Mapping Mountains – Hill Reclassifications – 200m Twmpau

 

Maes y Cefn (SN 797 954) – 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 Maes y Cefn (SN 797 954)

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

The 200m Twmpau by Myrddyn Phillips

The name the hill is listed by is Maes y Cefn and this was derived from the Tithe map, 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 A489 road to its north, the A487 road to its west and the B4518 road to its east, and has the village of Aberhosan towards the north north-east. 

When the original 200m 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 30m of drop, based on the 252m summit spot height that appears on the Ordnance Survey 1:25,000 Explorer map and an estimated c 222m bwlch height, based on interpolation of 10m contouring between 220m – 230m 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 were later re-assessed and its bwlch height was amended to an estimated c 221m with this giving the hill an estimated c 31m 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. 

LIDAR gives the height of the critical point of a field boundary that crosses the bwlch in a hill to hill direction as 221.0m positioned at SN 79489 95118, as this is considered a relatively recent man-made construct it is discounted from the height of the bwlch.  LIDAR also gives the natural ground at the immediate base of this field boundary as being 220.9m in height; therefore it is this height that is taken for that of the bwlch. 

Therefore, the confirmation of the reclassification of this hill from 200m Sub-Twmpau status is due to LIDAR analysis, resulting in a 250.9m summit height and a 220.9m bwlch height, with these values giving this hill 30.0m of drop, which is sufficient for it to be classified as a 200m Twmpau. 

 

The full details for the hill are: 

Group:  Pumlumon 

Name:  Maes y Cefn 

OS 1:50,000 map:  135

Summit Height:  250.9m (LIDAR) 

Summit Grid Reference:  SN 79761 95462 (LIDAR) 

Bwlch Height:  220.9m (LIDAR) 

Bwlch Grid Reference:  SN 79484 95138 (LIDAR) 

Drop:  30.0m (LIDAR) 

 

Myrddyn Phillips (March 2021)