DEFINITIONS
OF TERMS
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.
A run is the levelling between two or more points measured in one direction only. The outward run
is from known to unknown points and the return run is the check levelling in the opposite
direction.
A close is the difference between the starting level of the initial point for the outward run and that
determined at the end of the return run. If the levels have been reduced correctly this value should
be the same as the difference between the sum of the rises and falls and also the difference
between the sum of the backsights and foresights.
Height of Collimation is the elevation of the optical axis of the telescope at the time of the setup.
The line of collimation is the imaginary line at the elevation.
Orders of levelling refer to the quality of the levelling, usually being defined by the expected
maximum closing error. These are given in Table 1
Order Purpose Maximum close (m)
Precision order Deformation surveys 0.001 x km
First order Major levelling control 0.003 x km
Second order Minor levelling control 0.007 x km
Third order Levelling for construction 0.012 x km
Table 1 Levelling
The accuracy requirements for water-level stations relate to the standards; for further information
refer to section 1.
Change points are points of measurement which are used to carry the measurements forward in a
run. Each one will be read first as a foresight, the instrument position is changed, and then it will
be read as a backsight.
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