Lan (SN 972 529) – 200m Sub-Twmpau reclassified to 200m
Twmpau
There has been confirmation of a reclassification
to the list of 200m 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.
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| LIDAR image of Lan (SN 972 529) |
The criteria for the list that this
reclassification applies to are:
200m Twmpau
– Welsh hills at or above
200m and below 300m in height that have 30m minimum drop, with an accompanying sub list entitled the 200m
Sub-Twmpau with the criteria for this sub category being all Welsh hills at or
above 200m and below 300m 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 200m Twmpau by Myrddyn Phillips |
The name the hill is listed by is Lan and this was
derived from the Tithe map, and it is adjoined to the Drygarn Fawr group
of hills, which are situated in
the central part of the South Wales Region (Region B, Sub-Region B1), and it is positioned encircled by minor
roads, with the B4358 road farther to its north-west and the A483 road farther
to its south, and has the town of Llanfair-ym-Muallt (Builth Wells) towards the
east south-east.
When the original 200m height band of Welsh P30 hills were published on Geoff Crowder’s v-g.me website, this hill was included
in the Hills to be surveyed sub list
that accompanied the main P30 list, as it was considered not to meet the
criteria then used for the main P30 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 27m of drop, based on an
estimated c 284m summit height and an estimated c 257m bwlch height, with both
heights based on interpolation of 10m contouring that appear on the Ordnance
Survey 1:25,000 Explorer map.
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| Extract from the Ordnance Survey 1:25,000 Explorer map |
The details for this hill were re-assessed when
the Ordnance Survey Vector Map Local hosted on the Geograph website and which
was entitled the Interactive Coverage Map became available online. This mapping had many spot heights not on
other publicly available Ordnance Survey maps and for this hill it had a 252m
spot height on the area of the bwlch, and when coupled with a reassessment of
the estimated summit height of this hill of c 285m, these values gave this hill
an estimated c 33m of drop.
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 reclassification
of this hill from 200m Sub-Twmpau status is due to LIDAR analysis, resulting in
a 284.9m summit height and a 251.2m bwlch height, with these values giving this
hill 33.7m of drop, which is sufficient for it to be classified as a 200m
Twmpau.
The full details for the
hill are:
Group: Elenydd
Name: Lan
OS 1:50,000 map: 147
Summit Height: 284.9m (LIDAR)
Summit Grid
Reference: SN 97265 52939 (LIDAR)
Bwlch Height: 251.2m (LIDAR)
Bwlch Grid
Reference: SN 97057 53035 (LIDAR)
Drop: 33.7m (LIDAR)
Myrddyn Phillips
(January 2022)