Sunday, 9 October 2016

Prospective Ordnance Survey Datum Change and the effect on Mountain Heights


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.

 
The Leica set-up position on Beinn Teallach, sometimes it can get a wee bit windy on these Munros!


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.

Alan Dawson beside the Leica RX1250 at the summit of Y Gribin


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.


Thack Moor

Calf Top

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.


Jo Barnard beside the Leica equipment on the summit of Ffridd yr Allt Llwyd


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 of Moel Tywysog
 
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

A related article published on The Express website

A related article published on the WalesOnline website

A related article published on The Courier Mail website 

A related article published on The Daily Post website

A related article published on The Mail Online website



2 comments:

Chris Watson said...

Dramatic? Come, come, Myrddyn. Only one of the major lists you have analysed loses more than 0.5% of it's total (Nuttalls) - that's not statistically significant, is it ;-)

Myrddyn said...

Depends upon one's personal viewpoint, when coupled with the hills that are marginal that have not yet been accurately surveyed by GPS/GNSS receiver it could be argued that the effect will be dramatic. Another way to view this is by the overall effect such a datum point change would have on the combined listings that use minimum height within their qualification, by this analogy I think it would be dramatic.