Mapping Mountains – Significant Height Revisions – 200m Twmpau

Allt Mes (SN 795 397)

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 conducted by Myrddyn Phillips. 

LIDAR image of Allt Mes (SN 795 397)

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. 

200m Twmpau by Myrddyn Phillips

The name the hill is listed by is Allt Mes, and it is adjoined to the Drygarn Fawr group of hills, which are situated in the central part of the South Wales Region (Region B, Sub-Region B1), and it is positioned with minor roads to its north-east, north-west and south-west, and the A483 road to its south-east, and has the town of Llanymddyfri (Llandovery) towards the south south-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-evaluated and it was listed with an estimated c 33m of drop, based on the 217m summit spot height that appears on the Ordnance Survey 1:25,000 Explorer map and an estimated c 184m bwlch height, based on interpolation of 10m contouring between 180m – 190m that appear on the Ordnance Survey 1:25,000 Explorer map. 

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

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 Allt Mes (SN 795 397)

The summit height produced by LIDAR analysis is 219.4m and 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 219.4m and this was derived from LIDAR analysis, this is 2.4m higher than the originally listed summit height of 217m which appears as a spot height on the Ordnance Survey 1:25,000 Explorer map. 

 

The full details for the hill are: 

Group:  Drygarn Fawr 

Name:  Allt Mes 

OS 1:50,000 map:  146, 160

Summit Height (New Height):  219.4m (LIDAR) 

Summit Grid Reference:  SN 79575 39715 (LIDAR) 

Bwlch Height:  182.8m (LIDAR) 

Bwlch Grid Reference:  SN 79199 39937 (LIDAR) 

Drop:  36.6m (LIDAR) 

 

Myrddyn Phillips (January 2022)