In laser levelling techniques a laser levelling unit is set up usually on a tripod at a levelling station which gives a clear view of all points to be leveled in a building site. The relative height of a viewing plane is then determined from a known benchmark level. A measuring staff can then be held vertically on a particular location on a site and the visible laser line which intersects the staff provides a reading from a graduated scale on the staff which enables the measurement to be read off by the staff bearer. Often an elongated length of wood is used instead of a graduated measuring staff and the line at which the laser beam intersects the staff is marked with a pencil or the like. If the same staff is used for taking multiple measurements at different locations around the site, many pencil or pen marks will appear on the staff and in these circumstances, ensuring that the correct levels are established can become difficult.
The laser levelling unit can also be used with a laser receiver or detector which is releasably clamped onto a graduated staff via a slidable clamp having a pointer and the clamp may be released and slid along the staff to adjust the position of the laser detector until the detector detects the laser beam at which position, the pointer indicates a measurement on the staff. Use of a laser detector in this manner is particularly awkward and does not guarantee that correct readings are taken. Every time the laser detector is moved, there is a chance it is moved too much, too little or in the wrong direction.
With a normal staff, the way to determine how high or low the ground etc. is relative to a datum plane is to either move the laser detector on the staff up or down as above or lift the staff off the ground if the ground is too low and estimate the height. Alternatively, if the ground is too high, the staff may be leant over until the laser detector receives the laser beam and an estimate is made of the height. These methods are clearly inaccurate.
Similar problems to those described above arise when an optical level such as a dumpy level is used in combination with a sighting marker which is mountable to the staff for movement therealong.