shifting arrangement and move the instrument until the plumb-bob is exactly over the station-mark. The pointer end of the plumb-bob should hang within 3 mm vertically above the station mark. Then clamp the shifting head.

Note:

On a hill side, place two legs down-hill and ane leg-up hill. This will ensure greater stability.

(ii) Levelling:

Having centered and approximately levelled the instrument, it is accurately levelled with reference to the plate levels by means of foot-screws so that the vertical axis is made truly vertical. To level the instrument,

(a) Loosen all clamps and turn the instrument about either of its axis until the longer plate level is parallel to any pair of foot-screws, the other plate level will then be parallel to the line joining the third foot-screw and the mid-point of the line joining the first pair.

(b) Bring the long bubble to the centre of its run by turning both screws equally, either both inwards or both outwards.

(c) Similarly, bring the other bubble to the centre of its run by turning only the third foot-screw.

(d) Repeat this until both the bubbles are exactly centered.

Now rotate the instrument about the vertical axis through a completes revolution. Each bubble will now remain central provided the plate levels are in correct adjustment. The vertical axis is thus made truly vertical.

If the vertical angles are to be measured, the instrument should be levelled with reference to the altitude level fixed on the index arm.

To do this:

(a) First level the instrument by plate levels. Then turn the telescope so that the altitude level is parallel to the line joining a pair of foot-screws and bring the bubble to the centre of its run by means of these screws.

(b) Tum the telescope through 90° in the horizontal plane and make the bubble central by the third foot-screw.

(c) Repeat this until the bubble remains central in these two positions.

(d) Bring the altitude level over the third foot-screw and swing the telescope through 180°. If now the bubble does not remain central, correct half its deviation by clip screw and the other half by the third foot-screw.

Swing the telescope through 90° so that it is again parallel to the two foot- screws and then make the bubble central by means of these screws. The bubble should now remain central for all positions of the telescope. If not repeat the whole process until the bubble remains central when the telescope is rotated about the vertical axis.

Note:

When the levelling has been completed, it is well to look at the plumb-bob to see that the levelling has not disturbed the centering.

(iii) Focussing:

This is done in two steps viz:

(a) Focussing of the eye-piece for distinct vision of the cross-hairs at diagram, and

(b) Focussing the object-glass for bringing the image of the object into the plane of the diagram.

(a) Focussing the Eye-Piece:

Point the telescope to the sky or hold a piece of white paper in front of the telescope. Move the eye-piece in and out until a distinct and sharp black image of the cross-hairs is seen.

(b) Focussing the Object-Glass:

Direct the telescope towards the object and turn the focussing screw until a clear and sharp image of the object is obtained. It may be noted that parallax is completely eliminated if there is no movement of the image of the object when the eye is moved up and down.