Mapping Mountains – Hill Reclassifications – 30-99m Twmpau

Allt Fawr (SH 346 685) – 30-99m Sub-Twmpau addition 

There has been confirmation of an addition to the list of 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 bwlch image of Allt Fawr (SH 346 685)

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 that have 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 Allt Fawr 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 the coast to its west, a minor road to its immediate east and the A4080 road to its north-east, and has the village of Aberffraw towards the east north-east. 

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 24m of drop, based on the 42m summit spot height adjoined to a triangulation pillar and an estimated c 18m bwlch height, based on interpolation of 5m contouring between 15m – 20m that appears 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 30-99m Sub-Twmpau status is due to detail on contemporary maps produced from Ordnance Survey data and LIDAR analysis, resulting in a 42m summit height and a 17.0m bwlch height, with these values giving this hill 25m of drop, which is sufficient for it to be classified as a 30-99m Sub-Twmpau. 

 

The full details for the hill are: 

Group:  Ynys Môn 

Name:  Allt Fawr 

OS 1:50,000 map:  114

Summit Height:  42m (triangulation pillar) 

Summit Grid Reference:  SH 34639 68534 (triangulation pillar)               

Bwlch Height:  17.0m (LIDAR) 

Bwlch Grid Reference:  SH 34974 69389 (LIDAR) 

Drop:  25m (triangulation pillar summit and LIDAR bwlch) 

 

Myrddyn Phillips (September 2021)