Levelling

For most site survey work tilting or automatic levels provide for quick and accurate results. As with all instruments, they should be checked regularly for accuracy and returned to the manufacturer for overhaul and re-setting at intervals. The levelling staff must be kept clean and its markings clear to reduce reading errors and it is worth spending time to ensure the chainman is clear as to where the foot of the staff should be placed and that it is held vertically. The tripod must be in good condition, and set up so as to avoid movement during operations. Once fixed to the tripod the instrument must be levelled using the centre bubble to avoid excessive tilt of the telescope. For the tilting level the split view of the tubular level bubble must be adjusted to coincide for each reading while the compensation mechanism in the automatic level ensures that the sighting is in the horizontal plane. On-site checks on accuracy can be made using the two-peg test.
Even with a good instrument and set up, care must be taken to ensure the levelling staff is correctly extended and that sighting lengths are not so great that readings are indistinct. Good visibility is necessary and may be reduced by heat haze or vibration in the wind. Accuracy of reading and of booking the readings are also sources of error. Some errors can be reduced by keeping sight lengths approximately the same and it is a good practice to close the run of levels back onto a known benchmark.