The Sunday Times have published a piece on the prospective reclassification to some of the hills in Britain if Ordnance Survey changed the height of their datum point based on current or future mean sea level.
|The Sunday Times article this morning|
Mean sea level is regarded to have increased in height since the second geodetic levelling was completed by Ordnance Survey in 1921 and the third in 1956, both of which were based from measurements taken at Newlyn.
It is now accepted that sea levels are rising by around 1½ inches (4cm) per decade, and it is almost 100 years since the second geodetic levelling at Newlyn was completed, based on these simplified figures mean sea level would be approximately 40cm higher nowadays compared to the height of the current mean sea level datum point.
If Ordnance Survey updated their datum point in a decade's time the height difference between the current point at Newlyn and an updated datum point would be approximately 40-45cm. The hills that have been accurately surveyed by GPS/GNSS equipment that will be affected if Ordnance Survey alter their datum point, and therefore all known heights throughout Britain are reduced by 40-45cm appear below.
The Munros (Scotland, 3,000ft minimum height and above):
Beinn Teallach 914.6m summit height at NN 36139 85964.
Corbetts (Scotland, 2,500ft and above and below 3,000ft in height with 500ft minimum drop):
Beinn na h-Uamha 762.4m summit height at NM 91719 66415.
Simms (Britain, 600m minimum height with 30m minimum drop):
Y Gribin 600.4m summit height at SH 84356 17712.
Foel Lwyd 600.1m summit height at SH 72040 72326.
Hewitts (England and Wales, 2,000ft minimum height with 30m minimum drop):
Nuttalls (Englland and Wales, 2,000ft Minimum height with 15m minimum drop):
Bloodybush Edge 609.8m summit height at NT 90223 14342.
Thack Moor 609.65m summit height at NY 61166 46278.
Calf Top 609.61m summit height at SD 66450 85624.
Tal y Fan 610.0m summit height at SH 72940 72652.
Mynydd Graig Goch 609.75m summit height at SH 49732 48518.
|Tal y Fan|
Deweys (England, Wales and Isle of Man, 500m and above and below 609.6m (2,oooft) in height with 30m minimum drop):
Ffridd yr Allt Llwyd 500.4m summit height at SH 79724 29615.
Y Pedwarau (Wales, 400m and above and below 500m in height with 30m minimum drop):
Craig y Dduallt 400.3m (converted to OSGM15) summit height at SJ 23280 40062.
Moel Tywysog 400.2m (converted to OSGM15) summit height at SH 98480 65714.
Mynydd Deulyn 400.1m (converted to OSGM15) summit height at SH 75936 61265.
|The summit cairn atop Mynydd Deulyn|
The above only take in a small percentage of the listings that have been compiled, and there are other listings that use a minimum height in their criteria, such as the Grahams and The Fours, but there are no hills that have been accurately surveyed with GPS/GNSS receiver within these listings that would be affected. However, there are also a multitude of hills in all sorts of listings that are near to threshold minimum heights that have not yet been accurately surveyed by Differential GPS. Therefore the overall changes within hill listings caused by a change and updating by Ordnance Survey of their datum point would be dramatic.
Myrddyn Phillips (October 2016)
The Times website
A related article published on The Guardian website
A related article published on The Telegraph website
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A related article published on the WalesOnline website
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A related article published on The Mail Online website