Thursday, 17 August 2006 17:20
The following information was obtained by the Newlyn Harbour office:
Based at the South Pier near the lighthouse is the Ordnance Tidal Observatory from which the datum for all the latest ordnance survey maps is defined. Ordnance datum is actually the mean tide level and itâs situated 14.25 feet below the carriageway of the North Pier near the end. Tidal recordings are taken at the Ordnance Observatory by means of a brass strip with a float arrangement attached. This sets the benchmark for the whole of the United Kingdom and all heights are referenced to this point. The brass strip has holes in it which fits to a sprocket wheel, whilst on a cylinder rotated by a clock is the recording sheet. As on a barograph a pencil arrangement automatically records the inflow and ebb of the tides. As the float rises and falls with the tide the brass strip rotates the sprocket wheel.
The main bench mark is near Tolcarne School and frequent checks are made to determine if there is the slightest shrinkage between the observatory and the main bench mark. By deducting 9.9 feet from the Ordnance Datum the recordings on the admiralty chart for Mountâs Bay can be arrived by. Newlyn was chosen as the base for the tidal observatory because, after many years of constantly monitoring the tides, it was found to be the most constant. The first O.S. selected datum point in 1840 was 100 feet below a bench mark on St. Johnâs church, Liverpool, and a few years later at the Victoria Docks where tidal observations were taken every five minutes. From 1844 the approximate sea level was adopted from the Liverpool datum and was the accepted reading form 80 years.
Tidal observations were taken at Dunbar, Felixstowe and Newlyn where the first reading was taken in April 1915 when the Pier was extended. It was decided to base the O.S. datum on the figures from just one tidal observatory and Newlyn, where since 1915 hourly readings had been taken, was chosen as the most suitable.
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