Theodolite and its parts, Theodolite parts and functions, Theodolite function definitions

Different parts of theodolite given below:-

1. Telescope:-

• Telescope is an important part of theodolite

• It is fixed on spindle ( is known as horizontal axis or trunnion axis. 

• It may be internal focusing telescope or external focusing telescope. 

• In most of transit theodolite internal focusing telescope is provided. 

2. Vertical circle:-

• Vertical circle is a graduated arc which is attached with telescope and tissue with it. 

• Vertical circle is graduated continuously from 0 to 360 in clockwise direction or it is divided into four quadrants.

3. Index frame ( vernier frame) :-

• Index frame is a T- shaped frame consisting a vertical and horizontal bar(known as vernier arm or index arm).

• To read the vertical circle two verniers are fix at the vernier arm. 

4. Standards ( A- frame):-

• It is a A- framed structure,  two standards mounted on upper plate. 

• It supports the trunnion acids of telescope. 

• Also index frame and vertical circle are attached with it. 

5. Levelling head:-

• Levelling head consist of triangular plates parallaly, which is known as tribrach plates. 

• Upper tribrach having 3 arm and each having levelling screws.  

• Lower tribrach a circular whole in which plumb Bob suspended .

• Lower tribrach also known as foot plate.  

• Functions of levelling head:-

          ○To support the main play of instrument
          ○ To attached the theodolite to the tripod
          ○ To provide the mean for levelling the theodolite. 

6. Spindles:-

• Theodolite consist two spindles, inner spindles and outer spindles. 

• Inner spindles are solids and conical which fixed in the outer spindle which is hollow and conical toward ground. 

• Inner spindle carry the upper plate so it is also known as upper axis. 

• While outer spindle carry lower plate of theodolite so it is known as lower axis. 

• Both soundness having samne axis that is vertical axis of instrument. 

7. Lower plate ( Scale plate):-

• Lower plate consist a horizontal circle therefore it is also known as scale plate and it fixed with outer spindle and it also consist lower clamp screws. 

• When the lower clamp is tightened, the lower plate is fixed with upper tribrach of levelling head. 

• Size of theodolite determine by size of the scale plate. 

8. Upper plate ( Vernier plate):-

• Upper plate attached with inner arms. 

• It carries two vernier with magnifier.

• It supports the standards or A-frame.

• It consist clamp screw to fix it accurately to the lower plate. 

• When upper screw is clamped and lower screw is unclamped, instruments rotate on its outer spindle without any relative motion between plates.

• Whether,  upper screw is unclamped and lower screw is clamped,  the upper plate and instrument rotate on the inner spindle with relative motion between the vernier plate and scale plate. 

9. Plate level:-

• Plate level is attached with upper plate in no. Of 2 at right angle to each other. 

• It help to fix the vertical axis of instrument. 

• It can be centered by the help of levelling head. 

10. Tripod:-

• When theodolite is in use it supported by the tripod. 

• It consist of three legs, lower end of leg pointed with steel shoes. 

11. Plumb Bob:-

• Plumb Bob suspended from the hook which is attached with inner spindle. 

• It is usse to fix the instrument just over the station. 

12. Striding level:-

• Some theodolite fitted with striding level. 

• It is used to check horizontality of trunnion axis.