Mapping Mountains – Hill Reclassifications – The Welsh Highlands – Uchafion Cymru

Clogwyn Drain (SH 652 493) – Uchaf addition

There has been an addition to the listing of The Welsh Highlands – Uchafion Cymru due to analysis of 5m contouring on the OS Maps website coupled with spot height data on the contemporary Ordnance Survey 1:50,000 Landranger and 1:25,000 Explorer map.

The criteria for the list that this addition applies to are:

The Welsh Highlands – Uchafion Cymru – This is the revised draft title for the Welsh 500m P15s list that takes in all hills in Wales at or above 500m in height with 15m minimum drop.  The list is co-authored by Myrddyn Phillips and Aled Williams with the introduction to the list being published on Mapping Mountains in November 2015 and an update relating to the list appearing on Mapping Mountains on the 19th June 2019.

The name of the hill is Clogwyn Drain and it is adjoined to the Moelwynion group of hills, which are situated in the north-western part of North Wales (Region A, Sub-Region A1), and is positioned with the A498 road to its north-west and the A470 road to its east, and has the village of Beddgelert towards the west south-west and the town of Blaenau Ffestiniog towards the south-east.

During the initial compilation of the original list of Welsh 500m P15s that later became known as Yr Uchafion and latterly as The Welsh Highlands – Uchafion Cymru, this hill was included as an Uchaf with 19m of drop based on a basic levelling survey conducted by Myrddyn Phillips on the 25^th May 2004. 

However, this survey was conducted at the end of a long day on the hill which involved many such surveys and as contemporary Ordnance Survey maps of the day gave a 508m summit spot height and bwlch contouring between 490m – 500m, it meant that if this survey and the summit spot height were accurate the height of the bwlch would be below that of its lower 490m contour.  Because of this the survey result was questioned and the hill taken out of the list.

Extract from the Ordnance Survey 1:25,000 Explorer map

The details for this hill were re-examined when the Ordnance Survey Vector Map Local hosted on the Geograph website and which is entitled the Interactive Coverage Map became available online.  This mapping has many spot heights not on any other publicly available Ordnance Survey map and gives a 498m spot height positioned at SH 65212 49275 on the bwlch area of this hill.  This spot height is also shown on Ordnance Survey data that appears on the Magic Maps website.  When coupled with the 508m summit spot height these values gave this hill 10m of drop and the questionable accuracy of the basic levelling survey seemed to be substantiated.

Extract from the Magic Maps website

The details for this hill were re-assessed when the OS Maps website became available online.  This is the replacement for OS Get-a-map and has contours at 5m intervals which are proving consistently more accurate compared to the 5m contours that sometimes appear on Ordnance Survey 1:25,000 Explorer maps and the online Vector Map Local.  This mapping has an uppermost 505m contour and bwlch contouring between 490m – 495m and places the position of the 498m spot height above the 495m contour line, indicating that the spot height is not placed at the critical point of the bwlch.  This re-assessment resulted in the hill being listed with an estimated c 15m of drop.

Extract from OS Maps

Therefore, the addition of this hill to Uchaf status is due to the 508m summit spot height that appears on contemporary 1:50,000 Landranger and 1:25,000 Explorer maps and an estimated bwlch height of c 493m based on 5m contouring on the OS Maps website between 490m – 495m, with these values giving this hill c 15m of drop which is sufficient for Uchaf status.

The full details for the hill are:

Group:  Moelwynion

Name:  Clogwyn Drain

OS 1:50,000 map:  115

Summit Height:  508m (spot height)

Summit Grid Reference:  SH 65200 49356 (spot height)

Bwlch Height:  c 493m (interpolation)

Bwlch Grid Reference:  SH 65239 49257 (interpolation)

   

Drop:  c 15m (spot height summit and interpolated bwlch) 

For the additions, deletions and reclassifications to The Welsh Highlands – Uchafion Cymru reported on Mapping Mountains please consult the following Change Registers:

The Welsh 500m P15s (Yr Uchafion)

The Welsh 500m P15s (Yr Uchafion) – 500m Subs

The Welsh 500m P15s (Yr Uchafion) – 490m Subs

The Welsh 500m P15s (Yr Uchafion) – Double Subs

Aled Williams and Myrddyn Phillips (January 2020)