Mapping Mountains – Hill Reclassifications – 500m Twmpau

Foel y Geifr (SH 716 050) – 500m Twmpau reclassified to 500m Sub-Twmpau

There has been a reclassification to the listing of the 500m Twmpau, with the summit height, bwlch height and their locations, the drop and status of the hill derived from detail produced by Joe Nuttall in his surface analysis progamme, with subsequent LIDAR analysis conducted by the DoBIH team and independently by Aled Williams and Myrddyn Phillips. 

Foel y Geifr (SH 716 050)

The criteria for the list that this reclassification applies to are:

500m Twmpau - Welsh hills at or above 500m and below 600m in height with 30m minimum drop, with an accompanying sub list entitled the 500m Sub-Twmpau with the criteria for this sub category being all Welsh hills at or above 500m and below 600m 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. 

The 500m Twmpau by Myrddyn Phillips

The name the hill is listed by is Foel y Geifr and it is adjoined to the Tarren y Gesail group of hills, which are situated in the south-western part of North Wales (Region A, Sub-Region A3), and it is positioned with the B4405 road to its north-west, the A493 road to its south and the A487 road to its east, and has the town of Machynlleth towards the south-east.

When the list that later became known as the 500m Twmpau was first compiled, this hill was included in the main P30 list due to a basic levelling survey conducted by Myrddyn Phillips on the 26^th June 2000 that resulted in over 30m of drop, and listed with a 515m summit height, based on the spot height that appears on the Ordnance Survey 1:25,000 Explorer map and a 483m bwlch height; relative to the drop value ascertained from the basic levelling survey and the summit spot height. 

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

Since the original compilation of this list there have been a number of maps made available online.  Some of these are historic such as the series of Six-Inch maps on the National Library of Scotland website.  Whilst others were digitally updated such as the Ordnance Survey Vector Map Local that was hosted on the Geograph website and which was entitled the Interactive Coverage Map, whilst others are current and digitally updated such as the interactive mapping on the Magic Maps and WalkLakes websites.

The details for this hill were re-assessed when the Ordnance Survey Vector Map Local hosted on the Geograph website and which was entitled the Interactive Coverage Map became available online.  This mapping had many spot heights not on other publicly available Ordnance Survey maps and for this hill it had a 486m bwlch spot height and when coupled with its 515m summit spot height, these values gave this hill 29m of drop.  This emphasised the uncertainty applicable to the result produced by the basic levelling survey. 

LIDAR image of Foel y Geifr (SH 716 050)

However, it was not until LIDAR became available that the details for this hill could be accurately re-assessed.  The LIDAR (Light Detection & Ranging) technique produced highly accurate height data that is now freely available for much of England and Wales.

Therefore, the reclassification of this hill to 500m Sub-Twmpau status is due to LIDAR analysis, resulting in a 515.6m summit height and a 486.8m bwlch height, with these values giving this hill 28.8m of drop, which is insufficient for it to be classified as a 500m Twmpau. 

 

The full details for the hill are:

Group:  Tarren y Gesail

Name:  Foel y Geifr

OS 1:50,000 map:  124

Summit Height:  515.6m (LIDAR) 

Summit Grid Reference:  SH 71634 05092 (LIDAR) 

Bwlch Height:  486.8m (LIDAR) 

Bwlch Grid Reference:  SH 71726 05403 (LIDAR) 

Drop:  28.8m (LIDAR)

 

Myrddyn Phillips (July 2023)