LESSON NOTE ON PRECISE LEVELLING

PRECISE LEVELLING
Precise levelling may be required in certain instances in construction such as in deformation monitoring, the provision of precise height control for large engineering projects such as long-span bridges, dams and hydroelectric schemes and in mining subsidence measurements. For example, a dam that has been in place for many years is unlikely to be moving. However, should the dam fail the results would be catastrophic for those on the downstream side. Being under the pressure of water when full, the dam may be liable to distortion. The behaviour of the dam must therefore be monitored. One way of monitoring any vertical movement along the dam is by levelling. Since early warning of small movement is required, and since conclusions about movement must be made with statistical confidence, the levelling must be very precise.
There is more to precise levelling than precise levels. High quality equipment is very important, but so is the method by which it is used. Indeed the two components of precise levelling are precise equipment and precise procedures. Precise levelling uses the same principles as ordinary levelling but with:
(1) Higher quality instruments and more accurate staves
(2) More rigorous observing techniques
(3) Restricted climatic and environmental conditions
(4) Refined booking and reduction
(5) Least squares adjustment for a levelling net