SOURCES OF ERROR
All measurements have error. In the case of levelling, these errors will be instrumental, observational and natural.
(1) The main source of instrumental error is residual collimation error. As already indicated, keeping the horizontal lengths of the backsights and foresights at each instrument position equal will cancel this error. Where the observational distances are unequal, the error will be proportional to the difference in distances.
The easiest approach to equalizing the sight distances is to pace from backsight to instrument and then set up the foresight change point the same number of paces away from the instrument.
(2) Parallax error has already been described.
(3) Staff graduation errors may result from wear and tear or repairs and the staffs should be checked.
against a steel tape. Zero error of the staff, caused by excessive wear of the base, will cancel out on backsight and foresight differences. However, if two staffs are used, errors will result unless calibration corrections are applied.
(4) In the case of the tripod, loose fixings will cause twisting and movement of the tripod head. Overtight fixings make it difficult to open out the tripod correctly. Loose tripod shoes will also result in unstable set-ups.
(1) Levelling involves vertical measurements relative to a horizontal plane so it is important to ensure that the staff is held strictly vertical. It is often suggested that one should rock the staff back and forth in the direction of the line of sight and accept the minimum reading as the truly vertical one. However, as shown in Figure, this concept is incorrect when using a flat-bottomed staff on flat ground, due to the fact that the staff is not being tilted about its face.
Thus it is preferable to use a staff bubble, which should be checked frequently with the aid of a plumb-bob.
(2) There may be errors in reading the staff, particularly when using a tilting level which gives an inverted image. These errors may result from inexperience, poor observation conditions or overlong sights. Limit the length of sight to about 25–30 m, to ensure the graduations are clearly defined.
(3) Ensure that the staff is correctly extended or assembled. In the case of extending staffs, listen for the click of the spring joint and check the face of the staff to ensure continuity of readings. This also applies to jointed staffs.
(4) Do not move the staff off the CP position, particularly when turning it to face the new instrument position. Always use a well defined and stable position for CPs. Levelling plates (Figure ) should be used on soft ground.
The effect of rocking the staff on staff readings r1, r2, r3, r4
(5) Avoid settlement of the tripod, which may alter the height of collimation between sights or tilt the line of sight. Set up on firm ground, with the tripod feet firmly thrust well into the ground. On pavements, locate the tripod shoes in existing cracks or joins. In precise levelling, the use of two staffs helps to reduce this effect. Observers should also refrain from touching or leaning on the tripod during observation.
(6) Booking errors can, of course, ruin good field work. Neat, clear, correct booking of field data is essential in any surveying operation. Typical booking errors in levelling are entering the values in the wrong columns or on the wrong lines, transposing figures such as 3.538 to 3.583 and making arithmetical errors in the reduction process. Very often, the use of pocket calculators simply enables the booker to make the errors quicker. To avoid this error source, use neat, legible figures; read the booked value back to the observer and have them check the staff reading again; reduce the data as it is recorded.
(7) When using a tilting level remember to level the tubular bubble with the tilting screw prior to each new staff reading. With the automatic level, carefully centre the circular bubble and make sure the compensator is not sticking.
Residual compensator errors are counteracted by centring the circular bubble with the instrument pointing backwards at the first instrument set-up and forward at the next. This procedure is continued throughout the levelling.