Mapping Mountains – Significant Height Revisions – 200m Twmpau

 

Banc y Maen Rochorlem (SN 675 691)

There has been a Significant Height Revision to a hill that is listed in the 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 this height revision affects 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 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. 

The summit of this hill has now been surveyed with the Trimble GeoXH 6000, resulting in 299.3m at SN 67513 69184 and this height 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 summit 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 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 or by LIDAR analysis. 

The Trimble GeoXH 6000 gathering data from the natural high point of Banc y Maen Rochorlem

Therefore, this hill’s new listed summit height is 299.3m and this was derived from a Trimble GeoXH 6000 survey, this is positioned at SN 67513 69184 and is 3.3m higher than its originally listed height of c 296m which was based on interpolation of the uppermost 290m ring contour that appears on the Ordnance Survey 1:25,000 Explorer map. 

 

The full details for the hill are:

Group:  Elenydd 

Name:  Banc y Maen Rochorlem 

OS 1:50,000 map:  135

Summit Height (New 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)