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 – Craig Berwyn

26.06.15  Pt. 417.6m (SJ 214 396) and Craig y Dduallt (SJ 232 400)  

Craig y Dduallt (SJ 232 400)

The hills of the eastern Berwyn sweep down abruptly to the flatlands of the Shropshire plain, these eastern hills are high pasture with a feeling of openness and tranquillity; it was these hills that Mark had suggested for our latest evening walk.

We met in Llangollen and quickly moved to a pub/restaurant overlooking the river as it flowed through the town.  A mother duck and her ducklings were quaintly paddling in adjacent rock pools as the sun cast down from the western sky.  A heron flapped down to a small island in the river and proceeded to settle, extending its wings occasionally.

The pub was busy with early Friday evening activity; many people were eating, including ourselves.  It’s one of the delights of these evening walks to take in a good meal before or after the walk, and with daylight hours now extending to around 10.00pm it meant that we could take in a meal before venturing up to the hills.

Mark relaxing in the sunshine waiting for his nosh

Mmmmmmmm yummy, yummy - nosh time

Rrrrrrrrrmmmmmmmmmmmm this was stunningly yummy

I had a curry to start followed by a rather scrumptious vanilla cheesecake with rhubarb and ice cream with a delicate flavouring of ginger, it was all yummy, but the cheesecake was to die for.

Once heartily fed we set our sights on the hills, we planned to visit two and survey each bylchau and summit, one is listed in Y Pedwarau as having 30m of drop, whilst the other has a small uppermost 400m ring contour at its summit, so there was a possibility of losing two Pedwarau today.

We headed out of Llangollen on a minor road as it slowly gained height before leaving it to join the Allt y Badi, this was the old road that crossed the hills, now it is beautifully enclosed and rock strewn and gives a glimpse into times gone past.

Walking up the Allt y Badi

The Allt y Badi was the old road crossing the hills to the south of Llangollen

As the Allt y Badi gained height it became more enclosed with a canopy of summer growth shielding the evening sun.  Eventually the steepness and enclosed nature of the old road gave way to the paved section of a minor road next to a farm house.  This minor road led us to the first bwlch that needed surveying.

As Mark waited patiently on the lane I scampered into a field, over a fence and started assessing the lay of the land for Trimble placement.  During my scampering a tractor had appeared chugging up the lane and I was now in full view to its occupant, thankfully he was immersed in conversation on his mobile phone and didn’t seem to see me. 

As the Trimble gathered its data the evening colour started ebbing down to the illuminated hour when the richness of colour wants to scream out as it pierces the land.  Once five minutes of data were collected I quickly packed the Trimble away and scampered back to rejoin Mark on the lane.

Gathering data at the first bwlch

Hiding in the grass

After surveying the bwlch we walked directly east following the lane to a gate which gave access to the summit of our first hill, this hill is listed under the point (Pt.) notation in Y Pedwarau as no appropriate name could be found for it when the 1^st edition was being compiled.  We have now found an appropriate name for the hill and this will appear in the 2^nd edition when it is published by Europeaklist which is planned for 2016.

By the time we were walking up the field to the high point of the hill the evening colour illuminated the greens of field, with delicate evening cloud out to the west.  A breeze blew which swayed the long grass in adjacent fields and gave the impression of movement across the land.

Heading to the summit of the first hill

The high point of the hill proved to be a small mound close to a rusted iron fence, once Trimbled we walked back to the lane.  There is another potential high point to this hill which is situated further east, we judged this to be lower and as it was situated in the middle of an arable crop we thought it unwise to Trimble it tonight.

Gathering data at the summit of Pt. 417.6m

As we continued on the lane walking eastward toward the bwlch of our second hill I noticed a sundog in the sky casting its radiance out away from the sun.  These halos are formed when light interacts with ice crystals in the atmosphere and can be best seem when the sun is close to the horizon.

The sundog remained in view as we walked toward Craig y Dduallt

Our last view of the sundog

I had noted two possible positions for the next bwlch, we decided to survey this after visiting its accompanying summit, and this would also give us an opportunity to assess the lay of land at the bwlch from two directions.

Approaching the bwlch of Craig y Dduallt with the summit of the hill just beyond the masts

The two masts near the summit of Craig y Dduallt had been prominent on our easterly horizon during much of the walk and it was now time to head upto them.  The Ordnance Survey map gives the summit area of this hill as having two small 400m uppermost contour rings, with the two masts between them.  I had previously visited this hill in July 2003 and noted that the easterly, smaller of the two ring contours was the obvious higher point and the summit of the hill.  For completeness we planned on surveying the two, but once at the top of the easterly one we dismissed the westerly one as obviously lower.

Gathering data from the summit of Craig y Dduallt

By now dusk had set and the sun had sunk, with the first glimpse of pinkness in the sky appearing.  Once the summit of Craig y Dduallt had been Trimbled we walked back down the lane to the bwlch, Mark waited beside a gate whilst I headed into the field where the bwlch lay.  Within a few minutes I had estimated where the critical bwlch was positioned and Mark then directed me from his vantage point to the place where I positioned for Trimble. 

Gathering data from the bwlch of Craig y Dduallt

It was now dusk and the Trimble was submerged in long grass in a field, thankfully my route through the grass was evident and I retraced my footsteps back to the Trimble once it had gathered five minutes of data.  All that remained was to join the lane downward toward the lights of Llangollen.  We arrived back at Mark’s car at 10.45pm, it had been another excellent evening’s walk.

Survey Result:

Pt. 417.6m

Summit Height:  417.6m (converted to OSGM15)

Summit Grid Reference:  SJ 21464 39666 (summit relocation)

Bwlch Height:  385.7m (converted to OSGM15)

Bwlch Grid Reference:  SJ 20387 39418

Drop:  31.9m

Dominance:  7.63%

Craig y Dduallt

Summit Height:  400.3m (converted to OSGM15)

Summit Grid Reference:  SJ 23280 40062

Bwlch Height:  364.8m (converted to OSGM15)

Bwlch Grid Reference:  SJ 22554 39745

Drop:  35.5m

Dominance:  8.87%

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