Monday, 25 May 2020

Mapping Mountains – Hill Reclassifications – 200m Twmpau

Pt. 277.3m (ST 485 990) – 200m Sub-Twmpau reclassified to 200m Twmpau

There has been confirmation of a reclassification to the list of 200m 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 Pt. 277.3m (ST 485 990)

The criteria for the list that this reclassification 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 hill is being listed by the point (Pt. 277.3m) notation as an appropriate name for it either through local enquiry and / or historic research has not been found by the author, 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 B4293 road to its west, and has the village of Y Dyfawden (Devauden) towards the south-west.

When the original 200m height band of Welsh P30 hills were published on Geoff Crowder’s website this hill was included in the accompanying Hills to be surveyed sub list, as it was considered not to meet the criteria then used for the main P30 list.  

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 277m summit spot height that appears on the Ordnance Survey 1:50,000 Landranger and 1:25,000 Explorer map and an estimated c 247m bwlch height, with the latter based on interpolation of 5m contouring between 245m – 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. 

The confirmation of the reclassification of this hill from 200m Sub-Twmpau status is due to LIDAR analysis, resulting in a 277.3m summit height and a 246.3m bwlch height, with these values giving this hill 31.0m of drop, which is sufficient for it to be classified as a 200m Twmpau.

The full details for the hill are:

Group:  Gwent Is Coed

Name:  Pt. 277.3m

OS 1:50,000 map:  171

Summit Height:  277.3m (LIDAR)

Summit Grid Reference:  ST 48569 99063 (LIDAR)

Bwlch Height:  246.3m (LIDAR)

Bwlch Grid Reference:  ST 48475 98819 (LIDAR)

Drop:  31.0m (LIDAR)

Myrddyn Phillips (May 2020)

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