Cefn Sarnau (SO 014 341)
There has been a Significant Height Revision to a
hill that is listed in the Y Pedwarau – The 400m Hills of Wales and Y Trichant – The 300m
Hills of Wales, with the summit height, bwlch height and their locations,
the drop and status of the hill confirmed by LIDAR analysis initially conducted
by Aled Williams and subsequently by Myrddyn Phillips.
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| LIDAR image of Cefn Sarnau (SO 014 341) |
The criteria for the two listings this height revision
affects 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 being listed in the 390m Sub-Pedwar category. The criteria for 390m Sub-Pedwar status being
all Welsh hills at or above 390m and below 400m in height that have 20m or more
and below 30m of drop. The list is co-authored by Myrddyn Phillips and Aled Williams and is published on Mapping Mountains in Google Doc format.
Y Trichant
– The 300m Hills of Wales – 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 the hill is listed by is Cefn Sarnau and
it is adjoined to the Mynydd Epynt group of hills, which are situated in the south-eastern part of Mid
and West Wales (Region B, Sub-Region B2), and it is positioned with the Afon Honddu and the B4520 road to
its east, and has the small community of Capel Isaf (Lower Chapel) towards the north-east.
When the original 300m height band of Welsh P30 hills were published on Geoff Crowder’s v-g.me website the summit height of
this hill was listed as c 390m which was based on the uppermost contour that
appears on Ordnance Survey 1:25,000 Explorer maps.
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| Extract from the Ordnance Survey 1:25,000 Explorer map |
When the P30 lists were 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 395m summit height, and
these details were used when the hill was listed in the 1st edition
of the Y Pedwarau published by
Europeaklist in May 2013.
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.
 |
| LIDAR summit image of Cefn Sarnau |
The summit height produced by LIDAR analysis is 397.6m
and is positioned at SO 01415 34161, this is not a dramatic height revision
when compared to some revised heights, but it does come within the parameters
of the Significant Height Revisions used within this page heading, these
parameters are:
The term Significant Height Revisions applies to
any listed hill whose interpolated summit height and Ordnance Survey or Harvey
map summit spot height has a 2m or more discrepancy when compared to the survey
result produced by the Trimble GeoXH 6000 or analysis of data produced via
LIDAR. Also included are hills whose
summit map data is missing an uppermost ring contour when compared to the data
produced by the Trimble or by LIDAR analysis.
Therefore, this hill’s new listed summit height is
397.6m and this was derived from LIDAR analysis, this is 2.6m higher than the estimated
c 395m summit height that was previously given for this hill.
The full details for the hill are:
Group: Mynydd Epynt
Name: Cefn Sarnau
OS 1:50,000 map: 160
Summit Height (New Height):
397.6m (LIDAR)
Summit Grid Reference:
SO 01415 34161 (LIDAR)
Bwlch Height: 355.65m (LIDAR)
Bwlch Grid Reference:
SO 01482 34779 (LIDAR)
Drop: 42.0m (LIDAR)
Myrddyn Phillips and Aled Williams (March 2020)
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