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Errors in Differential Levelling
Errors in the equipment
1. Collimation error
This can be a serious source of error in levelling if sight lengths from one instrument position are not equal, since the collimation error is proportional to the difference in these. So, in all types of levelling, sight lengths should be kept equal, particularly back sights and fore sights and before using any level it is advisable to carry out a two peg test to ensure the collimation error is within acceptable limits. The two peg test. Step1: Set up a level between two staff positions keeping BS=FS Read RA and RB, the difference being the true height difference
Step 2 : Set up the level very close to one of the staff positions and take readings A and B
Example
5. Tripod defects The stability of tripods should be checked before any fieldwork commences by testing to see if the tripod head is secure, that the metal shoes at the base of each leg are not loose and that, once extended, the legs can be tightened sufficiently. **Field or on-site errors
- Staff not vertical** Since the staff is used to measure a vertical difference between the ground and the plane of collimation, failure to hold the staff vertical will give incorrect readings. If the staff is fitted with a circular bubble to aid it being held vertically, this should be checked at frequent intervals and adjusted if necessary. 2. Unstable ground When the instrument is set up on soft ground and bituminous surfaces on hot days, an effect is that the tripod legs may sink into the ground or rise slightly whilst readings are being taken. This alters the height of collimation and it is advisable to choose firm ground on which to set up the level and tripod, and to ensure that the tripod shoes are pushed well into the ground. Similar effects can occur with the staff, and for this reason it is particularly important that change points should be at stable positions such as manhole covers, kerbstones, concrete surfaces, and so on. This ensures that the base of the staff remains at the same height in between a FS and BS. For both the level and staff, the effect of soft or unstable ground is greatly reduced if readings are taken in quick succession. 3. Handling the instrument and tripod As well as vertical displacement, the plane of collimation of a level may be altered for any set-up if the tripod is held or leant against. When levelling, avoid contact with the tripod and only use the level by light contact through the fingertips. If at any stage the tripod is disturbed, it will be necessary to relevel the instrument and to repeat all the readings taken from that instrument position. 4. Instrument not level For automatic and digital levels this source of error is unusual, but for a tilting level in which the tilting screw has to be adjusted for each reading, this is a common mistake. The best procedure here is to ensure that the main bubble is centralised before and after a reading is taken.
5. Reading and booking errors Many mistakes can be made during the booking of staff readings taken with an automatic or tilting level, and the general rule is that staff readings must be carefully
entered into the levelling table or field book immediately after reading
Readings taken with a digital level are automatically stored by the instrument and there is no need for the operator to record anything by hand – this gives the digital level an advantage over automatic and tilting levels.
6. Weather conditions In strong winds, a level can become unusable because the line of sight is always moving and it also very difficult to hold the staff steady. For these reasons, it is not possible to take reliable readings under these conditions which should be avoided when levelling. In hot weather, the effects of refraction are serious and produce a shimmering effect near ground level that makes it very difficult to read the bottom metre of the staff. The effects of curvature and refraction on levelling When a level is set up, it defines a horizontal line for measurement of height differences. The level and horizontal lines through the instrument diverge because level lines follow the curvature of the Earth which is defined as mean sea level. If not accounted for, this is a possible source of error in levelling since all readings taken at A are observed along the horizontal line instead of the level line. However, for most levelling applications, sighting distances are relatively short and the curvature correction for a length of sight of 50 m is less than 1 mm. Consequently, when levelling, the difference between a horizontal line and a level line is small enough to be ignored. The effect of atmospheric refraction on a line of sight is to bend it either towards or away from the Earth’s surface. This has a value that can vary considerably, especially close to the ground, but it can be ignored for most levelling, where the length of sight is seldom greater than 50 m. Whatever sight lengths are used, the effects of curvature and refraction will cancel if the sight lengths are equal. How to reduce the chance of errors occurring When levelling, the following procedures should be used if many of the sources of error are to be avoided. _ Levelling should always start and finish at bench marks so that misclosures can be detected. When only one bench mark is available, levelling lines must be run in
