Surveying theodolite basics

INTRODUCTION

 Theodolite is used to measure the horizontal and
vertical angles.

 Theodolite is more precise than magnetic compass.

 Magnetic compass measures the angle up to as
accuracy of 30’. However a vernier theodolite
measures the angles up to and accuracy of 10’’, 20”.

 There are variety of theodolite vernier, optic,
electronic etc.

Vertical axis
VERNIER THEODOLITETraverse/horizontal axis
Vertical circle rigidly fixed to
the telescope

Line of Sight

Alidade assembly
 Vernier theodolite is
also known and transit.

Horizontal circle
assembly
 A transit theodolite is
one in which the
telescope can be
Levelling head assembly
rotated in a vertical
plane.

Three assemblies of Theodolite

 Main parts of a theodolite
 Levelling head (7): Levelling head
is used to attach the instrument to
tripod and attach the plumb bob
along the vertical axis of the
instrument.

MAIN PARTS-2
 Lower plate/circle plate (18): an annular
horizontal plate with the graduations provided
all around, from 0 to 360°, in a clockwise
direction. The graduations are in degree
divided in to 3 parts so that each division
equals to 20 min.

 Horizontal angles are measured with this plate.

 The size of the theodolite is defined by the
diameter of horizontal circle.

 Upper plate (17): Horizontal plate of smaller
diameter provided with two verniers. on
diametrically opposite parts of its
circumference. These verniers are designated
as A and B. They are used to read fractions of
the horizontal circle plate graduations. The
verniers are graduated in 20 min and each
minute is divided in 3 to 5 parts making least
count 20” or 10”.

MAIN PARTS-3
 Clamps and tangent screws (15, 19):
 There are two clamps and associated tangent
screws with the plate. These screws facilitate
the motion of the instruments in horizontal
plane.

 Lower clamp screw locks or releases the
lower plate. When this screw is unlocked both
upper and lower plates move together. The
associated lower tangent screw allows small
motion of the plate in locked position.

 The upper clamp screw locks or releases the
upper vernier plate. When this clamp is
released the lower plate does not move but
the upper vernier plate moves with the
instrument. This causes the change in the
reading. The upper tangent screw allows the
fine adjustment.

MAIN PARTS-4

 Plate level (5):
 Spirit level with the bubble and
graduation on glass cover.
 A single level or two levels fixed in
perpendicular direction may be provided.
 The spirit level can be adjusted with the
foot screw (21) of the levelling head (7).

 Telescope (10): The essential parts of the
telescopes are eye-piece, diaphragm with
cross hairs, object lens and arrangements
to focus the telescope.

MAIN PARTS-5

 Vertical circle (1): circular plate
supported on horizontal axis of the
instrument between the A-frames.
Vertical circle has graduation 0-90 in
four quadrants. Vertical circle moves
with the telescope when it is rotated in
the vertical plane.

 Vertical circle clamp and tangent
screw (11): Clamping the vertical circle
restrict the movement of telescope in
vertical plane.

 Altitude level (2): A highly sensitive
bubble is used for levelling particularly
when taking the vertical angle
observations.

Reading a
theodolite

Vernier scale graduation

Important Definition

Changing face
 Revolving the telescope by 180 in vertical plane
about horizontal axis
 Again revolving the telescope in horizontal plane
about vertical axis.

Adjustment of the theodolite
 Temporary Adjustment

 Setting up the theodolite

Measurement of horizontal angle
 Measurement of Angle ABC
 The instrument is set over B.
 The lower clamp is kept fixed and upper clamp is
loosened.
 Turn the telescope clockwise set vernier A to 0° and
vernier B to approximately 180°.
 Upper clamp is tightened and using the upper tangent
screw the vernier A and B are exactly set to 0° and 180°.
 Upper clamp is tightly fixed, lower one is loosened and
telescope is directed towards A and bisect the ranging rod
at A.
 Tightened the lower clamp and turn the lower tangent
screw to perfectly bisect ranging rod at A.
 Loose the upper clamp and turn the telescope clockwise to
bisect the ranging rod at C tightened the upper clamp and
do the fine adjustment with upper tangent screw.
 The reading on vernier A and B are noted. Vernier A gives
the angle directly and vernier B gives the reading by
subtracting the initial reading (180°) from final reading

 Read these two method
 Repetition method
 Reiteration method

Measurements of Deflection angle
magnetic bearing

P

B


A

N
B C



A

 Ranging and extending a line

 Method of traversing
 Included angle method
 Deflection angle method
 Fast angle (or magnetic bearing method)

Computation of latitude and departure
 Latitude of a line is the distances measured parallel
to the north south of the North-South direction
 Departure of the line is the distance measured
parallel to the east-west direction

Computing latitude and departure

REFERENCES
 Surveying Vol 1 by Dr B C Punmia,Ashok K
Jain,Arun K Jain.
 NPTEL

 Surveying Volume 1 by S K Duggal

Surveying theodolite basics

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