Mapping Mountains – Significant Height Revisions – 200m Twmpau

Cae’r Faen Mynach (SN 765 003) 

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 detail produced by Joe Nuttall in his surface analysis programme, with subsequent LIDAR analysis conducted by the DoBIH team and independently by Myrddyn Phillips. 

LIDAR image of Cae'r Faen Mynach (SN 765 003)

The criteria for the list that this height revision 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 now listed by is Cae’r Faen Mynach and this was derived from the Tithe map, and it is adjoined to the Fan Fawr group of hills, which are situated in the southern part of South Wales (Region B, Sub-Region B5), and it is positioned with a minor road to its west, the A474 road farther to its west, the A4230 and A465 roads to its south and the A4109 road to its east, and has the village of Aberdulais towards the south-east.

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 accompanying Hills to be surveyed sub list, as it was considered not to meet the criteria then used for the main P30 category. 

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

When the sub list was standardised, and interpolated heights and drop values also included the details for this hill were re-evaluated and it was listed with an estimated c 27m of drop, based on the 234m summit spot height that appears on the Ordnance Survey 1:50,000 Landranger and 1:25,000 Explorer map and an estimated c 207m bwlch height, based on interpolation of 10m contouring between 200m – 210m.

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 summit image of Cae'r Faen Mynach (SN 765 003) 

LIDAR analysis gives the highest ground on this hill as 237.6m positioned at SN 76549 00356, and when compared to its originally listed summit height of 234m 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 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.

Therefore, the new listed summit height of this hill is 237.6m and this was derived from LIDAR analysis, this is 3.4m higher than the originally listed summit height of 234m, which was based on the spot height that appears on the Ordnance Survey 1:50,000 Landranger and 1:25,000 Explorer map.

 

ills of Wales, and are reproduced below@

The full details for the hill are:

Group:  Fan Fawr 

Name:  Cae’r Faen Mynach 

OS 1:50,000 map:  170

Summit Height (New Height):  237.6m (LIDAR) 

Summit Grid Reference:  SN 76549 00356 (LIDAR) 

Bwlch Height:  207.0m (LIDAR) 

Bwlch Grid Reference:  SN 75759 01169 (LIDAR) 

Drop:  30.6m (LIDAR) 

 

Myrddyn Phillips (December 2023)