- Most precise / accurate
instruments used for measuring horizontal and vertical angles.
- Theodolites are classified as
transit and non-transit theodolite
- Transit theodolite or simply
transit:
> One in which the line of
sight can be reversed by revolving the telescope through 1800 in
vertical
plane.
- Non-transit theodolites or
Y-theodolites are those in which telescope cannot be reversed.
Essential parts of the
transit theodolite:
1)
The telescope:
>
it is an integral part of theodolite and mounted on a spindle known as
horizontal axis or trunion axis
2)
Vertical circle:
> It is a circular graduated
arc attached to the trunion axis or spindle.
>
Telescope can be set at any desired position in vertical plane by means of
vertical circle clamp and its corresponding slow motion or tangent screw.
>
Gradations on vertical circle are from, 00 to 3600 in
clockwise direction or it is divided into quadrants.
3)
Index frame or T-frame or vernier frame:
> It has are horizontal bar known as index arm. Or
vernier arm.
> Two verniers are fitted at
the extremities of the index arm to read the vertical circle.
> Vertical circle moves
relative to the verniers with the help of which reading can be taken.
4)
Standards
or A-frame: mounted on the upper plate and its shape resembles to letter A.
> Trunion axis is supported on A frame.
5)
Leveling head:
> Consists oftwo parallel triangular plates known
as tribrach plates
>
Function ofleveling head is to support the main part ofthe instrument, to
attach theodolite to tripod, to provide mean for leveling theodolite.
6)
Two spindles or Axes or centers.
7)
Lower plate or scale plate:
Attached
to the outer spindle and carries a horizontal scale / circle, and therefore
also known as scale plate.
Size of
the theodolite is represented by the size of scale plate. Exa. 12 cm theodolite
or 12 cm theodolite.
8)
Upper plate or vernier plate.
Carries two verniers,
diametrically opposite.
9)
Plate levels or level tubes.
10)
Tripod
11)
Plumb bob
12)
Compass
13)
Striding level
To check the horizontality oftransit or trunion axis Definitions and term:
1)
Vertical axis:
It is the axis about which
instrument can be rotated in horizontal plane.
2)
Horizontal axis or trunion axis
Horizontal axis is the axis about which the telescope
and the vertical circle rotates in vertical plane.
3)
Line of sight or line of collimation
The line passing through the intersection ofthe
horizontal & vertical cross-hairs and the optical centre ofthe object glass
and its continuation.
4)
Axis ofthe level tube: or bubble line
It is a straight line tangential to the longitudinal
curve ofthe level tube at its centre. It is horizontal align bubble is at
centre.
5)
Centrign:
Process of setting the theodolite
exactly aer station mark, done by using plumb bob.
6)
Transiting
Process of turning the telescope
in vertical plane through 1800 about the trunion axis. Line of sight
is reversed in this process so also known as plunging or reversing.
7)
Swinging the telescope:
The process ofturning the
telescope in horizontal plane.
8)
Face left observation:
When observation of vertical or horizontal angle is
taken by keeping the face of vertical circle to the left of observer it is FLO.
And vice versa for face right observation
9)
Face right observations
10)
Telescope normal: or direct
When face of the vertical circle
is to the left and the “bubble up” (of the telescope).
11)
Telescope inverted:
When face ofthe vertical circle
is to the right and the “bubble down”.
12)
Changing face:
Operation ofbringing the face of
the telescope from left to right and vice versa.
13)
Double sighting:
Taking observation angles of same
object once with telescope normal and ace with telescope
inverted. Also called as double
centering.
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