The vernier theodolite is also known as a transit. In a transit theodolite or simply transit the telescope can be rotated in a vertical plane. Earlier versions of theodolites were of the non-transit type and are obsolete now. Only the transit theodolite will be discussed here.
Two different views of a vernier theodolite are shown in Figs 6.1(a) and (b). The instrument details vary with different manufacturers but the essential parts remain the same. The main parts of a theodolite are the following.
(a) Sectional view (b) Another view
1. Trivet 7. Circle plate 13. Vertical circle clamp
2. Tribrach 8. A-frame 14. Telescope
3. Foot screw 9. Vertical circle clamp arm 15. Plate bubble
4. Inner axis 10. Vernier frame 16. Upper clamp
5. Outer axis 11. Vertical circle 17. Lower clamp
6. Vernier plate 12. Altitude bubble 18. Tripod head
Fig. 6.1 Vernier theodolite
Levelling head The levelling head is the base of the instrument. It has the provi-sion to attach the instrument to a tripod stand while in use and attach a plumb bob along the vertical axis of the instrument. The levelling head essentially consists of two triangular plates kept a distance apart by levelling screws. The upper plate of the levelling head, also known as the tribrach, has three arms, each with a foot screw. Instruments with four foot screws for levelling are also available. In terms of wear and tear, the three-foot-screw instrument is preferable. The lower plate, also known as the trivet, has a central hole and a hook to which a plumb bob can be attached. In modern instruments, the base plate of the levelling head has two plates which can move relative to each other. This allows a slight movement of the level-ling head relative to the tripod. This is called a shifting head and helps in centring the instrument over the station quickly. The functions of the levelling head are to support the upper part of the instrument, attach the theodolite to a tripod, attach a plumb bob, and help in levelling the instrument with the foot screws.
Lower plate The lower plate, also known as the circle plate, is an annular,
Horizontal plate with a beveled graduated edge fixed to the upper end of a hollow cylindrical part. The graduations are provided all around, from 0 o to 360 o , in the clockwise direction. The graduations are in degrees divided into three parts so that each division equals 20¢. An axis through the centre of the plate is known as the outer axis or the centre. Horizontal angles are measured with this plate. The diameter of the lower plate is sometimes used to indicate the size of or designate the instrument; for example, a 100-mm theodolite.
Upper plate The upper plate is also a horizontal plate of a smaller diameter attached to a solid, vertical spindle. The bevelled edge of the horizontal part carries two verniers on diametrically opposite parts of its circumference. These verniers are generally designated A and B. They are used to read fractions of the horizontal circle plate graduations. The centre of the plate or the spindle is known as the inner axis or centre. The upper and lower plates are enclosed in a metal cover to prevent dust accumulation. The cover plate has two glass windows longer than the vernier length for the purpose of reading. Attached to the cover plate is a metal arm hinged to the centre carrying two magnifying glasses at its ends. The magnify-ing glasses are used to read the graduations clearly.
Two axes or centres The inner axis as mentioned earlier is the axis of the conical spindle attached to the upper or vernier plate. The outer axis is the centre of the hollow cylindrical part attached to the lower or circle plate. These two axes coin-cide and form the vertical axis of the instrument, which is one of the fundamental lines of the theodolite.
Clamps and tangent screws There are two clamps
and associated tangent or slow-motion screws with the plates. The clamp
screws facilitate the motion of the instrument in a horizontal plane. The lower
clamp screw locks or releases the lower plate. When this screw is unlocked, the
lower and upper plates move together. The associated lower tangent screw allows
small motion of the plates in the locked position. The upper clamp screw locks
or releases the upper vernier plate. When this clamp is released (with the
lower clamp locked), the lower plate does not move
but the vernier plate moves with the instrument. This causes a change in the
reading. The upper tangent screw allows for a small motion of the vernier plate
for fine adjustments. When both the clamps are locked, the instrument cannot
move in the horizontal plane. The construction of the clamp and tangent screws
is shown in Fig. 6.2.
Plate level The plate level is a spirit level with a bubble and graduations on the glass cover. A single level or two levels fixed in perpendicular directions may be provided. The spirit level can be adjusted with the foot screw of the levelling head. The bubble of the spirit level can be moved with the foot screws of the levelling head, which is a very fundamental adjustment required for using the theodolite. A small circular bubble may be provided for rough adjustment before levelling.
Index frame The index frame, also known as a T-frame or vernier frame, is a T-shaped metal frame. The horizontal arm carries at its ends two verniers, which remain fixed in front of the vertical circle. These verniers are generally designedC and D. The vertical leg of the T-frame, known as the clipping arm, has clipping screws with which the frame can be titled. The altitude level is generally fixed on top of this frame. When the telescope is rotated in a vertical plane, the vertical circle moves and vertical angles are measured on the vertical circle with the help of these verniers.
Standard or A-frame Two standards in the shape of the letter A are attached to the upper plate. The horizontal axis of the instrument is attached to these stan-dards. The clipping arm of the index frame and the arm of the vertical circle clamp are also attached to the A-frame. The A-frame supports the telescope and the vertical circle.
Telescope The telescope is a vital part of the instrument. It enables one to see stations that are at great distances. The essential parts of a telescope are the eye-piece, diaphragm with cross hairs, object lens, and arrangements to focus the tele-scope. A focusing knob is provided on the side of the telescope. Earlier, external focusing telescopes were used. Today, only internal focusing telescopes are used in theodolites. These reduce the length of the telescope. The telescope may carry a spirit level on top in some instruments.
Vertical circle The vertical circle is a circular plate supported on the trunnion or horizontal axis of the instrument between the A-frames. The vertical circle has a bevelled edge on which graduations are marked. The graduations are generally quadrantal, 0 o -90 o in the four quadrants as shown in Fig. 6.2. The full circle system of graduations can also be seen in some instruments. The vertical circle moves with the telescope when it is rotated in a vertical plane. A metal cover is provided to protect the circle and the verniers from dust. Two magnifying glasses on metal arms are provided to read the circle and verniers. The cover has glass or plastic windows on which the magnifiers can be moved.
Vertical circle clamp and tangent screw The vertical circle is provided with a clamp and tangent screw as in the case of the horizontal plate, Upon clamping the vertical circle, the telescope cannot be moved in a vertical plane. The tangent screw allows for a slow, small motion of the vertical circle.
Altitude level is used for levelling, particularly when taking vertical angle observations.
A circular or trough magnetic compass is generally fitted to the odolite for measuring the magnetic bearing of lines. It is fitted on the cover of the horizontal plates. Two plates with graduations are provided in the compass box for ensuring that the needle ends are centred. The needle can be locked or released by a pin. When released, the telescope can be turned in azimuth to make the north end of the needle point to the north by making it read zero.
Tripod One accessory essential with the theodolite is the tripod on which it is mounted when it has to be used. The tripod head is screwed onto the base or the lower part of the levelling head. Its legs should be spread out for stability. The legs of the tripod are also used for rough levelling.
Plumb bob A heavy plumb bob on a good string with a hook at the end is required for centring the theodolite over a station. The plumb bob is fixed to the hook or other device projecting from the centre of the instrument in a central opening in the levelling head.
Main circle and vernier graduations In most of the instruments, the vernier enables readings up to 20? of the arc. This is made possible by marking the gradu-ations on the circle and the vernier suitably as follows. As shown in Fig. 6.2(b), the main circle is graduated into degrees and each degree is divided into three parts. Each main scale division thus represents 2¢. For the vernier, 59 main scale divisions are taken and divided into 60 parts. 59 main scale divisions form 59 ¥ 20¢. Therefore, each vernier scale division represents 59 ¥ 20/60 minutes. As you would have studied earlier, least count of the vernier = difference between a main scale division and a vernier scale division = main scale division - vernier scale division. Hence, in this case,
Least count = 20¢ - 59 ¥ 20/60 = 1/3 = (1/3) ¥ 60? = 20?
Thus the least count of the vernier in common theodolites is 20?.