-==>>LEVLLING==>>

-==>>LEVLLING

The primary reference at water-level recording stations is a set of stable bench-marks, installed in locations where their level should not change. Upon initial set-up of a station, the levels of the  relevant parts of the installation are established and recorded by means of accurate levelling. At least every two years, the levels of the staff gauges, sensor level, internal gauge, tower structure and benchmarks should be measured relative to each other as a check that records are not in error
due to bank subsidence or other movement. Accurate levelling is thus a particularly important part of site establishment, installation and quality assurance. All staff shall be thoroughly familiar with its theory and practice. Levelling and surveying methods are also used for measurements of river channel and lake
configurations. Often, less accurate methods can be used for this work than for water-level recording stations, although the techniques are common.

==>>TERMS USE IN LEVLLING


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.

 level surface=>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.

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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.

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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.

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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