Mapping Mountains – Hill Reclassifications – 200m Twmpau

 

Cae Pen y Maen (SN 656 694) – 200m Twmpau reclassified to 200m Sub-Twmpau

There has been confirmation of a reclassification to the list of 200m Twmpau, with the summit height, bwlch height and their locations, the drop and status of the hill derived from LIDAR analysis and a subsequent Trimble GeoXH 6000 survey conducted by Myrddyn Phillips. 

LIDAR image of Cae Pen y Maen (SN 656 694)

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

200m Twmpau – Welsh hills at or above 200m and below 300m in height that have 30m minimum drop, with an accompanying sub list entitled the 200m Sub-Twmpau with the criteria for this sub category being all Welsh hills at or above 200m and below 300m 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 200m Twmpau by Myrddyn Phillips

The name the hill is listed by is Cae Pen y Maen and this was derived from the Tithe map, and it is adjoined to the Elenydd group of hills which are situated in the central part of the Mid and West Wales Region (Region B, Sub-Region B2), and it is positioned with the A485 road to its west, the B4575 road farther to its north, the B4340 road to its east and a minor road to its immediate north, and has the village of Lledrod towards the north-west. 

When the original 200m height band of Welsh P30 hills were published on Geoff Crowder’s v-g.me website this hill was included in the main P30 list with a summit height of 297m. 

When the accompanying sub list was standardised, and interpolated heights and drop values also included the details for this hill were re-assessed and it was listed with an estimated c 90m of drop, based on the 297m summit spot height and an estimated c 207m bwlch height, based on interpolation of 10m contouring between 200m – 210m that appear on the Ordnance Survey 1:25,000 Explorer map.  With an adjacent hill now listed as Banc y Maen Rochorlem (SN 675 691) given an estimated c 28m of drop, based on an estimated c 296m summit height and an estimated c 268m bwlch height, with both heights based on interpolation of 10m contouring that appear on the Ordnance Survey 1:25,000 Explorer map. 

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

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 Banc y Maen Rochorlem a 299m spot height was given on the area of its summit and as this is higher than the 297m spot height given Cae Pen y Maen the bylchau were swapped and each hill was reclassified. 

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. 

LIDAR analysis confirmed Banc y Maen Rochorlem as the higher summit and gives Cae Pen y Maen a summit height of 296.6m, and as the summit has now been surveyed with the Trimble GeoXH 6000 it is this result that is being prioritised for listing purposes. 

The Trimble GeoXH 6000 gathering data at the bwlch of Cae Pen y Maen

The Trimble GeoXH 6000 gathering data at the summit of Cae Pen y Maen

Prior to the Trimble GeoXH 6000 summit survey of this hill, the summit of Banc y Maen Rochorlem had also been surveyed and each result is given below: 

 

Banc y Maen Rochorlem:  299.3m at SN 67513 69184 

Cae Pen y Maen:  296.6m at SN 65690 69496 

 

Therefore, the confirmation of the reclassification of this hill to 200m Sub-Twmpau status is due to LIDAR analysis and a subsequent Trimble GeoXH 6000 survey, resulting in a 296.6m summit height and a 268.8m bwlch height, with these values giving this hill 27.9m of drop, which is insufficient for it to be classified as a 200m Twmpau. 

 

The full details for the hill are: 

Group:  Elenydd 

Name:  Cae Pen y Maen 

OS 1:50,000 map:  135

Summit Height:  296.6m (converted to OSGM15) 

Summit Grid Reference:  SN 65690 69496 

Bwlch Height:  268.8m (converted to OSGM15) 

Bwlch Grid Reference:  SN 66401 68910 

Drop:  27.9m 

 

Myrddyn Phillips (May 2021)