THEODOLITE
A theodolite is an important instrument
used for measurement horizontal and
vertical angles in surveying.
It can also be used for a number of
surveying operations, such as
prolonging a line, measuring distances
indirectly and leveling.
CLASSIFICATION
Theodolites can be classified into transit
and non-transit theodolites.
1. Transit Theodolite
A theodolite is known as transit when
its telescope can be rotated through 180° in
a vertical plane about its horizontal axis,
thus directing the telescope in exactly
opposite direction.
2. Non-Transit Theodolite
A theodolite is known as a non-transit
when its telescope cannot be rotated
through 180° in a vertical plane about its
horizontal axis. Such theodolites are
obsolete nowadays.
Theodolite can also be classified into
two types as follows
1. Vernier Theodolites
2. Precise Optical Theodolites
• In a vernier theodolites, verniers are
used in taking the readings. These
• theodolites are most commonly used in
general work. Most of the vernier
theodolites can read angles up to 20’’.
(i.e. the least count of theodolite is 20’’)
• Precise optical theodolite is fitted with
an optical system which is used to read
both horizontal and vertical angles
precisely. These theodolites are having
a micrometer for taking readings and
are also called as Microptic Theodolites.
• These theodolites are used for precise
work. Most of these theodolites can
read angles up to 1’’ or less.
• The size of a theodolite is defined by the
size its lower graduated circle. For
example, a 20 cm theodolite means the
diameter of the graduated circle of the
lower plate is 20 cm. Generally the size of the theodlites varies from 8 to 25 cm
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PARTS OF VERNIER THEODOLITE
1. Telescope
2. Trunnion Axis
3. Vernier Frame
4. Verticle circle
5. Plate level
6. Standard
7. Upper Plate
9. Inner Axis
10. Outer spindle
11. Inner Spindle
12. Altitude bubble
13. Levelling screw
14. Clip screw
15. Hook
16. Tribrach
17. Verticle Clamp Screw
18. Vertical Tangent Screw
19. Trivet
20. Tripod
21. Tripod Leg
22. Target Sight
23. Rifle Sight
ESSENTIALS OF THE TRANSIT THEODOLITES.
Following are some essential parts of a
transit theodolite.
i. Telescope
ii. Vertical Circle
iii. The index frame
iv. The plumb-bob
v. Standards
vi. The leveling head
vii. Lower plate and upper-plate
1. Telescope:
Telescope is mounted on
the horizontal axis of theodolite. The
telescope may be internal focusing type
or external focusing type.
2. Vertical Circle:
Circular graduated are
and to horizontal axis of telescope.
Circle & either graduated continuously
from of to 360° or divided into four
quadrants.
3. The index Frame:
The index frame is T-shaped frame
consisting of vertical leg known as
clipping arm and a horizontal bar
known as index arm.
4. Standards:
Two frames resembling ‘A’
stand when upper plate to support
horizontal axis are called standards.
5. Leveling head:
Consist of two parallel
triangular plates known as tribrach
plates, upper tribes has leveling screws
and lower tribachor tool plate has hole
provisioned for plumb bab. Leveling
head serves following purposes.
i. Support main part of the instruments.
ii. Attachment of theodolite to tripad.
iii. Leveling of the theodolite.
6. Plumb bob:
It is suspended by a hook.
A is used for centering work.
7. Upper and lower plates:
The upper and lower plate supports the standards and
carries vernier scales hence also known
as vernier plate.
Upper plate is attached to the inner
axis.Lower plate is attached to outer
spindle. It carries the clamp & tangent
screw which controls the relative
motion of two spindles.
CENTERING
It is the process of setting up the
instrument exactly over the station mark. A plumb bob suspended from a small hook attached to the underside of the inner
HORIZONTAL AXIS
It is also called the Trunnion axis or
transverse axis. It is the axis about which
the telescope can be rotated in a vertical
plane.
VERTICAL AXIS
axis along which the telescope can
be rotated in a horizontal plane. It is also
known as azimuth axis.
LINE OF SIGHT
It is the imaginary line passing through the
intersection of the cross-hairs of the
diaphragm and the optical centre of the
objective. It is also known as line of
collimation.
When line of sight comes in horizontal
plane it is called as line of collimation.
AXIS OF LEVEL TUBE
• It is a line tangential to the longitudinal
curve of the level tube at its centre.
• Axis of plate level is horizontal when
bubble is centered.
FACE RIGHT
When the vertical circle of a theodolite is
right hand side of the viewers, the
position is called face right and the
observation made is called face right
observation.
FACE LEFT
When the vertical circle of a theodolite is
left hand side of the viewers, the
position is called face left and the
observation made is called face left
observation.
Note: By taking the man of both of the face
readings, the collimation error is eliminated.
TELESCOPE NORMAL
The telescope is known normal or direct
when its vertical circle is to the left hand
side of the observer and the bubble is up.
TELESCOPE INVERTED
The telescope is known inverted when
its vertical circle is to the right hand side of
the observer and the bubble is down.
TRANSIT
It is also called as plunging or reversing.
This is the operation of revolving the
telescope by 180° in a vertical plane about
its horizontal axis, thus making it point
exactly in the opposite direction.
SWINGING OF THE TELESCOPE
• Revolving the telescope in the
horizontal plane, about its vertical axis
is called swinging of telescope or simply
swing. A right swing means clockwise
rotation of the telescope, whereas a left
swing means anticlockwise rotation of
the telescope.
• By taking the mean of the left swing and
the right swing observations, the effects
of error due to friction or backlash in
the moving parts is eliminated.
CHANGING FACE
• It is the operation of bringing the
telescope from the face left condition to
the face right condition and vice versa.
• The face is changed by plunging the
telescope and swinging it by 180°.
The errors that are eliminated by
changing face are as below:
1. Error due to line of collimation not
being perpendicular to the
horizontal axis.
2. Error due to horizontal axis not
being perpendicular to the vertical
axis
3. Error due to line of collimation not
being parallel to the axis of the
altitude level.
DOUBLE SIGHTING
It is the process of measurement of a
horizontal angle or a vertical angle twice;
once with the telescope in the normal
condition and once with the telescope in
the inverted condition.
LINING IN
It is the process of establishing
intermediate points with the help of a
theodolite on a given straight line whose
ends are intervisible.
BALANCING IN
It is the process establishing intermediate
points with the help of a theodolite on a
given straight line whose ends are not
intervisible.
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Theodolite
Reviewed by Digitech education
on
April 11, 2020
Rating:
Reviewed by Digitech education
on
April 11, 2020
Rating:
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