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) |
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 |
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 |
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)
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