An Ordnance Survey Bench Mark is a surveying mark made by the Ordnance Survey to record heights above Ordnance Datum. They are typically found on buildings, bridges, or other structures and are used as reference points for altitude measurements.

above ordnance datum!

Above Ordnance Datum is a reference elevation used in surveying that is based on the mean sea level. It provides a standard benchmark for measuring height or altitude across different locations. This datum is commonly used in the UK and other countries for mapping and surveying purposes.

Sea level rises and falls, so a datum line was put by the British Ordnance Survey on a harbour wall at Newlyn, Cornwall. It is from this datum line that mean sea level is measured.

The highest motorway in the UK is the M62 near the boundry between Yorkshire and Lancashire which reaches 372 metres (above ordnance datum)at it's peak.

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