Bwlan (SH 946 255) - Uchaf deletion
There has been a deletion from the listing of Yr Uchafion instigated from analysis of
LIDAR data by Aled Williams and subsequently confirmed via a survey with the Trimble GeoXH 6000. Yr Uchafion is the draft title for a
list of the Welsh 500m P15s that takes in all hills in Wales at or above 500m
that have a minimum drop of 15m, the list is a joint compilation between
Myrddyn Phillips and Aled Williams, with details relating to this list being published
on the Mapping Mountains site in November 2015.
Accompanying the main Yr
Uchafion list are three sub lists; 500m
Sub-Uchaf, 490m Sub-Uchaf and the
Double Sub-Uchaf categories, with the
deletion of this hill affecting the main P15 Yr Uchafion list.
Prior to analysis of LIDAR data the hill was
listed with 16m of drop based on a 51ft 6˝ (15.7m) basic levelling survey conducted my
Myrddyn Phillips on the 10.05.05. The
hill is given a 528m summit spot height on Ordnance Survey 1:50,000 Landranger
and 1:25,000 Explorer maps and has a 514m bwlch spot height on the Ordnance
Survey enlarged mapping hosted on the Geograph website. There are two options for where the critical
bwlch of this hill is placed and between each is a 523m map heighted bump, the
spot height on the enlarged map on the Geograph website appears on the eastern
side of this bump and this is where the basic levelling survey started from,
with the alternate westerly bwlch being visually dismissed as higher. However, the basic levelling survey was conducted
to the top of the 523m map heighted bump and not to the summit of the 528m map
heighted Bwlan, this survey came to 10.7m, and 5m was then added to the drop
value to compensate for the difference in map heights. To ascertain a drop value for a hill using
this method is not ideal and the accuracy and therefore this hill’s
qualification as an Uchaf was always
open to question.
The hill is situated in the Y Berwyn group of hills with its Cardinal Hill being Foel y Geifr (SH 918 246) and is placed
in the Region of North Wales (Region A, Sub-Region A4), and is positioned between
the stream valleys of the Afon Nadroedd
to the north and the Afon Eiddew to
the south, with Llyn Efyrnwy (Lake
Vyrnwy) towards the hill’s south-east.
The hill is situated on open access land and can
be accessed from the north either by following the boundary of a conifer
plantation to cross the Nant Nadroedd
Fawr before following a track to the hill’s bwlch, or it can be ascended
starting lower in the valley from where the same track leaves the minor road
and heads through the conifer plantation.
As the latter route through the conifer plantation is not on open access
land permission to use this route should be sought.
The name of the hill is Bwlan and its deletion from Uchaf
status is due to the analysis of LIDAR data by Aled Williams. LIDAR (Light Detection & Ranging) is highly
accurate height data that is now freely available for much of England and
Wales.
Aled’s analysis of LIDAR data gives the hill the
following details:
Bwlan
Summit Height: 526.1m
(converted to OSGM15)
Summit Grid Reference:
SH 94690 25541
Bwlch Height: 512.6m
(converted to OSGM15)
Bwlch Grid Reference:
SH 94180 25715
Drop: 13.6m
Therefore, the 526.1m (converted to OSGM15) LIDAR
data produced for the summit position at SH 94690 25541 and the 512.6m (converted to OSGM15) LIDAR data
produced for the bwlch position at SH 94180 25715 gives
this hill 13.55m of drop which is insufficient for its retention as an Uchaf and also for it to be reclassified as a 500m Sub-Uchaf, with the details from the Trimble GeoXH 6000 survey being 525.9m (converted to OSGM15) summit at SH 94692 25540 and 512.1m (converted to OSGM15) bwlch at SH 94179 25714, giving this hill 13.8m of drop.
The full details for the hill are:
Cardinal Hill: Foel y
Geifr
Summit Height: 525.9m
(converted to OSGM15, Trimble data)
Name: Bwlan
OS 1:50,000 map: 125
Summit Grid Reference:
SH 94692 25540
Drop: 13.8m (converted
to OSGM15, Trimble data)
The overall total for Yr
Uchafion / The Welsh 500m P15s is now 63o hills with the total for the 500m Sub-Uchaf category remaining at 25.
For the additions, reclassifications and deletions to Yr Uchafion / The Welsh 500m P15s
reported on Mapping Mountains please consult the following Change Registers:
 |
| Bwlan (SH 946 255) confirmed as a deleted Uchaf |
Myrddyn Phillips and Aled Williams (April 2017)
No comments:
Post a Comment