Thursday, 5 July 2018

Mapping Mountains – Hill Reclassifications – 30-99m Twmpau


Fegla Fach (SH 638 153) – Double Sub-Twmpau reclassified to 30-99m Twmpau

There has been a reclassification to the 30-99m Twmpau list due to LIDAR analysis, an on-site visit and a survey with the Trimble GeoXH 6000 which took place on the 17th May 2018, with the criteria for this list being:

30-99m TwmpauWelsh hills at or above 30m and below 100m in height with 30m minimum drop, this height band of hills has two accompanying sub lists, the first of which is entitled the 30-99m Sub-Twmpau with the criteria for this sub category being all Welsh hills at or above 30m and below 100m in height with 20m or more and below 30m of drop, and the second sub category which this hill is a part of is entitled the Double Sub-Twmpau with the criteria for this sub category being all Welsh hills at or above 20m and below 30m 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.

This hill did not appear in the accompanying Hills to be surveyed sub list when the original Welsh P30 hills were published on Geoff Crowder’s v-g.me website as it did not meet the criteria for this sub category; however this sub list has now been standardised including the addition of interpolated drop values and heights.

Prior to LIDAR analysis, an on-site visit and the Trimble GeoXH 6000 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 spot height that appears on the area of this hill’s bwlch 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 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 west and north and the A 493 road to its south-east, and has the village of Y Friog (Fairbourne) to the south-west.  

Fegla Fach from across the Afon Mawddach


Fegla Fach from its connecting bwlch

If wanting to visit the hill permission to do so should be sought as it is not a part of designated open access land, for those wishing to do so the minor road to the south of the hill joins a public footpath that then heads toward it.

The addition of Fegla Fach to 30-99m Twmpau status is in part due to LIDAR analysis conducted by Myrddyn Phillips.  The LIDAR (Light Detection & Ranging) technique produced highly accurate height data that is now freely available for much of England and Wales. 

The 1m DTM LIDAR analysis gives the hill the following details:


Fegla Fach

Summit Height:  31.5m

Summit Grid Reference:  SH 63818 15311

Bwlch Height:  1.6m

Bwlch Grid Reference:  SH 63691 15131 and SH 69693 15130

Drop:  29.9m


This hill and others in the vicinity used to be tidal islands before sea defences were constructed, with the Afon Mawddach positioned to the immediate north, west and east of the hill, this has resulted in this hill now being landlocked by its connecting bwlch which is a part of the larger Arthog Bog. 

The area of the connecting bwlch of this hill is complicated to analyse via LIDAR as it is low lying and the land has been terraformed with embankments and drainage ditches.  However, extensive LIDAR analysis pinpoints the position of what can be construed as this hill’s remaining natural bwlch to be on or just below what is now a small land bridge.

LIDAR image of Fegla Fach (top right corner) and the land making up the Arthog Bog

This land bridge crosses one of many drainage ditches in the Arthog Bog and this position was surveyed using the Trimble GeoXH 6000, with three measurement offsets noted, the first to ground making up the land bridge, the second to water level in the drainage ditch and the third to ground mid-way between the land bridge and the water level, the survey results appear below:


1st survey, land bridge:  1.577m

2nd survey, water level in drainage ditch:  1.337m

3rd survey, mid-point between land bridge and water level:  1.457m


The Trimble GeoXH 6000 gathering data at the bwlch of Fegla Fach

When these survey results are combined with the LIDAR result for the summit of this hill the drop values for each of the three points at the bwlch are:


1st survey, land bridge:  29.912m

2nd survey, water level in drainage ditch:  30.152m

3rd survey, mid-point between land bridge and water level:  30.032m


The summit of this hill was analysed using the 1m DTM LIDAR model and as the high point consists of a large rock with a number of trees growing directly beside it, it is probable that the LIDAR technique did not model the very highest part of this rock, and when coupled with the result produced by the Trimble bwlch survey it is deemed sufficient to list this hill with a drop value of 30m, which is sufficient for 30-99m Twmpau status.


The summit of Fegla Fach

The full details for the hill are:

Group:  Cadair Idris

Name:  Fegla Fach

Summit Height:  31.5m (LIDAR)

Summit Grid Reference:  SH 63818 15311 (LIDAR)

Bwlch Height:  1.5m (converted to OSGM15)

Bwlch Grid Reference:  SH 63692 15129

OS 1:50,000 map:  124

Drop:  30.0m (LIDAR summit and Trimble bwlch)


For details on the survey of Fegla Fach

Myrddyn Phillips (July 2018)







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