Thursday, January 30, 2014

Setting Out - Levelling

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 5m length. Staff graduations are in 10mm 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

Application ~ methods to determine differences in ground levels for calculation of site excavation volumes and costs.

Setting Out---Levelling

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