Mapping Mountains – Hill Reclassifications – 200m Twmpau

 

Cefn Uchaf (SJ 157 072) – 200m Twmpau reclassified to 200m Sub-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 and a subsequent Trimble GeoXH 6000 survey conducted by Myrddyn Phillips. 

Cefn Uchaf (SJ 157 072)

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 Cefn Uchaf and this was derived from the Tithe map, and it is adjoined to the Carnedd Wen group of hills which are situated in the south-eastern part of North Wales (Region A, Sub-Region A4), and it is positioned with the B4392 road to its north-west and the A458 road to its south, and has the town of Y Trallwng (Welshpool) towards the 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 main P30 list, as map details on the contemporary Ordnance Survey 1:25,000 Explorer map gave this hill a 262m spot height, whilst its adjacent hill now listed as Quarry Field and which is positioned at SJ 15530 07552 was not given a spot height and remained with just an uppermost 260m ring contour. 

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

After 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 54m of drop based on the 262m summit spot height that appears on the Ordnance Survey 1:25,000 Explorer map and an estimated c 208m bwlch height based on interpolation of 10m contouring.  With an adjacent hill now listed as Quarry Field (SJ 155 075) listed with an estimated c 25m of drop, based on an estimated c 262m summit height and an estimated c 237m bwlch height, with both heights based on interpolation of 10m contouring that appear on the Ordnance Survey 1:25,000 Explorer map. 

The details for this hill were 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 the area taking in this hill it also had contouring at 5m intervals with an uppermost 265m contour given the adjacent hill of Quarry Field, with interpolation giving an estimated c 266m summit height and as this is higher than the 262m summit spot height given Cefn Uchaf the bylchau were swapped and each hill was reclassified. 

LIDAR image of Cefn Uchaf (SJ 157 072)

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 Trimble GeoXH 6000 gathering data at the summit of Cefn Uchaf

LIDAR analysis confirms Cefn Uchaf as lower than Quarry Field and as the summit of both hills have now been surveyed with the Trimble GeoXH 6000 it is this result that is being prioritised for listing purposes. 

The Trimble GeoXH 6000 survey result for each summit is given below:  

Cefn Uchaf:  261.8m at SJ 15739 07264 

Quarry Field:  268.5m at SJ 15530 07552 

 

Therefore, the confirmation of the reclassification of this hill from 200m Twmpau status is due to LIDAR analysis and a subsequent Trimble GeoXH 6000 survey, resulting in a 261.8m summit height and a 235.1m bwlch height, with these values giving this hill 26.6m of drop, and therefore it is reclassified to a 200m Sub-Twmpau. 

 

The full details for the hill are: 

Group:  Carnedd Wen 

Name:  Cefn Uchaf 

OS 1:50,000 map:  125

Summit Height:  261.8m (converted to OSGM15, Trimble GeoXH 6000) 

Summit Grid Reference:  SJ 15739 07264 (Trimble GeoXH 6000) 

Bwlch Height:  235.1m (LIDAR) 

Bwlch Grid Reference:  SJ 15745 07416 (LIDAR) 

Drop:  26.6m (Trimble GeoXH 6000 summit and LIDAR bwlch) 

 

Myrddyn Phillips (June 2021)