Wednesday, 30 November 2016

Mapping Mountains – Hill Reclassifications – Yr Uchafion

Pt. 637.1m (SO 267 322) - Uchaf reclassifed to 500m Sub-Uchaf

There has been a reclassification to 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 of 15m of drop, the list is a joint compilation with Aled Williams.  Details relating to this list were published on the Mapping Mountains site in November 2015.

The hill is situated in the Mynyddoedd Duon (Black Mountains) group of hills in the eastern part of the Parc Cenedlaethol Bannau Brycheiniog (Brecon Beacons National Park) in the Region of South Wales (C-3) and is situated on the most easterly of the main elongated ridges of this hill range with Waun Fach (SO 215 299) being its Cardinal Hill, and it is positioned above the small community of Capel-y-ffin to its south-west and the Olchon Valley to its east and the Vale of Ewyas to its south.  The hill can be accessed on good paths from these two valleys or from its south-east or north-west via the Offa’s Dyke Path that follows the ridge crest.

As an appropriate name for this hill is not in the public domain it is being listed under the point (Pt. 637.1m) notation, and its reclassification from an Uchaf to a 500m Sub-Uchaf is due to the analysis of LIDAR data.  LIDAR (Light Detection & Ranging) is highly accurate height data that is now freely available for much of England and Wales, and the hill was first analysed with this data by Joseph Nuttall, independent of this, Aled also analysed LIDAR data using a slightly different method and ascertained the details for this hill’s reclassification.

If Pt. 637.1m does not have the minimum of 15m of drop it would be taken out of the listing of Welsh hills at or over 500m in height that has a minimum of 15m of drop, and has the working title of Yr Uchafion, and if having between 14m and below 15m of drop the hill would be classified as a 500m Sub-Uchaf.

Aled’s analysis of LIDAR data gives the hill the following details:

Pt. 637.4m

Summit Height:  637.4m (converted to OSGM15, and middle of three equivalent height points)

Summit Grid Reference:  SO 26696 32273

Bwlch Height:  622.7m (converted to OSGM15)

Bwlch Grid Reference:  SO 26711 32628

Drop:  14.7m

As the stated philosophy for our co-authored lists is to take the most accurate data available on any given day, and as Ordnance Survey map data uses a 637m summit spot height attained from an aerial survey that has a prescribed margin of uncertainty of +/- 3m applicable to it and as the previously listed bwlch height is based on contour interpolation, it is viewed that LIDAR data is sufficient to reclassify this hill as it is quoted as having a prescribed margin of uncertainty of +/- 0.15m. 

Therefore, the 637.4m LIDAR data produced for the summit position at SO 26696 32273 and the 622.7m LIDAR data produced for the bwlch position at SO 26711 32628 is insufficient for this hill to retain its Uchaf status, with the details from the Trimble GeoXH 6000 survey being 637.1m (converted to OSGM15) summit at SO 26709 32261 and also SO 26695 32273 and 622.5m (converted to OSGM15) bwlch at SO 26710 32627, and therefore with 14.5m of drop Pt. 637.1m is reclassified to a 500m Sub-Uchaf.

This now revises the total in Yr Uchafion which will be updated accordingly and the hill is now listed as a 500m Sub-Uchaf.

The full details for the hill are:

Cardinal Hill:  Waun Fach

Summit Height:  637.1m (converted to OSGM15)

Name:  Pt. 637.1m

OS 1:50,000 map:  161

Summit Grid Reference:  SO 26709 32261 
Drop:  14.5m (converted to OSGM15)

The reclassifications to Yr Uchafion / The Welsh 500m P15s reported on Mapping Mountains are ills of wales as follows:



Pt. 637.1m (SO 267 322) Uchaf reclassified to 500m Sub-Uchaf with 14.5m of drop.


Pt. 637.1m (SO 267 322) is now reclassified from an Uchaf to a 500m Sub-Uchaf

Myrddyn Phillips and Aled Williams (November 2016)

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