Mapping Mountains – Hill Reclassifications – 200m Twmpau

Cefn Bwlch Cennant (SN 802 424) – 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 derived from LIDAR analysis conducted by Myrddyn Phillips. 

LIDAR image of Cefn Bwlch Cennant (SN 802 424)

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 Bwlch Cennant and this was derived from the Tithe map, and it is adjoined to the Drygarn Fawr group of hills, which are situated in the central part of the South Wales Region (Region B, Sub-Region B1), and it is positioned encircled by minor roads, with the A483 road farther to its south-east, and has the town of Llanymddyfri (Llandovery) towards the south south-west. 

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-evaluated and it was listed with an estimated c 29m of drop, based on an estimated c 283m summit height and an estimated c 254m bwlch height, with 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 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 this hill it had a 282m summit spot height and a 251m bwlch spot height, with these values giving this hill 31m of drop. 

Another of the mapping resources now available online is the Magic Maps website which hosts an interactive map originated from Ordnance Survey data.  This mapping also shows a 282m spot height on the summit area of this hill. 

Extract from the Magic Maps website

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 200m Sub-Twmpau status is due to LIDAR analysis, resulting in a 281.2m summit height and a 250.55m bwlch height, with these values giving this hill 30.6m of drop, which is sufficient for it to be classified as a 200m Twmpau.  With the caveat that LIDAR gives a 281.6m height to a raised boundary bank that is considered a relatively recent man-made construct and therefore is not taken as a part of the height of this hill. 

 

The full details for the hill are: 

Group:  Drygarn Fawr 

Name:  Cefn Bwlch Cennant 

OS 1:50,000 map:  147, 160

Summit Height:  281.2m (LIDAR) 

Summit Grid Reference:  SN 80249 42439 (LIDAR) 

Bwlch Height:  250.55m (LIDAR) 

Bwlch Grid Reference:  SN 80310 42683 (LIDAR) 

Drop:  30.6m (LIDAR) 

 

Myrddyn Phillips (January 2022)