Geirth Mawr (SH 535 393) – 30-99m Twmpau reclassified to 30-99m Sub-Twmpau
There has been confirmation of a reclassification 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 Geirth Mawr (SH 535 393) |
The criteria for the list that this reclassification
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 listed by is Geirth Mawr, and it is adjoined to the Moel Hebog 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 A497 road to its north
and minor roads to its west and east, and has the town of Porthmadog towards
the east south-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 included
in the Hills to be surveyed sub list,
as it was considered not to meet the criteria then used for the main P30 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 30m of drop, with an
estimated c 58m summit height, based on interpolation of its uppermost 50m ring
contour, and a 28m bwlch spot height that appears on the Ordnance Survey
1:25000 Explorer map.
Extract from the Ordnance Survey 1:25,000 Explorer map |
Since the original publication of the Welsh P30
lists on Geoff Crowder’s v-g.me website there have been a number of maps made
available online. Some of these are
historic such as the series of Six-Inch maps on the National Library of
Scotland website. Whilst others were digitally
updated such as the Ordnance Survey Vector Map Local that was hosted on the
Geograph website and which was entitled the Interactive Coverage Map, whilst
others are current and digitally updated such as the interactive mapping on the
Magic Maps and WalkLakes websites.
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 56m
summit spot height and when coupled with the 28m bwlch spot height, these
values gave this hill 28m of drop and therefore it was reclassified back to 30-99m
Sub-Twmpau status.
One of the mapping resources now available online
is the WalkLakes website which hosts an interactive map originated from the
Ordnance Survey Open Data programme.
This map has many spot heights not on other publicly available maps and for
this hill it has a 57m summit spot height which when coupled with the 28m bwlch
spot height would give this hill 29m of drop.
Extract from the interactive mapping hosted on the WalkLakes website |
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 to 30-99m Sub-Twmpau status is due to LIDAR analysis, resulting in
a 57.4m summit height and a 27.9m bwlch height, with these values giving this
hill 29.5m of drop, which is insufficient for it to be classified as a 30-99m Twmpau.
The full details for the hill are:
Group: Moel Hebog
Name: Geirth Mawr
OS 1:50,000 map: 124
Summit Height: 57.4m (LIDAR)
Summit Grid Reference: SH 53511 39363 (LIDAR)
Bwlch Height: 27.9m (LIDAR)
Bwlch Grid Reference: SH 53302 39651 & SH 53311 39649 (LIDAR)
Drop: 29.5m (LIDAR)
Myrddyn Phillips (May
2024)
No comments:
Post a Comment