1. Level Adjustment (Two Peg Test)

2.  All instruments are subject to errors of calibration and adjustment.  In the case of levelling instruments, the main source of instrumental error is where the line of sight, produced by the cross hairs in the telescope, is not parallel to the horizontal line of collimation produced by the manufacture of the instrument. This error is known as collimation error.  The test is known as the two peg test.

4. α

6. D α δ Correct Reading (can’t be observed. Will be calculated.) Observed Reading Correct Reading (can’t be observed. Will be calculated.) Give Examples

7.  The main idea of performing the Two Peg Test is to measure the level difference between the same two points from two different positions. a1a1 b1b1 a2a2 b2b2 Line of Sight  Otherwise, a collimation error exits.

8.  Formulae of correct readings and correct level difference are now performed. a1a1 b1b1 a2a2 b2b2 Line of Sight

9.  The correct level difference from the two positions are not known, but they are equal.  Now, any reading taken by the level can be corrected.