>Not really mining but they sound like 'broad arrow' survey marks -
>indicating a survey point used in the past by the Ordnance Survey. If you
>look at the earlier editions of the large scale OS maps, the marks will
>probably relate to spot heights.
The correct term is a Bench Mark and they reprisent heights above ordanance
datum. These are shown on most OS maps marked as a height preceeded with the
letters B.M. spot heights are less accurate and are marked with a height and
a dot. The height relates to the bar above the arrow. Depending on the date
of the map the datum can be measured from Newlyn or Liverpool and this will
be marked on the margin of the map. They are still used by surveyors to
establish height.
Martin Roe
Conservation Officer NAMHO
National Association of Mining History Organisations http://www.namho.org
Lead Mining in the Yorkshire Dales
http://www.mroe.freeserve.co.uk
The Industrial Heritage of Calderdale
http://www.halifaxcouriertoday.co.uk/ftpinc/calderheritage
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