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

other useful links

nearby UK OSGR/LatLong conversions OSi maps (Eire & NI) old maps online  

ffff info
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