At the beginning and end of each levelling run a stable and precise benchmark is required. Intermediate points are not observed. To avoid accidental damage or vandalism wall mounted benchmarks can be removed from the wall leaving the barrel, which has been fixed with epoxy resin, capped for protection . The size of the levelling team depends upon the observing conditions and the equipment available. In ordinary levelling an observer and staff holder are required. In precise levelling there are two staves and therefore two staff holders are required.
If a programmed data logger is available then the observer can also do the booking. If the observations are to be recorded on paper a booker should also be employed. The booker’s task, other than booking, is to do a series of quality control checks at the end of each set of observations, before moving to the next levelling bay. Finally, in sunny weather, an umbrella holder is required because it is necessary to shield the instrument and tripod from the heating effects of the sun’s rays.
Just as with ordinary levelling, a two-peg test is required to confirm that the instrumental collimation is acceptable. Precise levelling procedures are designed to minimize the effect of collimation, but even so, only a well-adjusted instrument should be used. Precise level lines should follow communication routes where possible because they generally avoid steep gradients; they are accessible and have hard surfaces. However, there may be vibration caused by traffic, especially if using an automatic level.
The following procedures should be adhered to when carrying out precise levelling:
(1) Precise levelling can be manpower intensive, and therefore expensive to undertake. It is important to carry out a full reconnaissance of the proposed levelling route prior to observations being taken to ensure that the best possible route has been chosen.
(2) End and intermediate benchmarks should be constructed well before levelling starts to prevent settling during levelling operations.
(3) Steep slopes are to be avoided because of the unequal and uncertain refraction effects on the tops and bottoms of staves.
(4) Long lines should be split into workable sections, usually each section will not be more than about 3 km, because that is about as much as a team can do in one day. There must be a benchmark at each end of the line to open and close on. The length of each line will depend upon terrain, transport, accommodation and other logistical considerations.
(5) Each section is to be treated as a separate line of levelling and is checked by forward and backward levelling. This will isolate errors and reduce the amount of re-levelling required in the case of an unacceptable misclosure.
(6) On each section, if the forward levelling takes place in the morning of day 1, then the backward levelling should take place in the afternoon or evening of day 2. This will ensure that increasing refraction on one part of the line in one direction will be replaced by decreasing refraction when working in the other direction. This will help to compensate for errors due to changing refraction effects.
7) On bright or sunny days an observing umbrella should be held over the instrument and tripod to avoid differential heating of the level and of the tripod legs.
(8) Take the greatest care with the base plate of the staff. Keep it clean. Place it carefully onto the change plate and do not drop the staff. This will avoid any change in zero error of the staff. When the staff is not being used, it should be rested upon the staff-man’s clean boot.
15) The rounded centre on the change plate should be kept polished and smooth to ensure that the same staff position is taken up each time it is used.
(16) The change plate must be firmly placed and not knocked or kicked between foresight and backsight readings. Remember there is no check on the movement of a change plate between these observations. The staff holder should stand clear between observations.
(17) The observation to the back staff must be followed immediately by an observation to the forward staff, both on one scale. This is to ensure that refraction remains constant during the forward and back observations of one bay. Then, an observation to the forward staff is followed immediately by an observation to the back staff on the other scale. This procedure helps to compensate for unknown changes in refraction, by balancing the errors. Using two double scale rods the sequence of observation would be:
(1) BS left-hand scale on staff A
(2) FS left-hand scale on staff B
(3) FS right-hand scale on staff B
(4) BS right-hand scale on staff A