Mapping Mountains – Hill Reclassifications – 100m Twmpau

Bryn (SN 274 401) – 100m Sub-Twmpau reclassified to 100m Twmpau

There has been a reclassification to the list of 100m Twmpau, with the summit height, bwlch height and their locations, the drop and status of the hill derived from detail on contemporary maps produced from Ordnance Survey data. 

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 name the hill is now listed by is Bryn and this was derived from the Tithe map, and it is adjoined to the Mynydd Pencarreg group of hills, which are situated in the south-western part of South Wales (Region B, Sub-Region B1), and it is positioned with minor roads to its north-east and south, the A484 road farther to its north-east and the B4332 road to its north-west, and has the town of Castellnewydd Emlyn (Newcastle Emlyn) towards the 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, 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 178m summit spot height adjoined to a triangulation pillar that appears on the contemporary Ordnance Survey 1:50,000 Landranger and 1:25,000 Explorer map and an estimated c 158m bwlch height based on interpolation of 10m contouring between 150m – 160m. 

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

Since the original publication of the Welsh P30 lists on Geoff Crowder’s v-g.me website there have been a number of maps made available online.  Some of these are historic such as the series of Six-Inch maps on the National Library of Scotland website.  Whilst others were digitally updated such as the Ordnance Survey Vector Map Local that was hosted on the Geograph website and which was entitled the Interactive Coverage Map, whilst others are current and digitally updated such as the interactive mapping on the Magic Maps and WalkLakes websites.

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 148m spot height positioned on the area of the bwlch, and when coupled with the 178m summit spot height, these values gave this hill 30m of drop.

One of the mapping resources now available online is on the Magic Maps website which hosts an interactive map originated from Ordnance Survey data.  Until recently this mapping had many spot heights not on other publicly available maps and for this hill it also had the 148m spot height positioned on the area of the bwlch. 

Extract from the Magic Maps website

One of the resources recently available online is the mapping on the OS Maps website and the details for this hill were subsequently re-assessed against this mapping.  This is the replacement for OS Get-a-map and until recent times had contours at 5m intervals which were proving consistently more accurate compared to the 5m contours that sometimes appear on the Ordnance Survey 1:25,000 Explorer map and used to appear on the online Vector Map Local.  This mapping had bwlch contouring between 145m – 150m, with interpolation placing the height of the bwlch as an estimated c 148m, with this position favoured over that of the 148m spot height.

Therefore, the reclassification of this hill from 100m Sub-Twmpau status is due to detail on contemporary maps produced from Ordnance Survey data, , resulting in a 178m summit height and an estimated c 148m bwlch height, with these values giving this hill an estimated c 30m of drop, which is sufficient for it to be classified as a 100m Twmpau. 

 

The full details for the hill are: 

Group:  Mynydd Pencarreg 

Name:  Bryn 

OS 1:50,000 map:  145

Summit Height:  178m (triangulation pillar) 

Summit Grid Reference:  SN 27426 40150 (triangulation pillar) 

Bwlch Height:  c 148m (interpolation) 

Bwlch Grid Reference:  SN 28020 40064 (interpolation) 

Drop:  c 30m (triangulation pillar summit and interpolated bwlch) 

 

Myrddyn Phillips (July 2022)