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 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, with confirmation from subsequent LIDAR analysis conducted by Aled Williams.

LIDAR image of Clogwyn Drain (SH 652 493)

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

Welsh Highlands – Uchafion Cymru – Welsh hills at or above 500m in height with 15m minimum drop, with an accompanying sub list entitled the Welsh Highland Subs, the criteria for which is all Welsh hills at or above 500m in height with 10m or more and below 15m of drop.  This list is authored by Myrddyn Phillips and Aled Williams with the Introduction to the list published on Mapping Mountains in November 2015 and the latest update relating to the list published on Mapping Mountains in January 2023.

Welsh Highlands - Uchafion Cymru by Aled Williams and Myrddyn Phillips

The name of the hill is Clogwyn Drain and it is adjoined to the Moelwyn Mawr group of hills, which are situated in the north-western part of North Wales (Region A, Sub-Region A2), 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 was entitled the Interactive Coverage Map became available online.  This mapping had many spot heights not on any other publicly available Ordnance Survey map and it gave 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

However, it was not until LIDAR became available that the details for this hill could be accurately re-assessed.  The LIDAR (Light Detection & Ranging) technique produced highly accurate height data that is now freely available for much of England and Wales. 

Therefore, the confirmation of the addition of this hill to Uchaf status is due to LIDAR analysis, resulting in a 508.6m summit height and a 490.2m bwlch height, with these values giving this hill 18.4m of drop, which is sufficient for it to be classified as an Uchaf.

The full details for the hill are:

Group:  Moelwyn Mawr

Name:  Clogwyn Drain

OS 1:50,000 map:  115

Summit Height:  508.6m (LIDAR)

Summit Grid Reference:  SH 65205 49362 (LIDAR)

Bwlch Height:  490.2m (LIDAR)

Bwlch Grid Reference:  SH 65234 49270 (LIDAR)

   

Drop:  18.4m (LIDAR) 

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

 Welsh Highland P15s

Welsh Highland Subs

Aled Williams and Myrddyn Phillips (January 2020)