Mapping Mountains – Hill Reclassifications – 30-99m Twmpau

Ifton Great Wood (ST 455 894) – 30-99m Sub-Twmpau reclassified to 30-99m Twmpau

There has been confirmation of a reclassification to the list of 30-99m 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 Ifton Great Wood (ST 455 894)

The criteria for the list 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.

The 30-99m Twmpau by Myrddyn Phillips

The name the hill is listed by is Ifton Great Wood, 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 M48 motorway to its south, and has the city of Casnewydd (Newport) towards the 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 Hills to be surveyed sub list that accompanied the main P30 list, as it was considered not to meet the criteria then used in 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-evaluated and it was listed with an estimated c 30m of drop based on the 82m summit spot height and an estimated c 52m bwlch height, with the latter based on interpolation of 5m contouring that appears on Ordnance Survey 1:25,000 Explorer maps.  The estimated drop was later changed to 30m based on the 52m spot height that appears on the Ordnance Survey 1:25,000 Explorer map with its dominance listed as 36.59%.

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 confirmation of the reclassification of this hill from 30-99m Sub-Twmpau status is due to LIDAR analysis, resulting in an 84.3m summit height and a 51.2m bwlch height, with these values giving this hill 33.1m of drop and 39.25% dominance, which is sufficient for it to be classified as a 30-99m Twmpau. 

The full details for the hill are:

Group:  Gwent Is Coed

Name:  Ifton Great Wood

OS 1:50,000 map:  171, 172

Summit Height:  84.3m (LIDAR)

Summit Grid Reference:  ST 45565 89483 (LIDAR)

Bwlch Height:  51.2m (LIDAR)

Bwlch Grid Reference:  ST 43958 89132 (LIDAR)

Drop:  33.1m (LIDAR)

Myrddyn Phillips (October 2020)