Levelling Field procedure
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.
(9) The distances of foresight and backsight must be as nearly equal as possible so as to limit the effect of the Earth’s curvature, refraction and bad instrumental collimation. This will also avoid the need to re-focus the level between sightings.
(10) Take care when levelling along roads or railways. Stop levelling when traffic or vibrations are heavy.
When the staff is not being used, it should be rested upon the staff-man’s clean boot. Vibration may
damage the staff base plate and so change its zero error.
(11) On tarmac and soft ground the instrument or staff may rise after it has been set up. This may be
apparent to the observer but not by the staff person.
(12) In gusty or windy conditions stop levelling because there will be uncertainty in the readings. In variable weather conditions consider levelling at night.
(13) The bottom 0.5 m of the staff should not to be used because of unknown and variable refraction
effects near the ground.
(14) If a precise automatic level is to be used, it should be lightly tapped and rotated before each reading to ensure that the compensator is freely operative. This will reduce errors by ensuring that the
compensator always comes from the same direction. Some automatic levels have a press button for
this purpose.
(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
Then (1)−(2) = H1 and (4)−(3) = H2; if these differences agree within the tolerances specified,
the mean is accepted. Staff A is now leapfrogged to the next position and the above procedure repeated starting with staff A again
(18) If the back staff is observed first at one set-up, then the forward staff is observed first at the
next set-up. This ensures that changing refraction will affect each successive bay in an equal and
opposite manner. The order of observations this time will be:
(1) FS left-hand scale on staff A
(2) BS left-hand scale on staff B
(3) BS right-hand scale on staff B
(4) FS right-hand scale on staff A
Note that in each case the first observation of a bay is to the same staff, which is alternately the back
and then the forward staff.
(19) The same staff that was used for the opening backsight must also be used for the closing foresight.
This will eliminate the effect of different zero errors on the two staves. This means that there must
always be an even number of set-ups on any line.
(20) Levelling should always be carried out in both directions, forward and back. If, on the forward
levelling, the A staff was used to open and close the line, then the B staff should be used to open and
close the line on the backward levelling. This will equalize the number of readings on each staff.
(21) Lines of sight should not exceed 50 m, especially in haze, or on sloping ground. This will minimize the effects of refraction, curvature of the Earth and difficulty of reading the staff. A good average length of sight is 35 m.
(22) Use the procedure already outlined for levelling the circular bubble on automatic levels. This will happen as a matter of course if the telescope is aimed at staff A each time when centring the circular bubble.
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