Abney's level- Introduction, Testing and Adjustment

Abney level is a most commonly used clinometers. Abney level is very useful for measuring vertical angles, slopes of ground, tracing a garde contour in topographic survey. It used in rapid and rough work. It can be used as hand level by setting the vernier to zero of scale.

It consists of 

• A square sight tube with eye piece or small telescope and cross- wire at opposite end.
• A Mirror at angle of 45° to axis, behind cross-wire and occupy half of width of tube is placed inside tube.
• A semicircular graduated arc, marked with zero at middle point and graduation upto 90° at both side.
• A small bubble tube attached to vernier, arm. It can be rotated by worm wheel and milled-head screw. The vernier can read angles upto 5-10 minutes.
  

Testing of Abney's level

Method 1
Sight a well distant object and measure angle of elevation (say a1) and angle of depression (say a2).
If both a1 and a2 are same then level is correct. If not, then adjustment is needed.

Method2
Take two ranging rod and make marks on it at same height.
Hold one ranging rod at top of slope and another at bottom of slope.
Stand at foot of slope and hold the instrument at height of mark on ranging rod and sight the mark on ranging rod present at top of slope and observe the angle of elevation say a1.
Then move at top of slope and repeat the process and measure angle of depression say a2.
If both a1 and a2 are not same then level needs adjustment.

Adjustment of Abney's level

1. Obtain the correct value of angle of inclination by taking mean of two reading i.e (a1+a2)/ 2.
2. Set the vernier to read (a1+a2)/2 and sight the object in test and also bring bubble in centre by turning adjustment of screws.

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