Mapping Mountains – Hill Reclassifications – Humps

Pen y Graig-fawr (SN 773 996) – Recommended Subhump deletion

This is the thirtieth in a series of Hill Reclassification posts that detail hills whose status has either been altered in the listing of the Humps (HUndred Metre Prominences) through map study and / or surveys that I have instigated, or it is the recommendation that their status is altered.

Pen y Graig-fawr (SN 773 996)

Many preceding posts detailing these alterations to the Humps are retrospective as these hill reclassifications were either initiated from studying the Ordnance Survey enlarged mapping that is hosted on the Geograph website, or initiated from a survey with the Trimble GeoXH 6000, and for the recommended reclassification that affects the Humps the survey of this hill took place on the 29.10.17.

The listing of Humps was published in book format by Lulu in 2009 and entitled More Relative Hills of Britain, its author; Mark Jackson gives credit to a number of people who contributed toward the formation of this list, these include; Eric Yeaman, Alan Dawson, Clem Clements, Rob Woodall, Bernie Hughes, Pete Ridges and others.  When the list was published in book format there were 2987 Humps listed with their criteria being any British hill that has 100m or more of drop, accompanying the main list is a sub category entitled Subhumps, with the criteria being any British hill that has 90m or more and below 100m of drop.

More Relative Hills of Britain by Mark Jackson

The details for the recommended reclassification appear below:

There has been a recommended reclassification to the listing of the Humps (HUndred Metre Prominences) instigated by a survey with the Trimble GeoXH 6000 which took place on the 29.10.17.

Mark Jackson lists this hill with 99m of drop based on the 217m summit spot height given to a triangulation pillar that appears on the Ordnance Survey 1:50,000 Landranger and 1:25,000 Explorer map and an estimated bwlch height of 118m based on bwlch contouring at 10m intervals and between 120m – 130m. 

The height of the above estimated bwlch is at odds with Ordnance Survey contouring.

Interpolation of 10m bwlch contouring suggests a height of c 217m, giving this hill the minimum of c 90m of drop for continued Subhump status.  However, OS Maps which has recently replaced OS Get-a-map gives bwlch contouring at 5m intervals and between 125m – 130m, with interpolation suggesting a height of c 128m, which would give this hill c 89m of drop.

Pen y Graig-fawr also has an adjacent summit named Copa Shon (SN 781 993) of equal 217m map height, with this summit height appearing on the Ordnance Survey 1:50,000 Landranger map and the enlarged mapping that is hosted on the Geograph website.

Copa Shon (SN 781 993)

These two equal map heighted summits are adjoined to the Pumlumon group of hills and are situated between the A 489 road to the north and the Afon Dulas to the south, and they are positioned with the town of Machynlleth to the west north-west and the small community of Penegoes towards the north.

These two hills were surveyed using a Trimble GeoXH 6000 by Myrddyn Phillips on the 29.10.17 resulting in a 216.833m (converted to OSGM15) summit height for Pen y Graig-fawr and a 217.336m (converted to OSGM15) summit height for Copa Shon, therefore as Copa Shon is higher than Pen y Graig-fawr their respective critical bylchau are swapped.  Each bwlch was surveyed using the Trimble GeoXH 6000 with the following results:

127.885m (converted to OSGM15) at SN 80033 98740

182.393m (converted to OSGM15) at SN 77600 99583

Gathering data with the Trimble GeoXH 6000 at the summit of Pen y Graig-fawr

Gathering data with the Trimble GeoXH 6000 at the summit of Copa Shon

Gathering data at the critical bwlch

The bwlch positioned at SN 800 987 is that for the higher of these two summits and therefore as Pen y Graig-fawr is lower than Copa Shon it is recommended that its Subhump status is swapped to the higher summit, but as Copa Shon only has 89.5m of drop it is recommended that the status of Subhump is deleted.

The full details for the hill listed as a Subhump are:

Name:  Pen y Graig-fawr

Summit Height:  216.8m (converted to OSGM15) (Copa Shon is higher)

OS 1:50,000 map:  135

OS 1:25,000 map:  23, 215

Summit Grid Reference:  SN 773 996 (as listed in the Humps)

Drop:  99m (as listed in the Humps) (surveyed as having only 34.4m of drop with Copa Shon having 89.5m of drop)

Myrddyn Phillips (January 2018)