Cerrig Llwydion (SN 909
731)
There has been confirmation of a Summit Relocation
to a hill that is listed in the Y Pedwarau
– The 400m Hills of Wales, 500m
Twmpau and The Welsh Highlands –
Uchafion Cymru, with the summit
height, bwlch height and their locations, the drop and status of the hill
confirmed by LIDAR analysis conducted by Aled Williams.
 |
| LIDAR image of Cerrig Llwydion (SN 909 731) |
The criteria for the three listings that this
summit relocation applies to are:
Y Pedwarau – The 400m Hills of Wales.
Welsh hills at or above 400m and below 500m in height that have 30m
minimum drop, accompanying the main Y Pedwarau list are five categories of sub
hills, with this hill now included in the 500m Sub-Pedwar category. The criteria for 500m Sub-Pedwar status being
all Welsh hills at or above 500m and below 510m in height that have 30m minimum
drop. The list is co-authored by Myrddyn Phillips and Aled Williams and is published on Mapping Mountains in Google Doc format.
500m Twmpau – Welsh hills at or above
500m and below 600m in height that have 30m minimum drop, with an accompanying
sub category entitled the 500m Sub-Twmpau consisting of all Welsh hills at or
above 500m and below 600m in height that have 20m or more and below 30m of
drop. With the word Twmpau being an
acronym standing for thirty welsh metre prominences and upward. The list is authored by Myrddyn Phillips and is published on Mapping Mountains in Google Doc format.
The Welsh Highlands –
Uchafion Cymru
– This is the revised draft title for the Welsh 500m P15s list that takes
in all hills in Wales at or above 500m in height with 15m minimum drop. The list is co-authored by Myrddyn Phillips
and Aled Williams with the Introduction to the list being published on Mapping Mountains in November 2015 and an update relating to the list appearing on Mapping Mountains on the 19th June 2019.
The name the hill is listed by is Cerrig Llwydion and it is adjoined to the Elenydd group of hills, which
are situated in the northern part of Mid and West Wales (Region B, Sub-Region
B2), and it is positioned with the Afon Gwy (River Wye) and the A470 road to
its north-east, and has the village of Llangurig towards the north.
When the listing that is now named the 500m Twmpau was first compiled the
qualifying hill was listed as Sychnentydd with a 508m summit height based on
the spot height that appears on contemporary Ordnance Survey 1:50,000
Landranger and 1:25,000 Explorer maps and which is positioned at SN 909 724. As the adjoining summit of Cerrig Llwydion
only had an uppermost 500m ring contour and did not possess a spot height it
was regarded as lower in height compared to the adjacent hill of Sychnentydd.
 |
| Extract from the Ordnance Survey 1:25,000 Explorer map |
It was David Purchase who first brought to
attention that Cerrig Llwydion may be higher than Sychnentydd, a rudimentary
survey using a basic levelling staff was then conducted by Myrddyn Phillups on
the 9th November 2005 resulting in Cerrig Llwydion being approximately
2m higher than Sychnenydd and with 13m of drop from the connecting bwlch
between the two summits to the higher hill.
This was followed by an on-site visit from John Kirk who took readings
during a basic survey and concluded that Cerrig Llwydion is the higher hill by
approximately 1.5m – 2m.
The details for this hill were also re-assessed
when the Ordnance Survey Vector Map Local hosted on the Geograph website and
which is entitled the Interactive Coverage Map became available online. This mapping had many spot heights not on
other publicly available Ordnance Survey maps and showed a 509m summit spot
height for Cerrig Llwydion. This spot
height is also shown on Ordnance Survey data that appears on the Magic Maps
website.
 |
| Extract from the Magic Maps website |
The qualifying summit was subsequently relocated
to Cerrig Llwydion based on the rudimentary surveys and the 509m spot height as
detailed above.
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.
The LIDAR result for each summit is given below:
Sychnentydd:
508.0m summit at SN 91001 72465
Cerrig Llwydion:
509.4m summit at SN 90965 73141
 |
| LIDAR image of Cerrig Llwydion and Sychnentydd |
The result produced by LIDAR analysis comes within the
parameters of the Summit Relocations used within this page heading, these
parameters are:
The term Summit Relocations applies to any listed
hill whose summit meets the following criteria; where there are a number of
potential summit positions within close proximity and the highest point is not
where previously given, or a relocation of approximately 100 metres or more in
distance from either the position of a map spot height or from where the summit
of the hill was previously thought to exist, or when the summit of the hill is
in a different field compared to where previously given, or when it is
positioned to a different feature such as in a conifer plantation, or when the
high point of the hill is placed within a different map contour compared to its
previous listed position, or when the natural and intact summit of a hill is
confirmed compared to a higher point such as a raised field boundary that is
judged to be a relatively recent man-made construct.
Therefore, the summit
height produced by LIDAR analysis for Cerrig Llwydion is 509.4m and is
positioned at SN 90965 73141, and as this summit has been confirmed as being
1.4m higher than the 508.0m summit of Sychnentydd positioned at SN 91001 72465
the relocation of the qualifying summit is confirmed.
The full details for the hill are:
Group: Elenydd
Name: Cerrig Llwydion
OS 1:50,000 map: 136 147
Summit Height: 509.4m
(LIDAR)
Summit Grid Reference (new position): SN 90965 73141 (LIDAR)
Bwlch Height: 462.0m
(LIDAR)
Bwlch Grid Reference:
SN 92807 72279 (LIDAR)
Drop: 47.4m (LIDAR)
Myrddyn Phillips and Aled Williams (February 2020)
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