An Auto Level should be checked at least every week or before any critical works. This can be done easily and quickly by carrying out a two peg test.
The two peg test should be carried out by the team who will be undertaking the levelling task. This has the added benefit of establishing the total ability of the levelling team. If the team can’t accurately use the equipment, then even the most accurate of equipment will not yield satisfactory results.
Before carrying out the two peg test check the levelling bubble on the Auto Level. With the Auto Level set up, check that the bubble in the vial stays within the inner circle when rotated through 90 degree intervals. If the bubble stays in the middle, then proceed with the two peg test.
· Set up the Auto Level on the tripod roughly centre from two fixed, stable points. The two fixed stable points should be at least 30m apart, ideally 50m apart.
· With the staff being held carefully take readings from first point A and then point B. Note down your staff readings for both points A and B.
· Now move and set up the Auto Level near either point A or Point B. This should be about 2m from the point, thus allowing the staff to be read easily.
· With the staff being held carefully take readings from the same points as before but record your staff readings as point A1 and point B1.
· Work out the difference in the level between point A and point B (A-B=X). Record value X.
· Work out the difference in the level between point A1 and point B1 (A1-B1=X1). Record value X1.
· Work out the difference in values between X and X1. This should be less than 3mm ideally.
· For example:
o Reading A is 1.254
o Reading B is 1.053
o Reading A1 is 1.568
o Reading B1 is 1.366
o (A-B=X) (1.254-1.053=0.201) X=0.201
o (A1-B1=X1) (1.568-1.366=0.202) X1=0.202
o (X1-X=Accuracy) (X1-X=0.001) Accuracy is 1mm over 50m.
If the two peg test does not yield the satisfactory accuracy, then it is best to have it adjusted or repaired and calibrated by your specialist survey equipment supplier. You should also ensure that your Auto Level is calibrated yearly to ensure the correct accuracy is maintained.