Thursday, August 22, 2013

Theodolite and Useful Terms


  • Theodolite: Theodolite is the most precise instrument designed for the measurement of the horizontal and vertical angles.

Commonly used terms in a Theodolite Survey:

  • (a) Centering: It means setting the Theodolite exactly over an instrument station so that its vertical axis passes through the station mark. It is done with the help of a Plumb bob suspended from the vertical axis of the Theodolite.
  • (b) Transiting: It is the process of reversing the telescope/line of sight about the horizontal axis in the vertical plane.
  • (c)Face Left and Face Right : If the vertical circle of the Theodolite is on the left side of the observer, then the observation taken are known as Face Left observations. Similarly if the vertical circle is on the right, the observation taken such as are known as Face Right observations.
  • (d) Line of collimation : Line of collimation or also known as the line of sight is the imaginary line joining the center of the cross hairs and the center of the object glass and its continuation.
  • (e)Axis of level Tube: It is a line tangential to the longitudinal curve of the level tube at the center of the tube, it is also known as the bubble line. When the instrument is level in horizontal plane, i.e. bubble is central this axis is horizontal.
  • Classification of Theodolite:

There are two broad classification of Theodolite
(a) Transit Theodolite   (b) Non Transit Theodolite

  • (a) Transit Theodolite: In this type of theodolite the line of sight can be revolved about trunion  axis/ horizontal axis through 180 degrees in the vertical plane.
  • (b) Non- Transit Theodolite: It can not be transited as in the case of a transit Theodolite.

Another classification of the Theodolite can be done on the basis of the type of the scale fitted to the Theodolite i.e. vernier or micrometer.
(i) Vernier Theodolites.
(ii) Micrometer Theodolites.

Diameter of the lower plate of the graduated circle determines the size of the Theodolite. Commonly they are available in 8 cm or 12 cm sizes while 14 cm and 25 cm instruments are used for triangulation work.

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