Ynys Deullyn (SM 844
340)
There has been a Significant Height Revision to a hill that appears
in the following lists; 30-99m Twmpau
and Y Trechol – The Dominant Hills of
Wales, with the hill’s height, drop, dominance and status confirmed by LIDAR
analysis conducted by Myrddyn Phillips.
The criteria for the two listings that this significant
height revision applies to are:
30-99m Twmpau – Welsh hills at or above 30m and below 100m in height that
have a minimum 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.
The name of the hill is Ynys
Deullyn and as its name implies it is an island, or more strictly speaking a tidal
island that is attached to mainland Wales at low tide. The hill is adjoined to the Carn Llidi group of hills, which are situated in the south-western
part of Mid and West Wales (Region B, Sub-Region B4), with the island being
positioned on the northern part of the Pembrokeshire coast, and it has the small
community of Abercastell towards its west south-west.
If wanting to visit the island permission
to do so should be sought as it is not a part of designated open access land,
for those wishing to do so caution is advised as the easiest approach will
probably necessitate some form of scramble.
This hill did not appear in the main P30 list or
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 was thought not to have 30m of drop and did not meet the
criteria then used for the sub category; however this sub list has now been
standardised including the addition of interpolated drop and height values.
This hill was included as a P30 shortly after the
Ordnance Survey Interactive Coverage Map became available on the Geograph
website, and prior to LIDAR analysis it was listed with 30m of drop based on
the 30m summit spot height that appears on this map.
 |
| Extract from the Ordnance Survey Interactive Coverage Map hosted on the Geograph website |
The summit height produced by 2m DSM LIDAR
analysis is 32.6m, 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:
 |
| 2m DSM LIDAR image of Ynys Deullyn (SM 844 340) |
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 by LIDAR analysis, 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.
Therefore, this hill’s new summit height is 32.6m
and this was produced from LIDAR analysis, this is 2.6m higher than its
previously listed height of 30m which appears on the Ordnance Survey Interactive
Coverage Map hosted on the Geograph website.
The full details for the hill are:
Group: Carn Llidi
Name: Ynys Deullyn
OS 1:50,000 map: 157
Summit Grid Reference:
SM 84479 34095 (LIDAR)
Drop: 30.5m (LIDAR)
Dominance: 93.51%
(LIDAR)
Myrddyn Phillips (September 2018)
