Mapping Mountains – Hill Reclassifications – 200m Twmpau

 

Banc y Maen Rochorlem (SN 675 691) – 200m Sub-Twmpau reclassified to 200m 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. 

The summit of Banc y Maen Rochorlem (SN 675 691)

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 name the hill is listed by is Banc y Maen Rochorlem 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 B4340 road to its north and east, and has the village of Lledrod towards the west 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 Hills to be surveyed sub list that accompanied the main P30 list, as it was considered not to meet the criteria then used for the main P30 category. 

When the 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 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.  With an adjacent hill now listed as Cae Pen y Maen (SN 656 694) given a summit height of 297m which was based on the spot height that appears on contemporary Ordnance Survey 1:25,000 Explorer maps. 

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 this hill 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 image of Banc y Maen Rochorlem (SN 675 691)

LIDAR analysis confirmed Cae Pen y Maen as the lower summit and gives Banc y Maen Rochorlem a summit height of 299.4m, 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 natural high point of Banc y Maen Rochorlem

Since the Trimble GeoXH 6000 summit survey of this hill, the summit of Cae Pen y Maen has 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 from 200m Sub-Twmpau status is due to LIDAR analysis and a subsequent Trimble GeoXH 6000 survey, resulting in a 299.3m summit height and a 206.2m bwlch height, with these values giving this hill 93.1m of drop, which is sufficient for it to be classified as a 200m Twmpau.

 

The full details for the hill are: 

Group:  Elenydd 

Name:  Banc y Maen Rochorlem 

OS 1:50,000 map:  135

Summit Height:  299.3m (converted to OSGM15, Trimble GeoXH 6000) 

Summit Grid Reference:  SN 67513 69184 (Trimble GeoXH 6000) 

Bwlch Height:  206.2m (LIDAR) 

Bwlch Grid Reference:  SN 69578 68344 (LIDAR) 

Drop:  93.1m (Trimble GeoXH 6000 summit and LIDAR bwlch) 

 

Myrddyn Phillips (April 2021)