Prior to undertaking a levelling exercise it is necessary to check that level is in correct adjustment. The two peg test method is employed for this task. The test procedure determines the difference between the actual line of sight ( line of collimation) and the true horizontal line. This enables the user to adjust the line of sight so that it coincides with the horizontal plane (See fig 1. in the top right window). The objective is to adjust the line of sight to eliminate the e component -the collimation error. The steps used to perform the task are outlined below.

1. Select a suitable test range.
• Flat and about 70 to 100 metres long.
2. Place a semi permanent marks such as short wooden pegs ( eg 300*25*25mm) firmly into the ground at each end of the range. The pegs must be hammered into the ground firmly to ensure they do not settle when the survey staff is placed on them.
   • One peg will be identified as BS ( backsight) and the other as FS ( foresight) (fig1).
3. Set the level up approximately midway online between the BS and FS marks.
4. Sight to the staff on the BS point and book the reading.
5. Sight to the staff on the FS point and book the reading.
6. Move the level to a point near the BS point (eg. 3 metres) and online between the BS and FS points.
7. Sight to the staff on the BS point and book the reading.
8. Sight to the staff on the FS point and book the reading.
9. Calculate the collimation error - see booking sheet example
10. Adjust the reticule ( cross hairs) to the correct staff reading.
11. Cross check the booking sheet and the corrected staff reading.

View the Two peg test movie fully integrated with other graphic media flash player 6; 5.6mB

View the Two peg test movie full quality streaming video; 11.2mB