Amongst all the various carvings on the stones at Stonehenge, from the modern graffiti of the 17th to the 20th century and the ancient axe heads and daggers from about 1700BC, there are three that are not often noticed.
These are the Ordnance Survey (OS) benchmarks.
The OS website says: “Bench marks are the visible manifestation of Ordnance Datum Newlyn (ODN), which is the national height system for mainland Great Britain and forms the reference frame for heights above mean sea level.”
The original reference datum levelling survey was begun in Liverpool in 1840 using a benchmark on St. John’s Church, and in 1844 it was changed to the tidal pole in Victoria Dock. The reference Mean Sea Level (MSL) for the datum was established over a nine day period of tidal observations.
A second levelling survey was carried out in 1912-21 and the datum was changed to…
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