Mapping Mountains – Significant Height Revisions – 30-99m Twmpau and Y Trechol – The Dominant Hills of Wales

Fegla Fawr (SH 629 146)

There has been a Significant Height Revision to a hill that is listed in the 30-99m Twmpau and Y Trechol – The Dominant Hills of Wales, with the summit height, drop and dominance of the hill confirmed by a Trimble GeoXH 6000 survey and LIDAR analysis conducted by Myrddyn Phillips.

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

30-99m Twmpau – Welsh hills at or above 30m and below 100m in height that have a minimum 30m of drop, with the word Twmpau being an acronym standing for thirty welsh metre prominences and upward.

Y Trechol – The Dominant Hills of Wales – Welsh P30 hills whose prominence equal or exceed half that of their absolute height.

The name of the hill is Fegla Fawr and it is adjoined to the Cadair Idris group of hills, which are situated in the south-western part of North Wales (Region A, Sub-Region A3), with the hill being positioned between the Afon Mawddach to its north-west and north and the A 493 road to its south-east, and has the village of Y Friog (Fairbourne) to the south-west.  

As the hill is not a part of designated open access land permission to visit should be sought, however there are paths leading toward its summit which is an indicator that local concession exists to visit it summit.

Prior to LIDAR analysis and the Trimble survey this hill was listed with c 55m of drop based on an estimated c 57m summit height with the hill having an uppermost 50m ring contour on contemporary Ordnance Survey maps, and a bwlch height of 2m based on the spot height that appears on the Ordnance Survey Interactive Coverage Map hosted on the Geograph website.

Extract from the Ordnance Survey 1:25,000 Explorer map

LIDAR image of Fegla Fawr

The summit height produced by the Trimble GeoXH 6000 survey is 59.5m (converted to OSGM15), 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 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 is 59.5m (converted to OSGM15) and this was produced by the Trimble GeoXH 6000 survey, this is 2.5m higher than its previously listed height of c 57m which was estimated from the uppermost 50m ring contour on the Ordnance Survey 1:50,000 Landranger and 1:25,000 Explorer map.

The full details for the hill are:

Group:  Cadair Idris

Summit Height (New Height):  59.5m (converted to OSGM15)

Name:  Fegla Fawr

OS 1:50,000 map:  124

Summit Grid Reference:  SH 62966 14665  

 

Drop:  57.6m (Trimble summit and LIDAR bwlch)

Dominance:  96.81% (Trimble summit and LIDAR bwlch)

Gathering data at the summit of Fegla Fawr (SH 629 146) which resulted in this hill's significant height revision

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

Myrddyn Phillips (August 2018)