Differential levelling is the term applied to any method of measuring directly with a graduated
staff the difference in elevation between two or more points.
Precise levelling is a particularly accurate method of differential levelling which uses highly
accurate levels and with a more rigorous observing procedure than general engineering levelling. It
aims to achieve high orders of accuracy such as 1 mm per 1 km traverse.
A level surface is a surface which is everywhere perpendicular to the direction of the force of
gravity. An example is the surface of a completely still lake. For ordinary levelling, level surfaces
at different elevations can be considered to be parallel.
A level datum is an arbitrary level surface to which elevations are referred. The most common
surveying datum is mean sea-level (MSL), but as hydrological work is usually just concerned with
levels in a local area, we often use:
An assumed datum, which is established by giving a benchmark an assumed value (e.g. 100.000
m) to which all levels in the local area will be reduced. It is not good practice to assume a level
which is close to the actual MSL value, as it creates potential for confusion.
A reduced level is the vertical distance between a survey point and the adopted level datum.
A bench mark (BM) is the term given to a definite, permanent accessible point of known height
above a datum to which the height of other points can be referred.
It is usually a stainless steel pin embedded in a substantial concrete block cast into the ground. At
hydrological stations rock bolts driven into bedrock or concrete structures can be used, but
structures should be used warily as they themselves are subject to settlement. The locations of
benchmarks shall be marked with BM marker posts and/or paint, and recorded on the Station
History Form.
A set-up refers the position of a level or other instrument at the time in which a number of
observations are made without mooring the instrument. The first observation is made to the known
point and is termed a backsight; the last observation is to the final point or the next to be measured
on the run, and all other points are intermediates.
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