Thursday, 12 January 2017

Mapping Mountains – Significant Height Revisions – Y Trechol and Y Pedwarau



Clepyn Melyn (SN 961 464)

There has been a Significant Height Revision to a hill that is listed in the Y Pedwarau and Y Trechol - The Dominant Hills of Wales, and which was initiated by analysis of LIDAR data by Aled Williams and a subsequent survey with the Trimble GeoXH 6000, with the survey that confirmed this height revision being conducted on the 28th December 2016.

The criteria for the two listings that this height revision affects are:

Y Pedwarau - Welsh hills at and above 400m and below 500m in height that have 30m minimum drop. 

Y Trechol - The Dominant Hills of Wales - These are the Welsh P30 hills whose prominence  equal or exceed half that of their absolute height.  With the criteria for Lesser Dominant status being those addition Welsh P30 hills whose prominence is 33% or more and below 50% of their absolute height.

The name that this hill is listed by is Clepyn Melyn and this was derived from local enquiry (see Significant Name Changes) and it is situated in the hills of Mynydd Epynt in the south-easterly part of Mid and West Wales (Region B, Sub-Region B2), and the survey was conducted on a beautiful winter’s morning with blue sky, frost and just a breath of breeze.  Large swathes of Mynydd Epynt is out of bounds for much of the year as it forms part of the Sennybridge Artillery Range Firing Area, and the hill range is positioned between those of Mynydd Du, Fforest Fawr, Bannau Brycheiniog (Brecon Beacons) and Mynyddoedd Duon (Black Mountains) to its south and the wilds of the Elenydd to its north.

The hill is situated just above the high point of the B 4519 as it climbs from the small community of Garth to the north toward the village of Upper Chapel to the south-east, the hill should only be ascended when it is appropriate to do so due to the presence of the military, otherwise it would be an easy ascent on a track from a car parking area near to the top of the road to the north-east of the summit.

The survey with the Trimble GeoXH 6000 was preceded with analysis of LIDAR data.  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:


Clepyn Melyn

Summit Height:  475.5m

Summit Grid Reference:  SN 96120 46434


Prior to the analysis of LIDAR data and surveying the hill with the Trimble GeoXH 6000, Clepyn Melyn was listed with a 478m summit height based on the spot height that appears on the Ordnance Survey 1:50,000 Landranger and 1:25,000 Explorer maps, this height seems to be a relatively recent addition as no spot height is shown on any Ordnance Survey map examined prior to and including the historical 1:25,000 map.

Clepyn Melyn was surveyed with the Trimble GeoXH 6000 giving the hill the following details:


Summit Height:  475.7m (475.681m converted to OSGM15)

Summit Grid Reference:  SN 96124 46428


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:

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 survey result produced by the Trimble GeoXH 6000, 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 for detailing these revisions.

The summit of the hill consists of red grass that when last visited had been fenced with stakes and a single wire that is easy to either clamber under or as I found; over.  The ground hereabouts is mainly hidden by the length of the reed grass but the high point is accepted as being beside a small concrete pillar and this is where the Trimble GeoXH 6000 was set up.  Therefore this hill’s new summit height as surveyed by the Trimble GeoXH 6000 is 475.7m (converted to OSGM15) which is 2.3m lower than its previously listed height of 478m which appears on the Ordnance Survey enlarged mapping hosted on the Geograph website and the 1:50,000 Landranger and 1:25,000 Explorer maps.


The full details for the hill are:

Cardinal Hill:  Clepyn Melyn

Summit Height (New Height):  475.7m (converted to OSGM15)

Name:  Clepyn Melyn

OS 1:50,000 map:  147

Summit Grid Reference:  SN 96124 46428
  
Drop:  c 198

Dominance:  41.62%


The Trimble GeoXH 6000 gathering data at the summit of Clepyn Melyn (SN 961 464) which confirmed this hill's significant height revision


Myrddyn Phillips and Aled Williams (January 2017)

No comments: