Yr Allt (SN 552 024) – 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 LIDAR analysis conducted by Myrddyn Phillips.
LIDAR image of Yr Allt (SN 552 024) |
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.
30-99m Twmpau by Myrddyn Phillips |
The name the hill is now listed by is Yr Allt and this was derived from the Tithe map, and it is adjoined to the Mynydd Sylen group of
hills, which are situated in the southern
part of South Wales (Region B, Sub-Region B5), and it is positioned with minor roads to its north and west, and
the A4138 road to its south, and has the town of Llanelli towards the west
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.
Extract from the Ordnance Survey 1:25,000 Explorer map |
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 26m of drop, based on the 97m summit spot
height and the 71m bwlch spot height that appeared on the Ordnance Survey
Vector Map Local hosted on the Geograph website and which was entitled the
Interactive Coverage 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 LIDAR analysis, resulting in a 96.8m
summit height and a 71.05m bwlch height, with these values giving this hill 25.75m
of drop, which is sufficient for it to be classified as a 30-99m
Sub-Twmpau.
The full details for the hill are:
Group: Mynydd Sylen
Name: Yr Allt
OS 1:50,000 map: 159
Summit Height: 96.8m (LIDAR)
Summit Grid Reference: SN 55279 02446 (LIDAR)
Bwlch Height: 71.05m (LIDAR)
Bwlch Grid Reference: SN 54918 02542 (LIDAR)
Drop: 25.75m (LIDAR)
Myrddyn Phillips (July 2024)
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