Mapping Mountains – Hill Reclassifications – 100m Twmpau

 

Pt. 104.3m (SN 712 990) – 100m Sub-Twmpau reclassified to 100m Twmpau

There has been confirmation of a reclassification to the list of 100m 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. 104.3m (SN 712 990)

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, with the word Twmpau being an acronym standing for thirty welsh metre prominences and upward. 

The 100m Twmpau by Myrddyn Phillips

The hill is being listed by the point (Pt. 104.3m) 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 Pumlumon group of hills, which are situated in the north-western part of Mid and West Wales (Region B, Sub-Region B2), and it has the A487 road to its north and west, and a minor road to its immediate south-east, and has the town of Machynlleth towards the north north-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 that accompanied the main P30 list, as it was considered not to meet the criteria then used for 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-assessed and it was listed with an estimated c 25m of drop, based on an estimated c 102m summit height and an estimated c 177m bwlch height, both heights based on interpolation of 10m contouring that appear on the Ordnance Survey 1:25,000 Explorer map. 

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

The details for this hill we re-assessed when the Ordnance Survey Vector Map Local hosted on the Geograph website and which was entitled the Interactive Coverage Map became available online.  This mapping had many spot heights not on other publicly available Ordnance Survey maps and for this hill a 104m spot height was given on the area of its summit and with a re-appraisal of its estimated bwlch height to c 74m, these values gave this hill an estimated c 30m of drop. 

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 100m Sub-Twmpau status is due to LIDAR analysis, resulting in a 104.3m summit height and a 73.0m bwlch height, with these values giving this hill 31.2m of drop, which is sufficient for it to be classified as a 100m Twmpau.

 

The full details for the hill are: 

Group:  Pumlumon 

Name:  Pt. 104.3m 

OS 1:50,000 map:  135

Summit Height:  104.3m (LIDAR) 

Summit Grid Reference:  SN 71207 99002 (LIDAR) 

Bwlch Height:  73.0m (LIDAR) 

Bwlch Grid Reference:  SN 71498 98989 (LIDAR) 

Drop:  31.2m (LIDAR) 

 

Myrddyn Phillips (April 2021)