Pen y Bigil (SH 576 621) – Sub-Trichant reclassified to
Trichant
There has been confirmation of a reclassification
to a hill that is listed in the Y
Trichant, with the summit height, bwlch height and its location, and status
of the hill initially confirmed by LIDAR analysis and subsequently by a summit survey
with the Trimble GeoXH 6000 which were conducted by Myrddyn Phillips, with the
latter taking place on the 6th October 2018.
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| Pen y Bigil (SH 576 621) |
The criteria for the list that this hill
reclassification applies to are:
Y Trichant – Welsh hills at or above 300m and below 400m in
height that have 30m minimum drop, with an accompanying sub list entitled the Sub-Trichant with the criteria for this sub category being all
Welsh hills at or above 300m and below 400m in height with 20m or more and
below 30m of drop. The list is authored by Myrddyn Phillips, with the
Introduction to the list and the renaming of it appearing on Mapping Mountains
on the 13th May 2017.
The name of the hill is Pen y Bigil and it is
adjoined to the Glyderau group of hills, which are situated in the
north-western part of North Wales (Region A, Sub-Region A1), and it has the
A4244 road to its north-west and the A4086 road and Llyn Padarn to its south, and
has the villages of Deiniolen towards the north and Llanberis towards the south.
When the original 300m 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 thought not to meet the criteria then used for the main P30 list;
however this sub list has now been standardised and interpolated heights and
drop values also included in the main P30 and the accompanying sub list.
When this list was standardised and interpolated
heights also included this hill was listed with an estimated c 33m of drop,
based on an estimated c 326m summit height and an estimated c 293m bwlch
height, with each based on interpolation of 5m contouring on the contemporary
Ordnance Survey 1:25,000 Explorer map.
 |
| Extract from the Ordnance Survey 1:25,000 Explorer map |
The details for this hill were re-assessed when
the Ordnance Survey 1:25,000 historical map and the series of Six-Inch maps became
available online, the former has a 1081ft height and the latter has a 1080ft
height given to this hill’s summit, these heights equate to 329m in metric, and
when coupled with the c 293m estimated bwlch height gives this hill c 36m of
drop.
 |
| Extract from the Ordnance Survey series of Six-Inch maps |
 |
| Extract from the Ordnance Survey 1:25,000 historical map |
However, it was not until LIDAR became available
that the details for this hill were next re-assessed. The LIDAR (Light Detection &
Ranging) technique produced highly accurate height data that is now freely
available for much of England and Wales.
As the area of this hill’s bwlch has a road cutting running across it in
the valley to valley direction it was not until examining LIDAR that the
remaining natural bwch could be pinpointed.
 |
| LIDAR image of Pen y Bigil |
 |
| The road cutting splicing through the area of this hill's bwlch |
The confirmation of the reclassification of Pen y
Bigil to Y Trichant status is due to
LIDAR analysis and a subsequent survey with the Trimble GeoXH 6000 conducted by
Myrddyn Phillips, resulting in a 331.0m summit height and an 292.6m bwlch
height, with these values giving this hill 38.4m of drop which is sufficient
for it to be classified as a Trichant.
The full details for the hill are:
Group: Glyderau
Name: Pen y Bigil
OS 1:50,000 map: 114,
115
Summit Height: 331.0m (converted
to OSGM15)
Summit Grid Reference:
SH
57600 62119
Bwlch Height: 292.6m (LIDAR)
Bwlch Grid Reference: SH 58014 62043 (LIDAR)
Drop: 38.4m (Trimble summit and LIDAR bwlch)
Myrddyn Phillips (March 2019)
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