Theodolite

Theodolite is the most precise instrument used for measuring horizontal and vertical angles.
Horizontal and vertical distances can also be calculated indirectly.
(a) Transit Theodolite (or) Transit:
The Theodolite in which line of sight can be reversed by revolving the telescope through 180° in the vertical plane. Transit Theodolite is most commonly used.
(b) Non Transit Theodolite:
Either plain Theodolite or Y-Theodolite. This can not be used for most of the works.

Essential parts of a transit Theodolite:

a) Telescope: Mounted on a spindle known as horizontal axis or trunion axis. Internal focusing telescope is widely used.

b) Vertical Circle: It is circular graduated arc attached to the trunion axis of the telescope. Controlling by a vertical circle clamp and its corresponding slow motion or tangent screw.

c) Index frame or T-frame or Vernier frame: Two verniers are fitted to this to read the vertical circle. Clip screw is used for slight adjustment. Altitude bubble is placed on top of index frame.

d) Levelling head: It consists of two parallel triangular plates known tribrach plates.

e) Lower plate or scale plate: Size of a Theodolite is represented by the size of scale plate. i.e., 10cm Theodolite etc.

f) A-frame
g) Upper plate or vernier plate.
h) Plate levels
i) Tripod
j) Plumb bob
k) Compass

Note: Tangent Screw is operated for final adjustment of reading or bisection when both clamp screws tightened.

Some important Definitions and Terms:
(a) Centering: The process of setting up the instrument exactly over the station mark. Plumb bob is made use of.

(b) Vertical axis: Axis about which the instrument rotates in the horizontal plane. Also known as Azimuth axis.

(c) Horizontal axis (or) Trunion axis: Axis about which the telescope and vertical circle rotate in vertical plane.

(d) Line of sight (or) line of collimation: The line passing through the intersection of horizontal and vertical cross hairs and optical centre of object glass and its continution.

(e) Transiting: The process of turning the
telescope in vertical plane through 180°
about the trunion axis. It is also known as plunging or reversing.

(f) Axis of level tube (or) Bubble line:
A straight line tangential to the longitudinal curve of the level tube at its centre. It is horizontal when the bubble is central.

(g) Swinging the telescope: Process of turning the telescope in horizontal plane.

(h) Face left observation (FL): The observation of the angles (horizontal or Vertical), if the circle is to the left of the observer / bubble up.

(i) Face right observation (FR): If the face of vertical circle is to the right of the observer/bubble down.

(j)Telescope normal (or Direct): When the face of the vertical circle is to the left and the "bubble (of telescope) up"

(k) Telescope inverted or reversed: When the face of the vertical circle is and the "bubble down".

(l) Changing face: Operation of bringing the face of the telescope from left to right and vice versa. It is done by transitting.

(m) Double sighting or Double centering: Measurement of horizontal angle or vertical angle twice; once with the telescope in normal condition and once with the telescope in inverted condition.
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Theodolite

Theodolite is the most precise instrument used for measuring horizontal and vertical angles. Horizontal and vertical distances can also be...