Aquaculture Engineering

Unit 1- Surveying

Adjustment of a level

• Permanent adjustment
• Temporary adjustment

Permanent adjustment made only when the fundamental relations between some parts or lines are disturbed.

Temporary adjustment or station adjustment, which are made at every instrument setting and preparatory to taking observations with instrument.

Temporary adjustment consists of

• Setting up the level
• Levelling up.
• Elimination of parallax

Setting up the level

• The operation of setting up includes
• Fixing the instrument on the stand

Levelling the instrument approximately by leg adjustment
To fix the level to the tripod, the clamp is released; instrument is held in the right hand and is fixed on the tripod by turning round the lower part with the left hand. The tripod legs are so adjusted that the instrument is at the convenient height and the tribrach is approximately horizontal.

Levelling up
After having levelled the instrument approximately accurate levelling is done with the help of foot screws and with reference to the plate levels. The purpose of levelling is to make the vertical axis truly vertical. The manner of levelling the instrument by the plate levels depends upon whether there are three or four leveling screws.

Three screw head

• Loose the clam. Turn the instrument until the longitudinal axis of the plate level is roughly parallel to a line joining any two of the leveling screws (A and B)
• Hold these two leveling screws between the thumb and first finger of each hand and turn them uniformly so that the thumbs move either towards each other or away from each other, until the bubble is central. It should be noted that the bubble will move in the direction of movement of the left thumb

• Turn the upper plate through 900 until the axis on the level passes over the position of the third levelling screw, ‘C’
• Turn this levelling screw until the bubble is central
• Return the upper plate through 900 to its original position and repeat (step-2) till the bubble is central.
• Turn back again through 900 and repeat step (4)
• Repeat steps (2) and (4) till the bubble is central in both the positions.
• Rotate the instrument through 1800. The bubble should remain in the centre if its run provided it is in correct adjustment.