Mapping Mountains – Hill Reclassifications – 200m Twmpau

 

Penllan Oat Piece (SO 103 633) – 200m Sub-Twmpau addition

There has been of an addition to the list of 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 Penllan Oat Piece (SO 103 633)

The criteria for the list that this addition 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.  The list is authored by Myrddyn Phillips, 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 now listed by is Penllan Oat Piece, and this was derived from the Tithe map, and it is adjoined to the Hirddywel group of hills, which are situated in the eastern part of South Wales (Region B, Sub-Region B2), and it is positioned with the A44 road to its north, the A483 road to its west and minor roads to its south-west and south-east, and has the town of Llandrindod towards the west 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 not included in the accompanying Hills to be surveyed sub list, as it was considered not to meet the criteria then used for this sub category.

After 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 17m of drop, based on the 259m summit spot height that appears on the Ordnance Survey 1:25,000 Explorer map and an estimated c 242m bwlch height, based on interpolation of 10m contouring between 240m – 250m. 

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.

Therefore, the addition of this hill to 200m Sub-Twmpau status is due to LIDAR analysis, resulting in a 260.3m summit height and a 240.0m bwlch height, with these values giving this hill 20.3m of drop, which is sufficient for it to be classified as a 200m Sub-Twmpau. 

 

The full details for the hill are: 

Group:  Hirddywel 

Name:  Penllan Oat Piece 

OS 1:50,000 map:  148

Summit Height:  260.3m (LIDAR) 

Summit Grid Reference:  SO 10357 63307 (LIDAR) 

Bwlch Height:  240.0m (LIDAR) 

Bwlch Grid Reference:  SO 10742 63832 (LIDAR) 

Drop:  20.3m (LIDAR) 

 

Myrddyn Phillips (August 2026)