Mapping Mountains – Significant Height Revisions – Yr Uchafion, 700m Twmpau and Y Pellennig – The Remotest Hills of Wales

Fan y Big (SO 036 206)

There has been a Significant Height Revision to a hill that appears in the following lists; Yr Uchafion, 700m Twmpau and Y Pellennig – The Remotest Hills of Wales, and which was initiated by LIDAR analysis conducted by Aled Williams followed by a survey with the Trimble GeoXH 6000 conducted by Myrddyn Phillips, with the latter conducted over two days on the 30^th June 2018 and the 21^st August 2018 in clear conditions turning to mist on the first survey and misty conditions clearing on the second survey.

LIDAR image of Fan y Big

The criteria for the three listings that this height revision applies to are:

Yr Uchafion – Welsh hills at or above 500m in height that have 15m minimum drop.  The list is co-authored by Myrddyn Phillips and Aled Williams, with the Introduction to the list appearing on Mapping Mountains on the 4^th November 2015.

700m Twmpau - Welsh hills at or above 700m and below 800m in height with 30m minimum drop, with an accompanying sub list entitled the 700m Sub-Twmpau with the criteria for this sub category being all Welsh hills at or above 700m and below 800m in height with 20m or more and below 30m of drop, with the word Twmpau being an acronym standing for thirty welsh metre prominences and upward.  The list is authored by Myrddyn Phillips.

Y Pellennig –The Remotest Hills of Wales - Welsh hills whose summit is at least 2.5km from the nearest paved public road and the hill has a minimum 15m of drop.  The list is co-authored by Myrddyn Phillips and Aled Williams and is available as a downloadable e-booklet or print-booklet version on Mapping Mountains Publications with the up-to-date master list available on Mapping Mountains in Google Doc format.

The name of the hill is Fan y Big and it is placed in the central part of the main ridge that forms the Bannau Brycheiniog (Brecon Beacons) group of hills, which are situated in the South Wales Region (Region C, Sub-Region C2), and it is positioned between the towns of Aberhonddu (Brecon) to its north and Merthyr Tudful to its south.

The hill can be accessed from public footpaths originating from its north and from its south; or it can be combined as a longer expedition taking in the main Bannau Brycheiniog ridge either from its west or from its east.

Prior to LIDAR analysis and the subsequent survey with the Trimble GeoXH 6000 the height of the hill was listed as 719m based on the summit spot height that appears on the Ordnance Survey 1:50,000 Landranger and 1:25,000 Explorer map.

Extract from the Ordnance Survey 1:125,000 Explorer map

The summit of this hill was surveyed three times over two days, each 53 days apart resulting in the following:

1^st survey 30.06.18:  716.800m

2^nd survey 30.06.18:  716.676m

3^rd survey 21.08.18:  716.754m

Average of three summit surveys:  716.746m

Therefore, the average of three summit surveys produced by the Trimble GeoXH 6000 is 716.7m (converted to OSGM15), this is not a dramatic height revision when compared to some survey results, but it does come within the parameters of the Significant Height Revisions used within this page heading, these parameters are:

The term Significant Height Revisions applies to any listed hill whose interpolated height and Ordnance Survey or Harvey map summit spot height has a 2m or more discrepancy when compared to the survey result produced by the Trimble GeoXH 6000 or analysis of data produced via LIDAR, also included are hills whose summit map data is missing an uppermost ring contour when compared to the data produced by the Trimble or by LIDAR analysis.  As heights on different scaled Ordnance Survey maps are not consistent the height given on the 1:25,000 Explorer map is being prioritised in favour of the 1:50,000 Landranger map for detailing these revisions.

Therefore, this hill’s new summit height as surveyed with the Trimble GeoXH 6000 is 716.7m (converted to OSGM15) which is 2.3m lower than its previously listed height of 719m which appears on Ordnance Survey 1:50,000 Landranger and 1:25,000 Explorer map.

The Trimble GeoXH 6000 gathering data at the summit of Fan y Big which resulted in this hill's significant height revision

ills of Wales, and are reproduced below@

The full details for the hill are:

Group:  Bannau Brycheiniog

Summit Height (New height):  716.7m (converted to OSGM15)

Name:  Fan y Big

OS 1:50,000 map:  16o

Summit Grid Reference:  SO 03652 20658

  

Drop:  28.5m (converted to OSGM15)

For details on the 1st survey that significantly revised the summit height of this hill

For details on the 2nd survey that significantly revised the summit height of this hill

Myrddyn Phillips and Aled Williams (November 2018)