Mapping Mountains – Hill Reclassifications – 200m Twmpau

Rhandir Llwyn Piod (SN 593 281) – 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 detail on contemporary maps produced from Ordnance Survey data. 

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 now listed by is Rhandir Llwyn Piod and this was derived from the Tithe map, and it is adjoined to the Mynydd Mallaen group of hills, which are situated in the central part of South Wales (Region B, Sub-Region B1), and it is positioned encircled by minor roads, with the A40 road farther to its south, and has the town of Llandeilo towards the south south-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-evaluated and it was listed with an estimated c 31m of drop, based on the 257m summit spot height that appears on the Ordnance Survey 1:25,000 Explorer map and an estimated c 226m bwlch height, based on interpolation of 10m contouring between 220m – 230m. 

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. 

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 225m – 230m, with interpolation placing the height of the bwlch as an estimated c 227m, resulting in the drop value for this hill being amended to an estimated c 30m. 

Therefore, the reclassification of this hill from 200m Sub-Twmpau status is due to detail on contemporary maps produced from Ordnance Survey data, resulting in a 257m summit height and an estimated c 227m bwlch height, with these values giving this hill an estimated c 30m of drop, which is sufficient for it to be classified as a 200m Twmpau. 

 

The full details for the hill are: 

Group:  Mynydd Mallaen 

Name:  Rhandir Llwyn Piod 

OS 1:50,000 map:  146

Summit Height:  257m (spot height) 

Summit Grid Reference:  SN 59308 28166 (hand-held GPS via DoBIH) 

Bwlch Height:  c 227m (interpolation) 

Bwlch Grid Reference:  SN 60257 29082 (interpolation) 

Drop:  c 30m (spot height summit and interpolated bwlch) 

 

Myrddyn Phillips (March 2022)