FUNDAMENTAL LINES AND DESIRED RELATIONS

 

FUNDAMENTAL LINES AND DESIRED RELATIONS

·        A Theodolite consists of several fundamental lines in order the readings to be accurate, certain desired relationship must exist between the fundamental lines of the instrument.

·        But due to improper handling (or) excessive use, this relationship may be disturbed and hence from the theodolite may lead to erroneous results.

·        The fundamental lines of a theodolite are.

                           I.          The vertical axis 

                         II.          The axis of the plate level 

                       III.          The line of collimation

                      IV.          The horizontal axis 

                        V.          The bubble line of the altitude level 

·        The desired relationships between the fundamental lines are as follows.

Ø  The axis of the plate level must be perpendicular to the vertical axis.

Ø  The line of collimation should coincide with the optical axis of the Telescope and should also be perpendicular to the vertical axis.

Ø  The axis of telescope must be parallel to the line of collimation. The line of collimation must be perpendicular to the horizontal axis and the vertical circle should read zero when the line of collimation is horizontal.

The permanent adjustment should be made in the order.

* Adjustment of vertical cross hair.

* Adjustment of Plate level axis. 

* Adjustment of line of sight. 

* Adjustment of horizontal axis. 

* Adjustment of the axis of the telescope. 

* Adjustment of vertical circle index.

Temporary Adjustments:

* These are the adjustments which are done before making observations at every Setup of the instrument. 

* They consist of Setting up the Theodolite levelling it and focusing the eye Piece and objective.

Setting up theodolite:

It consists of centering the theodolite over the station and its approximate levelling by tripod legs.

Centering:

·        This involves setting the theodolite exactly over the station mark (or) on the station peg

·        The plum bob is suspended from a small hook attached to the vertical axis of the theodolite.

·        The Tripod legs are used to center the instrument over the peg by fixing two  legs into ground and one leg is for adjustment towards centering.

·        The centering process carried over by Tripod legs is known as “Approximate Levelling".

 

Levelling :

* This means making the vertical axis truly vertical. This is done with the help of the foot screws.

The process of levelling is carried by turning the screws with thumb fingers, as surveyor turns. 

* "A" & "B" are a turned uniformly either Inward (or) outward and is individually rotated to level the Theodolite.