Sunday, 20 November 2016

Mapping Mountains – Significant Height Revisions – Yr Uchafion

Craig Gwaun Taf (SO 005 207)

This is the thirty sixth post under the heading of Significant Height Revisions, and the Trimble survey that resulted in this height revision was conducted on the 25th October 2016.

The thirty sixth major height revision initiated from a survey with the Trimble GeoXH 6000 was conducted in the hills of the Bannau Brycheiniog in the northern part of south Wales on a becalmed misty morning.  The Bannau Brycheiniog are referred to in English as the Brecon Beacons and are positioned between the hills of Mynydd Epynt to their north and the hills of the south Wales valleys to their south.

The hill is situated between the high cwm that holds the infant Blaen Taf Fechan to its east and the busy A 470 road to its west and is positioned above the towns of Aberhonddu (Brecon) to its north north-east and Merthyr Tudful (Merthyr Tydfil) to its south south-east, and the hill can be easily accessed from the car park at Pont ar Daf to its south south-west with the path leading toward the bwlch just north of its summit; Bwlch Duwynt, being on the Beacons Way and one of the main paths leading to the summit of south Wales’ highest mountain; Pen y Fan.

The name of the hill is Craig Gwaun Taf and the survey conducted with the Trimble GeoXH 6000 was instigated due to communication with Joe Nuttall, who had analysed this hill’s summit and bwlch height with LIDAR dataLIDAR (Light Detection & Ranging) is highly accurate height data that is now freely available for much of England and Wales, and Joe has been diligently checking a number of hills for qualification to his parents 2,000ft list.  These lists are based on a minimum qualifying drop value of 15m and Joe found that LIDAR data gave Craig Gwaun Taf a drop of 15.783m.

However, if Craig Gwaun Taf has a minimum of 15m of drop it would also be included in the listing of Welsh hills at or over 500m in height that have a minimum of 15m of drop.  This list has the working title of Yr Uchafion and is co-authored with Aled Williams, and was detailed on the Mapping Mountains site on the 4th November 2015.

Craig Gwaun Taf is given an 824m summit spot height on current Ordnance Survey maps that derives from the 2,704ft height on the series of old Six-Inch maps and has current bwlch contouring between 800m – 810m, with an 815m spot height on the area of the bwlch on the Ordnance Survey enlarged mapping on the Geograph website.  These figures give this hill an approximate c 13m of drop based on interpolation of bwlch contours, or only 9m of drop if based on the 815m spot height.  However, when Joe Nuttall analysed this hill with LIDAR data he found that it gave the summit as 825.885m and the bwlch as 810.102m, giving a drop value of 15.783m, sufficient for both Uchaf and Nuttall qualification.

The 826.4m (converted to OSGM15) height produced by the Trimble is an average of three summit data sets and is 2.4m higher than the current Ordnance Survey map height of 824m, and therefore comes within the parameters of the Significant Height Revisions used within this page heading, these parameters are:

The term ‘significant height revision’ applies to any listed hill whose Ordnance Survey summit spot height has a 2m or more discrepancy when compared to the surveyed Trimble height, also included are hills whose summit map data is missing an uppermost ring contour when compared to the data produced by the Trimble.  As heights on different scaled Ordnance Survey maps are not consistent the height given on the 1:25,000 map is being prioritised in favour of the 1:50,000 for detailing these revisions.

The summit of this hill is easily identifiable and consists of ground at the top of a grassy hummock a few metres east of the ridge path, and therefore this hill’s new summit height as surveyed by the Trimble GeoXH 6000 is 826.4m (converted to OSGM15) which is 2.4m higher than the current map height of 824m which appears on the Ordnance Survey 1:50,000 Landranger and the 1:25,000 Explorer maps.

The full details for the hill are:

Cardinal Hill:  Pen y Fan

Summit Height (New Height):  826.4m (converted to OSGM15)

Name:  Craig Gwaun Taf

OS 1:50,000 map:  160

Summit Grid Reference:  SO 00548 20720  

Drop:  16.1m (converted to OSGM15)

The Trimble GeoXH 6000 gathering data at the summit of Craig Gwaun Taf (SO 005 207) which resulted in this hill's significant height revision

Myrddyn Phillips and Aled Williams (November 2016)

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