Mapping Mountains – Significant Height Revisions – 900m Twmpau

 

900m Twmpau – Significant Height Revisions

The 900m Twmpau (thirty welsh metre prominences and upward) are the Welsh hills at or above 900m and below 1000m in height that have a minimum drop of 30m.  Accompanying the main P30 list is a theoretical sub list entitled the 900m Sub-Twmpau with the qualification to this sub category being all Welsh hills at or above 900m and below 1000m in height with 20m or more and below 30m of drop.

The list is authored by Myrddyn Phillips and the posts that have appeared on Mapping Mountains detailing the Significant Height Revisions to the main P30 list and the sub list appear below presented chronologically in receding order.

Mapping Mountains - Significant Height Revisions - 900m Twmpau

Foel Fras (SH 696 681) - 1st significant height revision

Summit Relocations post for Foel Fras

 

There has been a Significant Height Revision to a hill that is listed in the Y Pellennig – The Remotest Hills of Wales, Welsh Highlands – Uchafion Cymru and 900m Twmpau, with the summit height, bwlch height and their locations, the drop and status of the hill derived from a Leica RX1250 survey conducted by Alan Dawson. 

Foel Fras (SH 696 681) from the summit of Trwsgl (SH 663 679)

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

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 to download in Google Doc format. 

Y Pellennig - The Remotest Hills of Wales by Myrddyn Phillips and Aled Williams

Welsh Highlands – Uchafion Cymru – Welsh hills at or above 500m in height with 15m minimum drop, with an accompanying sub list entitled the Welsh Highland Subs, the criteria for which is all Welsh hills at or above 500m in height with 10m or more and below 15m of drop.  This list is authored by Myrddyn Phillips and Aled Williams with the Introduction to the list published on Mapping Mountains in November 2015 and the latest update relating to the list published on Mapping Mountains in January 2023.

Welsh Highlands - Uchafion Cymru by Aled Williams and Myrddyn Phillips

900m Twmpau - Welsh hills at or above 900m and below 1000m in height with 30m minimum drop, with an accompanying sub list entitled the 900m Sub-Twmpau with the criteria for this sub category being all Welsh hills at or above 900m and below 1000m 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 and is available to download in Google Doc format from Mapping Mountains.

900m Twmpau by Myrddyn Phillips

The name the hill is listed by is Foel Fras and it is adjoined to the Carnedd Llywelyn group of hills, which are situated in the north-western part of North Wales (Region A, Sub-Region A1), and it is positioned with the A55 road to its north-west, the A5 road to its south-west and the B5106 and A470 roads to its east, and has the town of Bethesda towards the west south-west.

When the original list that later became known as the Welsh Highlands – Uchafion Cymru was first compiled, this hill was listed with 58m of drop, based on the 942m summit spot height adjoined to a triangulation pillar that appears on the Ordnance Survey 1:50,000 Landranger and 1:25,000 Explorer map, and the 884m bwlch spot height that appears on the Ordnance Survey 1:25,000 Explorer map.

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

However, the summit area of this hill is rocky and includes a number of substantial upright rock spikes.  One of the earliest Abney Level surveys to ascertain the high point of this hill was conducted in 2011 by Graham Jackson.  This survey concluded that a rock spike approximately 55 metres south-wesward of the trig pillar is the highest point of this hill.

This rock spike was subsequently surveyed by Alan Dawson using the Leica RX1250, resulting in a 944.1m summit height positioned at SH 69611 68118, and this comes 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, the Leica RX1250 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, Leica RX1250 or by LIDAR analysis. 

The Leica RX1250 gathering data at the summit of Foel Fras (SH 696 681).  Photo: Alan Dawson  

Therefore, the new listed summit height of this hill is 944.1m and this was derived from a Leica RX1250 survey, this is 2.1m higher than the previously listed height of 942m, which was based on the spot height adjoined to a trinagulation pillar that appears on the Ordnance Survey 1:50,000 Landranger and 1:25,000 Explorer map.

 

The full details for the hill are:

Group:  Carnedd Llywelyn

Name:  Foel Fras

OS 1:50,000 map:  115

Summit Height (New Height):  944.1m (converted to OSGM15, Leica RX1250) 

Summit Grid Reference:  SH 69611 68118 (Leica RX1250) 

Bwlch Height:  881.3m (converted to OSGM15, Leica RX1250) 

Bwlch Grid Reference:  SH 68799 66515 (Leica RX1250) 

Drop:  62.8m (Leica RX1250) 

Remoteness:  3.975km

 

Myrddyn Phillips (August 2024)