Adjustment of a Theodolite, Surveying and Levelling | Surveying and Levelling Notes- Agricultural Engg - Agricultural Engineering PDF Download

ADJUSTMENT OF A THEODOLITE
The adjustments of a theodolite are of two kinds:

1. Permanent adjustment

2. Temporary adjustment

PERMANENT ADJUSTMENT

• The permanent adjustment are made to establish the fixed relationships between the fundamental lines of the instrument, and once made, they last for long time. 
• They are essential for the accuracy of observations. The permanent adjustment in case of transit theodolite are:

  1. Adjustment of the Horizontal Plate Levels.

  2. Collimation Adjustment

  3. Horizontal Axis Adjustment

  4. Adjustment of the Telescope Level or the Altitude level.

  5. Vertical Circle Index Adjustment.

TEMPORARY ADJUSTMENT OF THEODOLITE

The temporary adjustments are made at each set up of the instrument before starting taking observations with the instrument. 

1) Setting up the Theodolite over the station
2) Levelling up the theodolite
3) Elimination of the parallax

1. Setting up :  Operation of setting up a theodolite includes:
         a)  centering the theodolite over the ground mark
         b) approximate levelling with the help of tripod legs.
2.  Levelling up of theodolite
    The operation of making the vertical axis truly vertical is known as levelling of Theodolite.

            i)  Turn the horizontal plate until the longitudinal axis of the plate level is   

                 approximately parallel to a line joining any two levelling screws.

ii) Bring the bubble to the centre of its run by turning both foot screws simultaneously in opposite directions either inwards or outwards.  The movement of the left thumb indicates the direction of movement of bubble.

iii) Turn the instrument through 180⁰ in azimuth.

iv) Note the position of the bubble.  If it occupies a different position, move it by means of the same two foot screws to the approx. mean of the two positions.v) Turn the theodolite through 90 in azimuth so that the plate level becomes perpendicular to the previous position.

vi) With the help of third floor screw, move the bubble to the approx. mean position already indicated.

vii) Repeat the process until the bubble, retains the same position for every setting of the instrument.

3. Elimination of Parallax :  Elimination of parallax may be done by focusing the eye piece for distinct vision of cross hairs and focusing the objective to bring the image of the object in the plane of cross hairs.