Mapping Mountains – Hill Reclassifications – 100m Twmpau

Graig yr Harris (SO 338 052) – 100m Sub-Twmpau reclassified to 100m Twmpau

There has been a reclassification to the list of 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 Graig yr Harris (SO 338 052)

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.  The list is authored by Myrddyn Phillips, 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 listed by is Graig yr Harris, and it is adjoined to the Cefn yr Ystrad group of hills, which are situated in the southern part of South Wales (Region B, Sub-Region B5), and it is positioned with minor roads to its north-west and south-west, the A4042 road farther to its west and the B4598 road to its east, and has the town of Brynbuga (Usk) towards the south-east.

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, as it was considered not to meet the criteria then used for the main P30 category.

After 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 28m of drop, based on the 122m summit spot height that appears on the Ordnance Survey 1:25,000 Explorer map and the 94m bwlch spot height that appeared on the Ordnance Survey Vector Map Local hosted on the Geograph website and which was entitled the Interactive Coverage 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.

Therefore, the reclassification of this hill from 100m Sub-Twmpau status is due to LIDAR analysis, resulting in a 121.2m summit height and an 84.9m bwlch height, with these values giving this hill 36.25m of drop, which is sufficient for it to be classified as a 100m Twmpau. 

 

The full details for the hill are: 

Group:  Cefn yr Ystrad 

Name:  Graig yr Harris 

OS 1:50,000 map:  161

Summit Height:  121.2m (LIDAR) 

Summit Grid Reference:  SO 33864 05205 (LIDAR) 

Bwlch Height:  84.9m (LIDAR) 

Bwlch Grid Reference:  SO 32374 04494 (LIDAR) 

Drop:  36.25m (LIDAR) 

 

Myrddyn Phillips (January 2025)