Fegla Fach (SH 638 153) – Dominant
addition
There has been an addition to the listing of Y Trechol – The Dominant Hills of Wales due to LIDAR analysis, an on-site visit and a survey with the Trimble GeoXH 6000 which took place on the 17th May 2018, with the criteria for this list being:
Y Trechol – The Dominant Hills of Wales - Welsh P30 hills whose prominence equal or exceed
half that of their absolute height.
This hill did not appear in the accompanying Hills to be surveyed sub list when the
original Welsh P30 hills were published on Geoff Crowder’s v-g.me website as it
did not meet the criteria for this sub category; however this sub list has now
been standardised including the addition of interpolated drop values and heights.
Prior to LIDAR analysis, an on-site visit and the
Trimble GeoXH 6000 survey this hill was listed with 25m of drop based on the
28m summit spot height that appears on the Ordnance Survey 1:25,000 Explorer
map and the 3m spot height that appears on the area of this hill’s bwlch on the
Ordnance Survey Interactive Coverage Map hosted on the Geograph website.
Extract from the Ordnance Survey Interactive Coverage Map hosted on the Geograph website |
The name of the hill is Fegla
Fach and it is adjoined to the Cadair Idris group of hills, which are situated in the south-western
part of North Wales (Region A, Sub-Region A3), with the hill being positioned
between the Afon Mawddach to its west and north and the A 493 road to its
south-east, and has the village of Y Friog (Fairbourne) to the south-west.
Fegla Fach from across the Afon Mawddach |
Fegla Fach from its connecting bwlch |
If wanting to visit the hill
permission to do so should be sought as it is not a part of designated open
access land, for those wishing to do so the minor road to the south of the hill
joins a public footpath that then heads toward it.
The addition of Fegla Fach to Dominant status is in part due to LIDAR analysis conducted by
Myrddyn Phillips. The LIDAR (Light
Detection & Ranging) technique produced highly accurate height data that is
now freely available for much of England and Wales.
The 1m DTM LIDAR analysis gives the hill the
following details:
Fegla Fach
Summit Height: 31.5m
Summit Grid Reference:
SH 63818 15311
Bwlch Height: 1.6m
Bwlch Grid Reference:
SH 63691 15131 and SH 69693 15130
Drop: 29.9m
This hill and others in the vicinity used to be
tidal islands before sea defences were constructed, with the Afon Mawddach
positioned to the immediate north, west and east of the hill, this has resulted
in this hill now being landlocked by its connecting bwlch which is a part of
the larger Arthog Bog.
The area of the connecting bwlch of this hill is
complicated to analyse via LIDAR as it is low lying and the land has been
terraformed with embankments and drainage ditches. However, extensive LIDAR analysis pinpoints
the position of what can be construed as this hill’s remaining natural bwlch to
be on or just below what is now a small land bridge.
LIDAR image of Fegla Fach (top right corner) and the land making up the Arthog Bog |
This land bridge crosses one of many drainage
ditches in the Arthog Bog and this position was surveyed using the Trimble
GeoXH 6000, with three measurement offsets noted, the first to ground making up
the land bridge, the second to water level in the drainage ditch and the third
to ground mid-way between the land bridge and the water level, the survey
results appear below:
1st survey, land bridge: 1.577m
2nd survey, water level in drainage
ditch: 1.337m
3rd survey, mid-point between land
bridge and water level: 1.457m
The Trimble GeoXH 6000 gathering data at the bwlch of Fegla Fach |
When these survey results are combined with the
LIDAR result for the summit of this hill the drop values for each of the three
points at the bwlch are:
1st survey, land bridge: 29.912m
2nd survey, water level in drainage
ditch: 30.152m
3rd survey, mid-point between land
bridge and water level: 30.032m
The summit of this hill was analysed using the 1m
DTM LIDAR model and as the high point consists of a large rock with a number of
trees growing directly beside it, it is probable that the LIDAR technique did
not model the very highest part of this rock, and when coupled with the result
produced by the Trimble bwlch survey it is deemed sufficient to list this hill
with a drop value of 30m, which when coupled with the summit height produced by
LIDAR analysis gives this hill 95.37% Dominance.
The summit of Fegla Fach |
The full details for the hill are:
Group: Cadair Idris
Name: Fegla Fach
Dominance: 95.37%
OS 1:50,000 map: 124
Summit Grid
Reference: SH 63818 15311 (LIDAR)
Summit Height: 31.5m
(LIDAR)
Drop Summit to Bwlch:
30.0m (LIDAR summit and Trimble bwlch)
Drop Bwlch to ODN: 1.5m
(LIDAR summit and Trimble bwlch)
Myrddyn Phillips (July 2018)
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