### DH1

Upendra

Generally three types of transit theodolite in the use.
1. Vernier theodolite 2. Microptic theodolite 3. Digital theodolite

First, we learn about Least Count.
least count of an instrument is the smallest unit reading read of the instrument.

1. Vernier theodolite:-
There are two circular plates considered on vernier theodolite.

A.  Lower plate:-Outer axis is attached with a lower plate and the lower plate is graduated from 0 to 360 in a clockwise direction. The lower plate is called the main scale .the lower plate is provided with a clamp or slow-motion screw. when the observation is taken then the lower plate is tightened with a lower clamp or tangent screw. In the above picture shown graduation of the lower circle are two types, one is big division and the other is small division. The value of one big division is1°because no. of big division in the lower plate is 360. The one big division or 1° is divided into three small division. Then the value of one small division on the main scale --->60'÷3=20' Or- --------------------------------->20' ×60''=1200''

B. Upper plate: The upper plate is called the vernier plate. It is attached with inner axis and A clamp and A tangent screw provided for fixing the vernier plate to the main scale. There are two vernier plates placed apart 180° on the upper plate with two magnifiers. In the above picture, the vernier plate is divided, 20 big division. The value of 1big division on the vernier plate is 1'.
The big division is divided into three small division then the value of 1 small division will be 1'÷3 or 60" ÷3=20".

Reading the above picture ----> First saw on main scale reading, The vernier indicator 0 is crossed between 9° 40' and 10 °.

Next, we saw vernier reading, The 14' of big division of the vernier scale is exact conside with the main scale. Then the vernier scale reading will be 14'0". Here is division of second is not Cons ide with the main scale.

Total reading of the above picture ----------> 9°40'+14'0"=9°54'0"

The same procedure does on vernier B.
True reading of vernier theodolite will be = Reding of vernier A + Reading of vernier B ÷2

Least count of the above picture--------------->Value of 1 small division of main scale ÷ total no. of division on the vernier scale
--> 20'×60" ÷60=20"
Then the least count of above picture of the instrument =20"

2. Microptic Theodolite:

Micrometre Scale
In the above picture (a) two type scale, one is the main scale and other is micrometre scale. The main scale show degree and minute. One degree is divided into 6 divisions, and the value of one division=60'÷6=10', on the main scale. In the micrometre scale, the one  minute is also divided into 6 division the value of one small division is 60'÷6 =10", In the above picture (a)  on the main scale index mark is position between 31°30' and 31°40',
For measuring fractional part of the main scale we turn the micrometre, thus moving the hairlines until the back division(30' division) is exactly midway between the hairlines as shown in figure (b) and read the drum.
In the drum or micrometre, the reading is 5'10" in figure (b). Then the total reading will be 31°30'+5'10" =31°35'10".

3.digital theodolite:- In the digital theodolite available a display panel.
This give direct reading in digital mode.