Theodolite: - It is an instrument used for
measuring horizontal and vertical angles in surveying. This is very important
instrument in surveying and can be used in large survey works.
There
are basically two types of theodolite:-
Transit
theodolite: - Transit theodolite is one which telescope revolve through 180° in a vertical plane about horizontal axis.
(It means revolving in direction of Y axis) Refer below image.
Non-Transit
theodolite: - When its telescope cannot be rotated through 180° in
a vertical plane about horizontal axis. These types are obsolete in these days.
These kind of theodolite can be classified in two types:-
- Vernier
theodolite: - In this theodolite vernier are used for taking the readings.
Least count of theodolite is 20 second
- Optical
theodolite:- It is fitted with optical system by which we can read both
horizontal and vertical angles. Least count of these theodolite is 1 second.
Note:-
Size of theodolite is measured from the size of its lower plate.
MAINS PARTS OF THEODOLITE:-
1) Telescope:
- It is used for sighting the object and mounted on the trunnion axis or
horizontal axis or horizontal spindle. As you can see in the above picture item
number 1(telescope) is mounted with item number 2(Trunnion axis).
- This telescope can be rotate in vertical plane about horizontal axis for sighting the object.
- Telescope
is majorly internal focusing type
- As
we can see in telescope image an additional double concave lens is fitted inside
the small tube which can move forward and backward between diaphragm and
objective with the help of focusing screw.
- By
moving the focusing lens, the image of the object formed by the objective is
brought in the plane of cross hairs of the diaphragm.
- Internal
focusing type prevent any dust to enter into the telescope.
2) Vernier
Frame: - It is also called Index frame or T frame. It consists of a vertical
leg called clipping arm. And a horizontal
bar called index frame. As we can see in the below image that at the two end of
index frame, Vernier C and Vernier D are fitted. Its vertical leg is called T frame
provided with fork and screw at its lower end.
3) Vertical
clamp screw: - It is used to clamp the vertical circle and telescope at any
desired vertical angle and rotation of telescope about horizontal axis is
prevented. We can use vertical tangent screw after tightening this clamp so
that small rotation can be given for proper sighting.
4) Altitude
bubble: - Altitude bubble is attached to index arm. It is used to make the
index arm truly horizontal during temporary adjustment. Altitude bubble is more
sensitive than plate bubble because while we are taking vertical angle,
altitude bubble will be at center
5) Upper
plate: - It is also called lower plate it supports the standard at its upper
surface At
its lower face, upper plate is attached to vertical spindle known as the inner
spindle .Upper
plate can be clamped to the lower plate with the use of upper clamp screw
5) Lower
plate: - It is also called main scale plate or horizontal circle. It is mounted
on the hollow tapered spindle or outer spindle. Size of theodolite id governed by
the size of lower plate
The
lower plate is graduated in degrees from 0 to 360 with least count is 20’.
These graduation increase in clockwise direction. The horizontal circle reading
are taken by means of Vernier A and B. The least count of Vernier is 20”
When
Both upper and lower clamp screw clamped then there will be no movement in the
theodolite.
When
lower clamp screw is tightened but upper clamp screw is lose then instrument
will rotates along upper plate and lower plate will be fixed.
When
lower clamp screw is lose and upper clamp screw is tightened, then both upper and
lower plate will move together.
6) Plate
level: - A level tube called plate level is mounted on upper plate. This bubble
is centered with the help of levelling screw. In
some instruments, there are two plate levels. These plate level are mounted
horizontally ta right angle to each other on the upper plate. One of the plate
levels is parallel to the trunnion axis and the other is right angle to it.
This
plate level are used to make the vertical axis of the instrument truly
vertical.
These
all are the main components of theodolite.
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