Countries tend to choose the mean sea level at one specific point to be used as the standard sea level for all mapping and surveying in that country . ( For example, in Great Britain, the national vertical datum, Ordnance Datum Newlyn, is based on what was mean sea level at Newlyn in Cornwall between 1915 and 1921 ).
42.
At ordnance datum ( OD ), the mean height above sea level, Dunkery Beacon is the highest natural point in Somerset, although the tip of the Mendip TV Mast is higher at OD . Dunkery is ranked 23rd in the UK in terms of dominance and is a Marilyn, meaning that it is a peak with or more of relative height.
43.
Samuel Murphy calculated that the three levels had been 659 metres above ordnance datum ( AOD ), 631 metres AOD and 581 metres AOD . These three old levels imply a lot of mining work had been done before William Sheffield abandoned the mine, some time before 1819 . In addition, a fourth old level ( the " Hush Level ", see below ) was driven in this early period, and a fifth level ( later known as the " High Level ", or the " High Horse Level " ) had been started before the mine was abandoned.
44.
The ground level before construction varied between 3.35m and 5.18m ( 11 and 17 ft ) ordnance datum ( O . D . ) In order to provide adequate protection against flooding, the area in which the main building is constructed was raised to 7.92m ( 26 ft ) O . D . by filling from borrow pits on site, but the coal store and cooling tower area remain at the original level of 4.87m ( 16 ft ) O . D . The nature of the sub-soil was investigated by trial bores and found to be good load bearing marl at depths between 4.26m and 12.19m ( 14 and 40 ft ) below the existing ground level and overlain by sand and gravel strata on top of which lay clay or silt and top soil.
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