| A - Original style top
These appear to be the earliest style -
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Rectangular (not square) granite pillar ...
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... with vertical sides - and plaque toward top edge of front face:
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ORDNANCE
SURVEY
HEIGHT OF TOP
OF PIN
<space for plate> Ft.
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- clearly designed to accommodate a rectangular height plate;
- but fitted with a curved tag at
Basingstoke
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Top has a circular bolt with no legend (as per the lower centre of a trig point?)
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B - 2GL "Classic" style (as installed for 2GL England & Wales) top
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Rectangular (not square) granite pillar ...
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... with vertical sides - and plaque toward the top edge of the front face:
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Top has circular bolt with legend "HEIGHT ABOVE DATUM" around the circumference, and space for a height tag at the front
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May or may not have metal railings - normally looped style painted green. Maybe some never had railings; possibly some
FBMs had railings which were removed in the war effort, or maybe even wooden railings that have long since decayed.
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Front of pillar is closest to railings; buried chamber is behind pillar; on this basis, the pillar at
Southampton FBM is back to front!
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This style was used for most of the original 115 FBMs installed for
2GL England & Wales.
A number of roadside verge pillars were modified around 1990
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C - Post-2GL style (as installed for 2GL Scotland and later extensions) top
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Square (with chamfered corners) concrete pillar
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... with tapering sides (some more vertical than others??)
and a plaque around half way down the front face:
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top has circular bolt with legend "HEIGHT ABOVE DATUM" and space for
height plate at the front, but height tags appear either not to have been
fitted or to have blank height tags. A few FBMs have a different
orientation on the bolt
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Only a small minority of these have marked (sometimes railed) enclosures
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D - 1942 style top
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The pillars built in 1942 have "O S B M" bolts instead of the 'Datum' type seen in most post-2GL cases
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| E - Gravity Stations top
Two pairs of "Gravity FBM's" were built in the mid-1960's - forming 'Short Calibration Lines' 'NRCF' from North Rode to
Cat & Fiddle, and 'HHP' from Hatton Heath to Prees
These consist of a tapered square pillar rather like a small version of a trig pillar, with a numbered Flush Bracket fitted,
but no spider or bolt.
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F - 1990 modifications top
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A number of roadside verge Pillars were modified (around 1990) to remove anything projecting above ground level
FBMs were modified by severing the pillar at ground level, embedding the top portion nearby, so that the "ORDNANCE / SURVEY / B M"
plate was uppermost and horizontally at ground level, and inserting a new bolt into the stump of the pillar.
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Brecon
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Cranford
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Dulverton (?)
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Hexham
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Linlithgow (?)
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Northallerton
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Wetherby
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Yelverton (?)
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GPS Plaque top
Most pillars were later given a GPS plaque dated July 1999, placed below the "ORDNANCE / SURVEY / B M" plate,
above or below the "ORDNANCE / SURVEY / BENCH MARK" plate, on the left, right or back face,
or the top face in the case of truncated pillars or pillars with no other surface exposed. Also available in Welsh ...
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This monument forms part of the Ordnance Survey National
GPS Network.
It is an offence to damage it.
If found damaged please ring our helpline on
08456 05 05 05
Thank you.
July 1999
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Mae'r gofgolofn hon yn ffurfio rhan o Rwydwaith Cenediaethol GPS
yr Ordnance Survey.
Mae'n drosedd i'w difrodi.
Os gwelir difrod ffoniwch ein llinell gymorth ar
08456 05 05 05
Diolch
Gorffennaf 1999
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| Known FBM replacements or rebuilds top
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G - Newtown and Waltham-on-the-Wolds top
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Concrete block with small chamber for standard height bolt
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Plastic ring
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Into which concrete cover sits. Presumably rebuilds.
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H - Navenby top
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Surface block with "ORDNANCE / SURVEY / BENCH MARK" plate (implying previous rebuild?)
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New bolt added to top surface. Unlike other "modified 1990" FBMs
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| Not yet Classified top
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