Mapping Mountains – Summit Relocations – 30-99m Twmpau

Bwlch y Fen & Tyddyn Gwyn (SH 417 783) 

There has been a Summit Relocation to a hill that is listed in the 30-99m Twmpau, with the summit height, bwlch height and their locations, the drop and status of the hill derived from detail on contemporary maps produced from Ordnance Survey data and LIDAR analysis conducted by Myrddyn Phillips. 

LIDAR image of Bwlch y Fen & Tyddyn Gwyn (SH 417 783)

The criteria for the list that this summit relocation 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

The name the hill is listed by is Bwlch y Fen & Tyddyn Gwyn and this was derived from the Tithe map, and it is adjoined to the Ynys Môn group of hills, which are situated in the north-western part of North Wales (Region A, Sub-Region A1), and it is positioned with minor roads to its north, west and east, and the B5109 road to its south, and has the village of Gwalchmai towards the south-west. 

When the original 30-99m height band of Welsh P30 hills were published on Geoff Crowder’s v-g.me website this hill was not included in the Hills to be surveyed sub list, as it was considered not to meet the criteria then used for this sub category. 

After 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 an estimated c 21m of drop, based on the 89m summit spot height positioned at SH 41706 78330 that appears on the Ordnance Survey 1:25,000 Explorer map and an estimated c 68m bwlch height based on interpolation of 5m contouring between 65m – 70m. 

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.  

The height produced by LIDAR analysis for the remaining natural summit is 88.150m and is positioned at SH 41738 78329, with LIDAR analysis giving a raised field boundary positioned at SH 41714 78319 a height of 88.169m, and this comes within the parameters of the Summit Relocations used within this page heading, these parameters are: 

The term Summit Relocations applies when the hill’s high point is found to be positioned; in a different field, within a different map contour, to a different feature such as in a conifer plantation, to a different point where a number of potential summit positions are within close proximity, 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, or when natural ground or the natural and intact summit is confirmed compared to a higher point such as a raised field boundary that is judged to be a relatively recent man-made construct. 

LIDAR summit image of Bwlch y Fen & Tyddyn Gwyn

Therefore, the summit height produced by LIDAR analysis is 88.15m and this is positioned at SH 41738 78329, this position is relatively close to where the 89m spot height appears on the contemporary Ordnance Survey 1:25,000 Explorer map and importantly to the remaining natural summit of this hill as opposed to the slightly higher raised field boundary. 

 

The full details for the hill are: 

Group:  Ynys Môn 

Name:  Bwlch y Fen & Tyddyn Gwyn 

OS 1:50,000 map:  114, 115

Summit Height:  88.15m (LIDAR) 

Summit Grid Reference (New Position):  SH 41738 78329 (LIDAR)               

Bwlch Height:  c 68m (interpolation) 

Bwlch Grid Reference:  SH 41140 80401 (interpolation) 

Drop:  c 20m (LIDAR summit and interpolated bwlch) 

 

Myrddyn Phillips (September 2021)