Levelling ~ the process of establishing height dimensions, relative
to a fixed point or datum. Datum is mean sea level, which varies
between different countries. For UK purposes this is established at
Newlyn in Cornwall, from tide data recorded between May 1915 and
April 1921. Relative levels defined by benchmarks are located
throughout the country. The most common, identified as carved
arrows, can be found cut into walls of stable structures. Reference
to Ordnance Survey maps of an area will indicate benchmark
positions and their height above sea level, hence the name
Ordnance Datum (OD).
On site it is usual to measure levels from a temporary benchmark
(TBM), i.e. a manhole cover or other permanent fixture, as an OD
may be some distance away.
Instruments consist of a level (tilting or automatic) and a staff. A
tilting level is basically a telescope mounted on a tripod for stability.
Correcting screws establish accuracy in the horizontal plane by air
bubble in a vial and focus is by adjustable lens. Cross hairs of
horizontal and vertical lines indicate image sharpness on an extending
staff of 3, 4 or 5 m length. Staff graduations are in 10 mm intervals,
with estimates taken to the nearest millimetre. An automatic level is
much simpler to use, eliminating the need for manual adjustment. It is
approximately levelled by centre bulb bubble. A compensator within
the telescope effects fine adjustment.
Setting Out---Levelling