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 72936 72648.

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

Mapping Mountains – Trimble Surveys – Mynydd Hiraethog

22.08.14  Moel Tywysog (SH 984 657)

Most evening’s just before sun set the hills can give up part of their beauty, when subtle accentuated deep colours wash over the landscape.  This period of time that the hill’s offer is all too brief and should be savoured as the illuminated colour with radiant greens and deep blacks of shadow is all too soon cast out. 

This evening atop Moel Tywysog in the hills of Mynydd Hiraethog the sun gave up part of this beauty as adjacent fields ran awash with colour and far off hills became grey silhouettes framed against a gentle sky, and all beside a hillside more reminiscent of a hay field with long beige grass swept aside leading up to a solitary trig atop mudstone.

The main reason for choosing Moel Tywysog as an evening wander was that the hill is conveniently placed next to a relatively high road and so access is made easy.  It is also conveniently placed in relation to Wrexham, where Mark had finished his daily chore of work and just eastward is Denbigh, where I met him for a fish supper in Morrisons café.  Another reason for visiting the hill is that it is given a map height of 400m with the flush bracket atop the trig being given a height of 400.668m in the OS Trig Database.  Therefore its Pedwar status had been questioned and Mark had done a reccie earlier in the year and examined the base of the trig which is built on local mudstone.  His verdict was that the base was definitely man-made.  Then, his visit was to a place with hillsides of close cropped green; whereas today the grass had grown and lost its luxuriant greenness, replaced with a yellowish tinge almost shining in the evening’s light.

Mark reccied the summit and found the trig built on a mud-stone base.  Photo: Mark Trengove

We set off for the summit of Moel Tywysog from a small gravelled area to the east where a tee junction of country lanes meet, the time was 6.45pm.  A gate gave us access to a vehicle track which headed uphill toward another gate and the higher part of the hill. 

Mark on the vehicle track that gave access to the higher part of the hill

As I walked up I looked around and took many photographs, some of Mark as he headed for the trig and others of the landscape which was now taking on that magical coloured radiance, to the east was the extended Clwyd ridge with Moel Fama and Foel Fenlli dominating the view.  To the west was Mynydd Tryfan with its three rocky tops being highlighted against the higher Eryri peaks that were cast in grey silhouette, all lined up with memories aplenty.

Heading for the trig

Mynydd Tryfan with the high peaks of Eryri as backdrop

To the north stood Moel Fodiar, a hill I have not yet visited, its upper hillsides of bracken and moor standing out against its lower reclaimed grazed fields of green, with the emeralds highlighted with evening colour.  Further to the west was what we had planned as our last hill for the evening; Bryn Trillyn, a 496m map heighted hill, now dark and foreboding showing an extended moorland top matched against lower pastoral fields of green.

Moel Fodiar with heather, fern and a patch of gorse contrasting the greens of lower fields

We examined the ground at the base of the trig from many angles and decided that the high point looked as if it was about one metre away from the mud-stone plinth, just on the eastern side of the trig.  As the Trimble quickly attained its 0.1m accuracy before data can be logged, Mark sat and watched the ever soothing view.

Mark enjoying the view

Once the ‘Log’ key was pressed I pottered around with camera in hand, taking photos of the trig as the richness of blue with western streaks of cloud met the tawny browns and yellows of swept grass, it was very fulfilling and all too soon over, as the illuminated colour for one evening had given itself and now the sun crept behind the western cloud and the colour dimmed, we also needed to press on toward Bryn Trillyn where we hoped to watch the sunset over the giants of Eryri.

Illuminated colour on the summit of Moel Tywysog

I took one or two quick photos of the Trimble position in relation to the trig and I then packed it away.  Before we headed down to the car we positioned an 18 inch wooden ruler flush with the top of the flush bracket, put a spirit level on the ruler and measured the height between the top of the flush bracket and the ground at the base of the mud-stone plinth with a steel tape.  This measurement came to 66.5cm, this ground is probably slightly lower when compared to the Trimble position, only a level and staff could determine by how much, this we didn’t have, but I’d guess the difference in height would be around 10cm.

The Trimble set-up position in relation to the trig

As we headed down the sky opened with streams of light cast out of a clouding sky.  The walk had only taken 45 minutes and much of that was setting up and then waiting for the Trimble to gather its data, as well as measuring the height from flush bracket to base of the trig plinthed mud-stone. 

Streams of light cast out of a clouding sky

The show of colour only ever lasts for 30 - 45 minutes or so, longer periods are rare, but if fortunate enough to experience the richness on offer, it is one aspect of the hills that can give up part of their beauty, it is to be savoured and appreciated.

   

Survey Result:

Moel Tywysog

Summit Height:  400.2m (converted to OSGM15, Trimble GeoXH 6000) 

Summit Grid Reference:  SH 98480 65714 (Trimble GeoXH 6000) 

Bwlch Height:  358m (spot height)

Bwlch Grid Reference:  SH 98077 65335 (spot height)

Drop:  42m (Trimble GeoXH 6000 summit and spot height bwlch)

Dominance:  10.50% (Trimble GeoXH 6000 summit and spot height bwlch)

  

For further details please consult the Trimble survey spreadsheet click {here}