Mapping Mountains – Summit Relocations – 100m Twmpau

Coed Penllyn (SH 786 696)

There has been a Summit Relocation to a hill that is listed in the 100m Twmpau, with the summit height, bwlch height and their locations, the drop and status of the hill derived from LIDAR analysis conducted by Myrddyn Phillips. 

LIDAR image of Coed Penllyn (SH 786 696)

The criteria for the list that this summit relocation applies to are:

100m Twmpau - Welsh hills at or above 100m and below 200m in height that have 30m minimum drop, with an accompanying sub list entitled the 100m Sub-Twmpau, with the criteria for this sub category being all Welsh hills at or above 100m and below 200m in height with 20m or more and below 30m of drop.  The list is authored by Myrddyn Phillips, with the word Twmpau being an acronym standing for thirty welsh metre prominences and upward. 

100m Twmpau by Myrddyn Phillips

The name the hill is now listed by is Coed Penllyn, and this was derived from the Ordnance Survey Vector Map Local hosted on the Geograph website and which was entitled the Interactive Coverage Map, and it is adjoined to the Mynydd Hiraethog group of hills, which are situated in the northern part of North Wales (Region A, Sub-Region A2), and it is positioned with the A470 road to its west and a minor road to its east, and has the town of Llanrwst towards the south.

When the original 100m 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 main P30 list, and listed with a 193m summit height positioned at SH 786 697 with an accompanying note stating; Height from 1989 1:50000 map. 

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

After the sub list was standardised, and interpolated heights and drop values also included the details for this hill were re-assessed and it was listed as a twin top with an estimated c 25m of drop, based on the 193m spot height that appears on the 1:50,000 Landranger map and a twin 193m map heighted top positioned at SH 78713 69823 that appeared on the Ordnance Survey Vector Map Local hosted on the Geograph website and which was entitled the Interactive Coverage Map and an estimated c 168m bwlch height, based on interpolation of 10m contouring between 160m – 170m. 

Extract from the Ordnance Survey 1:25,000 Explorer 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 Coed Penllyn (SH 786 696)

LIDAR analysis gives the highest ground on this hill as 198.0m positioned at SH 78630 69665, and compared to its previously listed twin summit position this comes within the parameters of the Summit Relocations used within this page heading, these parameters are:

The term Summit Relocations applies when the high point of the hill is found to be positioned; in a different field, to a different feature such as in a conifer plantation,  within a different map contour either on Ordnance Survey maps or interactive mapping, to a different point where a number of potential summit positions are within close proximity, when natural ground or the natural and intact summit of a hill is confirmed compared to a higher point such as a raised field boundary or covered reservoir that is considered a relatively recent man-made construct, or the listing of a new twin summit or de-twinning of a summit, or a relocation of approximately 100 metres or more in distance from either the position of a map spot height or from where the summit of the hill was previously thought to exist.

Therefore, the height produced by LIDAR analysis to the summit of this hill is 198.0m and is positioned at SH 78630 69665, this position is close to the spot height that appears on the Ordnance Survey 1:50,000 Landranger map and approximately 158 metres south-westward from where the twin map heighted spot height appeared on the Ordnance Survey Vector Map Local hosted on the Geograph website and which was entitled the Interactive Coverage Map. 

 

The full details for the hill are: 

Group:  Mynydd Hiraethog 

Name:  Coed Penllyn 

OS 1:50,000 map:  115

Summit Height:  198.0m (LIDAR) 

Summit Grid Reference (New Position):  SH 78630 69665 (LIDAR) 

Bwlch Height:  170.9m (LIDAR) 

Bwlch Grid Reference:  SH 78892 70134 (LIDAR) 

Drop:  27.1m (LIDAR)

Myrddyn Phillips (April 2026)