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

Fegla Fach (SH 638 153)

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, dominance and status of the hill confirmed by a combination of LIDAR analysis and a Trimble GeoXH 6000 survey 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 Fach 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 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, for those wishing to do so access to its summit can be found from the confines of a camp site which is situated at the base of the hill to its south-west.

Prior to LIDAR analysis and the Trimble survey this hill was listed with 25m of drop based on the 28m summit spot height that appears on the Ordnance Survey 1:25,000 Explorer map and the 3m bwlch spot height that appears on the Ordnance Survey Interactive Coverage Map hosted on the Geograph website.

Extract from the Ordnance Survey Interactive Coverage Map hosted on the Geograph website

The summit height produced by LIDAR analysis is 31.5m, 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:

LIDAR image of Fegla Fach (top right of photograph) and showing the Arthog bog

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 31.5 and this was produced by LIDAR analysis, this is 3.5m higher than its previously listed height of 28m which appears as a spot height on the Ordnance Survey 1:25,000 Explorer map.

The full details for the hill are:

Group:  Cadair Idris

Summit Height (New Height):  31.5m (LIDAR)

Name:  Fegla Fach

OS 1:50,000 map:  124

Summit Grid Reference:  SH 63818 15311 (LIDAR)   

Drop:  30.0m (LIDAR summit and Trimble bwlch)

Dominance:  95.37% (LIDAR summit and Trimble bwlch)

The summit of Fegla Fach (SH 638 153)

For details on the survey and on-site visit that significantly revised the summit height of this hill

Myrddyn Phillips (August 2018)