Mapping Mountains – Hill Reclassifications – 30-99m Twmpau and 100m Twmpau

The Knoll (ST 424 904) – 30-99m Twmpau reclassified to 100m Twmpau

There has been a reclassification to the list of 30-99m Twmpau and 100m Twmpau, with the summit height, bwlch height and their locations, the drop, dominance and status of the hill confirmed by LIDAR analysis conducted by Myrddyn Phillips.

LIDAR image of The Knoll (ST 424 904)

The criteria for the two listings that this reclassification applies to are:

30-99m Twmpau – Welsh hills at or above 30m and below 100m in height that have 30m minimum drop, with an accompanying sub list entitled the 30-99m Sub-Twmpau with the criteria for this sub category being all Welsh hills at or above 30m and below 100m 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 – 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 name the hill is listed by is The Knoll, and it is adjoined to the Gwent Is Coed group of hills which are situated in the south-eastern part of South Wales (Region C, Sub-Region C3), and it is positioned with the A48 road to its north and the M4 motorway to its south, and has the city of Casnewydd (Newport) towards the west south-west.

When the original 30-99m height band of Welsh P30 hills were published on Geoff Crowder’s v-g.me website this hill was included in the main P30 list and listed with a c 90m summit height, based on its uppermost contour that appears on the Ordnance Survey 1:50,000 Landranger and 1:25,000 Explorer map.

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

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 c 51m of drop, based on an estimated c 94m summit height based on interpolation of 5m contouring and a 43m bwlch spot height that appears on 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 close-up image of The Knoll

Therefore, the reclassification of this hill from 30-99m Twmpau to 100m Twmpau status is due to LIDAR analysis, resulting in a 101.9m summit height and a 43.1m bwlch height, with these values giving this hill 58.7m of drop and 57.66% dominance.  With the summit height of this hill sufficient for it now to be classified as a 100m Twmpau.

The full details for the hill are:

Group:  Gwent Is Coed

Name:  The Knoll

OS 1:50,000 map:  171, 172

Summit Height:  101.9m (LIDAR)

Summit Grid Reference:  ST 42467 90424 (LIDAR)

Bwlch Height:  43.1m (LIDAR)

Bwlch Grid Reference:  ST 42135 90345 (LIDAR)

Drop:  58.7m (LIDAR)

Myrddyn Phillips (July 2020)