Allt Wen (SH 745 772)
There has been a Significant Height Revision to a
hill that is listed in the 200m Twmpau and Y Trechol – The Dominant Hills of Wales, with
the summit height and its location, the drop and status of the hill confirmed
by LIDAR analysis, and a subsequent survey with the Trimble GeoXH 6000, both
conducted by Myrddyn Phillips with the latter taking place on the 15th
October 2018.
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| Allt Wen (SH 745 772) |
The criteria for the two lists that this height
revision applies to are:
200m Twmpau - Welsh hills at or above 200m and below 300m in height with 30m
minimum drop, with an accompanying sub list entitled the 200m Sub-Twmpau with the criteria for this sub category being all
Welsh hills at or above 200m and below 300m 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.
Y Trechol – The Dominant Hills of Wales – Welsh P30 hills whose
prominence equal or exceed half that of their absolute height. With the criteria for Lesser Dominant status being those additional Welsh P30 hills whose
prominence is between one third and half that of their absolute height. The list is authored by Myrddyn Phillips with
the Introduction to the start of the Mapping Mountains publication of this list
appearing on the 3rd December 2015.
The name of the hill is Allt
Wen, and it is adjoined to the Carneddau 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 A55 road and the coast to its north, and has the town of Conwy towards its east.
When the original Welsh 100m P30 list was
published on Geoff Crowder’s v-g.me website, this hill was listed with a 255m summit
height based on the spot height that appears on the Ordnance Survey 1:50,000
Landranger and 1:25,000 Explorer map.
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| Extract from the Ordnance Survey 1:25,000 Explorer map |
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
analysis was closely followed by the survey of this hill with the Trimble GeoXH
6000.
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| LIDAR image of Allt Wen |
The summit height produced by the Trimble GeoXH
6000 survey is 252.3m, this is not a
substantial revision when compared to some revised heights, and 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 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. As heights on different scaled
Ordnance Survey maps are not consistent the height given on the 1:25,000
Explorer map is being prioritised in favour of the 1:50,000 Landranger map for
detailing these revisions.
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| The Trimble GeoXH 6000 gathering data at the summit of Allt Wen |
Therefore, this hill’s new listed summit height is
252.3m and this was produced by surveying with the Trimble GeoXH 6000, this is
2.7m lower than its previously listed height of 255m which was based on the
spot height that appears on the Ordnance Survey 1:50,000 Landranger and
1:25,000 Explorer map.
The full details for the hill are:
Group: Carneddau
Name: Allt Wen
OS 1:50,000 map: 115
Summit Height (New height):
252.3m (converted to OSGM15)
Summit Grid Reference:
SH 74546 77264
Bwlch Height: 160.2m
(converted to OSGM15)
Bwlch Grid Reference:
SH 74855 77154
Drop: 92.1m
Dominance: 36.51%
Myrddyn Phillips (April 2019)
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