Parts  of a Vernier Theodolite

Given figure shows a sectional view of a typical vernier theodolite and plate shows photograph of such theodolite. Major parts of such a theodolite are following:

1. Telescope: A telescope is mounted on a horizontal axis (trunnian axis) therefore it can rotate in vertical plane. Its length varies in the range 100 mm 175 mm and its diameter is 38 mm at objective end. Its functions are to provide a line of sight.

1. Telescope               9.   Foot crews

2. Trunnian axis          10. Levelling head

3. Vernier frame         11. Tripod head

4. Standards or A-Frame         12. Tripod

5. Upper plate            13. Plumb bob

6. Lower plate            14. Altitude level

7. Vernier                  15. Focussing screw

8. Plate level              16. Vertical circle

                        Sectional view of a transit theodolite

                                 Plate Theodolite

2. Vertical Circle: A vertical circle graduated up to an accuracy of 20′ is connected rigidly to the telescope and therefore moves with it when the telescope is rotated in vertical plane.

The graduations are in quadrantal system, 0-0 line being

3. Vernier Frame: It is a T-shaped frame which consisting of a vertical arm and a horizontal arm. screws the vertical frame with the help of the climping and therefore the telescope can be clamped at wanted angle. Vertical frame is also called as T-frame or index frame.

The vernier arm is called as index arm. At the ends it carries verniers C and D so as to read graduations on vertical circle. With glass magnifiers they are provided. Altitude bubble tube is fitted over the horizontal arm.

4. Standards or A-Frame: The frames which supporting telescope are in the form of English letter 'A'. This frame permits telescope to rotate on its trunnian axis in vertical frame. T-frame & the clamps are also fixed to this frame.

5. Upper Plate: on its top surface Upper plate supports standards. On lower side it is associated to inner spindle which rotates in the outer spindle of lower plate. Upper plate can be clamped to lower plate by using upper clamp. It is possible to give slight relative motion between the two plates, using tangent screws, even after clamping. 2 diametrically opposite verniers A and B fixed to upper plate help in reading horizontal circle graduations. They are provided along magnifying glasses.

6. Lower Plate: The lower plate which is attached to the outer spindle carries a graduated circle at its bevelled edge. Graduations are up to an accurateness of 20′. It may be clamped at any desired position by using lower clamps. If upper clamp is locked & lower one is loosened the 2 plates rotate together. Upper plate alone rotates, if the upper clamp is loosened and lower clamp locked. This mechanism is used in measuring horizontal angle.

7. Plate Level: on the upper plate one or two plate level tubes are mounted. They will be at right angles to each other one of them being parallel to trunnion axis if the 2 level tubes are provided. These levels are helpful in making the vertical axis of the instrument truly vertical.

8. Levelling Head: It consists of 2 parallel triangular plates which are known as tribratch plates. With 3 levelling screws the upper tribratch plate is provided which each one carried by an arm of tribratch plate. Screws the levelling of upper plate by operating and therefore telescope can be ensured. Lower tribratch can be fitted into a tripod head.

9. Tripod: always Theodolite is used by mounting it on a tripod. The legs of tripod can be solid or framed. To get good grip with the ground at the lower end the legs are provided with steel shoes. With external screw the top of tripod is provided to which the lower tribratch plate can be screwed. When not in use tripod head can be protected with a steel cap which is provided for this purpose.

10. Plumb Bob: A hook is provided at the middle of lower tribratch plate from which a plumb bob may be suspended. It facilitates accurate centring of the theodolite on a station.

11. Shifting Head: It is provided below the lower plate. One plate slides over another over small area of approximate 10 mm radius in this. The 2 plates may be tightened in the desired position. It facilitates exact centring of the instruments.

12. Magnetic Compass: In some theodolites on one of the strands a magnetic compass is fixed. It is useful if readings are to be recorded with magnetic north as meridian.