Mapping Mountains – Significant Height Revisions – 100m Twmpau

 

Allt Pendugoedydd (SN 749 311) 

There has been a Significant Height Revision to a hill that is listed in the 100m 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 Pendugoedydd (SN 749 311)

The criteria for the list that this height revision applies to are:

100m Twmpau – Welsh hills at or above 100m and below 200m in height that have 30m minimum drop, with an accompanying sub list entitled the 100m Sub-Twmpau, with the criteria for this sub category being all Welsh hills at or above 100m and below 200m 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. 

100m Twmpau by Myrddyn Phillips

The name the hill is now listed by is Allt Pendugoedydd and this was confirmed by the Ordnance Survey series of Six-Inch maps, and it is adjoined to the Mynydd Du group of hills, which are situated in the southern part of South Wales (Region B, Sub-Region B5), and it is positioned with the A4069 road to its north-west and minor roads to its west and east, and has the village of Llangadog towards the south-west.

When the original 100m height band of Welsh P30 hills were published on Geoff Crowder’s v-g.me website, this hill was not included in the Hills to be surveyed sub list, as with only an uppermost 150m contour on the contemporary Ordnance Survey 1:25,000 Explorer map, it was considered not to meet the criteria then used for this sub category. 

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

Since the original publication of the Welsh P30 lists on Geoff Crowder’s v-g.me website there have been a number of maps made available online.  Some of these are historic such as the series of Six-Inch maps on the National Library of Scotland website.  Whilst others were digitally updated such as the Ordnance Survey Vector Map Local that was hosted on the Geograph website and which was entitled the Interactive Coverage Map, whilst others are current and digitally updated such as the interactive mapping on the Magic Maps and WalkLakes websites.

After the sub list was standardised, and interpolated heights and drop values also included the details for this hill were re-evaluated against the Ordnance Survey Vector Map Local hosted on the Geograph website and which was entitled the Interactive Coverage Map.  This mapping had many spot heights not on other publicly available maps and for this hill it had a 163m summit spot height positioned at SN 74989 31133.

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 Pendugoedydd (SN 749 311)

The height produced by LIDAR analysis to the summit of this hill is 162.4m positioned at SN 74992 31127, and this in relation to the detail on the contemporary Ordnance Survey 1:25,000 Explorer map 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 162.4m and this was derived from LIDAR analysis, this is 12.4m higher than the uppermost 150m contour on the contemporary Ordnance Survey 1:25,000 Explorer map.

 

ills of Wales, and are reproduced below@

The full details for the hill are:

Group:  Mynydd Du 

Name:  Allt Pendugoedydd 

OS 1:50,000 map:  146, 160

Summit Height (New Height):  162.4m (LIDAR) 

Summit Grid Reference:  SN 74992 31127 (LIDAR) 

Bwlch Height:  141.4m (LIDAR) 

Bwlch Grid Reference:  SN 75086 31369 (LIDAR) 

Drop:  21.0m (LIDAR) 

 

Myrddyn Phillips (April 2024)