Mapping Mountains – Hill Reclassifications – 30-99m Twmpau

 

Pt. 27.4m (SH 319 764) – Double Sub-Twmpau addition 

There has been confirmation of an addition to the list of Double Sub-Twmpau adjoined to the 30-99m 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 Pt. 27.4m (SH 319 764)

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

30-99m Twmpau - Welsh hills at or above 30m and below 100m in height with 30m minimum drop, this height band of hills has two accompanying sub lists, the first of which is 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, and the second sub category is entitled the Double Sub-Twmpau with the criteria for this sub category being all Welsh hills at or above 20m and below 30m 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 hill is being listed by the point (Pt. 27.4) notation as an appropriate name for it either through local enquiry and / or historic research has not been found by the author, 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 is positioned with the coast to its west, minor roads to its north-west and south-east and the A55 and A5 roads to its north-east, and has the village of Llanfihangel-yn-Nhywyn towards the north. 

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. 

When 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 with an estimated c 20m of drop, based on the 28m summit spot height that appears on the Ordnance Survey 1:25,000 Explorer map and an estimated c 8m bwlch height based on interpolation of 5m contouring between 5m – 10m that appear on the Ordnance Survey 1:25,000 Explorer map. 

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. 

Therefore, the confirmation of the addition of this hill to Double Sub-Twmpau status is due to LIDAR analysis, resulting in a 27.4m summit height and a 5.0m bwlch height, with these values giving this hill 22.35m of drop, which is sufficient for it to be classified as a Double Sub-Twmpau. 

 

The full details for the hill are: 

Group:  Ynys Môn

Name:  Pt. 27.4m

OS 1:50,000 map:  114

Summit Height:  27.4m (LIDAR)

Summit Grid Reference:  SH 31929 76427 (LIDAR)

Bwlch Height:  5.0m (LIDAR)

Bwlch Grid Reference:  SH 31835 76966 (LIDAR) 

Drop:  22.35m (LIDAR)

 

Myrddyn Phillips (August 2021)