Monday, 24 April 2017

Mapping Mountains – Hill Reclassifications – Yr Uchafion


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)


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