Establishing
a site master benchmark
a site master benchmark
One of your fist tasks as a site engineer on a new site is to establish
height control.
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
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.
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.
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.
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.
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.
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.
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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