Mapping Mountains – Significant Height Revisions – 30-99m Twmpau and Y Trechol – The Dominant Hills of Wales

 

Cefn Coch (SH 610 396) 

There has been a Significant Height Revision to a hill that is listed in the 30-99m Twmpau and Y Trechol – The Dominant Hills of Wales, with the summit height, bwlch height and their locations, the drop, dominance and status of the hill derived from LIDAR analysis conducted by Myrddyn Phillips. 

LIDAR image of Cefn Coch (SH 610 396)

The criteria for the two listings that this height revision applies to are:

30-99m Twmpau - Welsh hills at or above 30m and below 100m in height with 30m minimum drop, with an accompanying sub list entitled the 30-99m Sub-Twmpau with the criteria for this sub category being all Welsh hills at or above 30m and below 100m in height with 20m or more and below 30m of drop, with the word Twmpau being an acronym standing for thirty welsh metre prominences and upward. 

The 30-99m Twmpau by Myrddyn Phillips

Y Trechol - The Dominant Hills of Wales - Welsh P30 hills whose prominence  equal or exceed half that of their absolute height.  With the criteria for Lesser Dominant status being those additional Welsh P30 hills whose prominence is between one third and half that of their absolute height, with the Introduction to the Mapping Mountains publication of this list appearing on the 3^rd December 2015, and the list is now available in its entirety on Mapping Mountains in Google Doc format. 

Y Trechol - The Dominant Hills of Wales by Myrddyn Phillips

The name the hill is now listed by is Cefn Coch 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 it is positioned with the A498 road and the B4410 road to its north-west, the A497 road and the A487 road to its south-west and the A4085 road to its east, and has the town of Penrhyndeudraeth towards the south. 

When the original 30-99m height band of Welsh P30 hills were published on Geoff Crowder’s v-g.me website, this hill was included in the Hills to be surveyed sub list, as it was considered not to meet the criteria then used for the P30 category, and it was listed with an 81m summit height, with an accompanying note stating; 86m on 1984 1:50,000 map. 

When the sub list was standardised, and interpolated heights and drop values also included the details for this hill were re-evaluated and it was listed with 33m of drop and 40.74% dominance, based on the 81m summit spot height and the 48m bwlch spot height that appear on the Ordnance Survey 1:25,000 Explorer map. 

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

The details for this hill were subsequently re-assessed as the bwlch contouring on the Ordnance Survey 1:25,000 Explorer map is between 50m – 60m, therefore according to map contouring the 48m spot height is below the position of the bwlch.  This reassessment resulted in the hill being listed with an estimated c 26m of drop, based on the 81m summit spot height and an estimated c 55m bwlch height. 

The details for this hill were later reassessed and the 86m summit spot height that appears on the Ordnance Survey 1:50,000 Landranger map was favoured over that of the 81m spot height, and with the bwlch height amended to an estimated c 54m, these values gave this hill an estimated c 32m of drop and 37.21% dominance. 

Extract from the Ordnance Survey 1:50,000 Landranger map

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. 

LIDAR summit image of Cefn Coch (SH 610 396)

The summit height produced by LIDAR analysis is 86.6m and this comes within the parameters of the Significant Height Revisions used within this page heading, these parameters are: 

The term Significant Height Revisions applies to any listed hill whose interpolated height and Ordnance Survey or Harvey map summit spot height has a 2m or more discrepancy when compared to the survey result produced by the Trimble GeoXH 6000 or analysis of data produced via LIDAR, also included are hills whose summit map data is missing an uppermost ring contour when compared to the data produced by the Trimble or by LIDAR analysis. 

Therefore, the new listed summit height of this hill is 86.6m and this was derived from LIDAR analysis, this is 5.6m higher than the originally listed summit height of 81m which appears as a spot height on the Ordnance Survey 1:25,000 Explorer maps.

 

ills of Wales, and are reproduced below@

The full details for the hill are:

Group:  Moelwynion 

Name:  Cefn Coch 

OS 1:50,000 map:  124

Summit Height (New Height):  86.6m (LIDAR) 

Summit Grid Reference:  SH 61030 39620 (LIDAR)               

Bwlch Height:  51.5m (LIDAR) 

Bwlch Grid Reference:  SH 61461 39763 (LIDAR) 

Drop:  35.1m (LIDAR) 

Dominance:  40.51% (LIDAR) 

 

Myrddyn Phillips (December 2021)