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they mark a set (500,000) of places, each a known
datum in the locality. |
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they have a defined location (OS grid ref) |
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and height above "mean sea level" (measured
in Newlyn, Cornwall) |
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the first BMs were cut in 1835 |
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the last in about 1991 (GPS plus terrestial stations have
overtaken their original purpose) |
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technically it is still illegal to destroy or cover them. |
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historically, there are BMs in Ireland administered by OSi (eg
Bundorun) and many other countries (eg Barcelona) |
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sea level varies, Ireland shows 1 metre difference east/west
coasts (a gyre in the Irish Sea) |
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Panama has 2 metres difference (tidal drag & the Gulf) |
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this website links each BM to Streetmap (& Bing maps)
to show where they can be found (about 33% have been lost) |
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Links to satellite maps via what3words.com
are to 3 metre resolution - street level images can be seen. |
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other useful links |
nearby UK OSGR/LatLong
conversions |
OSi maps
(Eire & NI) |
old maps online |
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