Establishing a site master benchmark
One of your fist tasks as a site engineer on a new site is to establish height control.
It is usual to establish a site Master Bench Mark (MBM) from nearby Ordnance Bench marks and the following lists the usual procedure
a) The Master Bench Mark must be constructed in apart of the site clear of any construction activity and must be of very stable construction.
A typical MBM comprises a steel pin firmly driven into the ground and surrounded by concrete. Provide protection by surrounding it with a fence and the reduced level of the bench mark should be written on that fence as well being recorded in the field book.
b) The reduced level of the MBM is found by flying leveling from a nearby Ordnance Bench Mark (OBM) using the MBM as a change point (intermediate sights are not permitted) and then closing the circuit onto a second bench mark, if such exists. This not only provides the closing error but also checks the OBM for any possible movement.
c) The leveling operation must be carried out a minimum of three times, ensuring that a good closing error is obtained each time. The range of results must not exceed 5 mm, if it does then further leveling must be carried out until such a range is achieved. The mean value can then be adopted as the MBM level.
d) Having established the MBM, any number of temporary bench marks (TBM) can be located and leveled where and as required, working to the same order of accuracy as above.
The ideal situation regarding TBMs is to locate them in such positions that a level can be set up anywhere on the site and be within sighting distance from it, thus avoiding the need to fly level to the area of operation.
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