Mapping Mountains – Hill Reclassifications – 100m Twmpau

 

Allt Dderw (SN 638 832) – 100m Sub-Twmpau reclassified to 100m Twmpau

There has been confirmation of a reclassification to the list of 100m Twmpau, with the summit height, bwlch height and their locations, the drop and status of the hill confirmed by LIDAR analysis conducted by Myrddyn Phillips. 

LIDAR image of Allt Dderw (SN 638 832)

The criteria for the list that this reclassification 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. 

The 100m Twmpau by Myrddyn Phillips

The name the hill is listed by is Allt Dderw and it is adjoined to the Pumlumon group of hills, which are situated in the north-western part of Mid and West Wales (Region B, Sub-Region B2), and it is positioned with minor roads to its north, south and east, and the A4159 road to its west, and has the town of Aberystwyth towards the west 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 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 30m of drop, based on the 142m summit spot height that appears on the Ordnance Survey 1:25,000 Explorer map and an estimated c 112m bwlch height based on interpolation of 10m contouring between 110m – 120m. 

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 image of Allt Dderw (SN 638 832)

Therefore, the confirmation of the reclassification of this hill from 100m Sub-Twmpau status is due to LIDAR analysis, resulting in a 142.2m summit height and a 111.5m bwlch height, with these values giving this hill 30.7m of drop, which is sufficient for it to be classified as a 100m Twmpau. 

 

The full details for the hill are: 

Group:  Pumlumon 

Name:  Allt Dderw 

OS 1:50,000 map:  135

Summit Height:  142.2m (LIDAR) 

Summit Grid Reference:  SN 63838 83206 & SN 63838 83204 & SN 63840 83203 (LIDAR) 

Bwlch Height:  111.5m (LIDAR) 

Bwlch Grid Reference:  SN 64419 83203 & SN 64418 83202 & SN 64419 83198 (LIDAR) 

Drop:  30.7m (LIDAR) 

 

Myrddyn Phillips (June 2021)