ordnance datum

noun

1. mean sea level calculated from observation taken at Newlyn, Cornwall, and used as the official basis for height calculation on British maps OD

Example Sentences

If we consult a map which includes the lower Thames, and has the levels clearly marked or contoured, and follow the coast line from, say, Kew Bridge, we come to no higher ground for more than six miles, the surface varying from one foot above the ordnance datum of high water to seven.

From Project Gutenberg

At each visit the observer should pull the float wire down a short distance, and allow it to return slowly, thus making a vertical mark on the diagram, and should then measure the actual level of the surface of the water below the bench mark in the hut, so that the water line on the chart can be referred to ordnance datum.

From Project Gutenberg

Trinity high-water mark is the datum adopted for the Port of London by the Thames Conservancy; it is the level of the lower edge of a stone fixed in the face of the river wall upon the east side of the Hermitage entrance of the London Docks, and is 12 48 ft above Ordnance datum.

From Project Gutenberg

It is very desirable that for sewage work all tide levels should be reduced to Ordnance datum.

From Project Gutenberg

It may be of interest to mention that Ordnance datum for Ireland is the level of low water of spring tides in Dublin Bay, which is 21 ft below a mark on the base of Poolbeg Lighthouse, and 7.46 ft below English Ordnance datum.

From Project Gutenberg