Mapping Mountains – Hill Reclassifications – 30-99m Twmpau

 

Mynydd Pant y Gaseg (SH 414 944) – 30-99m Sub-Twmpau deletion 

There has been a deletion to the list of 30-99m 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 deletion applies to are: 

30-99m Twmpau – Welsh hills at or above 30m and below 100m in height that have 30m minimum drop, with an accompanying sub list entitled the 30-99m Sub-Twmpau with the criteria for this sub category being all Welsh hills at or above 30m and below 100m 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 30-99m Twmpau by Myrddyn Phillips

The name the hill is listed by is Mynydd Pant y Gaseg and it is adjoined to the Ynys Môn group of hills, which are situated in the north-western part of North Wales (Region A, Sub-Region A1), and it is positioned with the coast to its immediate north and the A5025 road to its south, and has the town of Amlwch towards the east south-east. 

When the original 30-99m 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, with an accompanying note stating; Height is also 80c.  1:50,000 and 1:25,000 maps differ. 

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

The contouring on the two publicly available Ordnance Survey maps of the time differed with the 1:50,000 Landranger giving an uppermost 80m contour and the 1:25,000 Explorer giving an uppermost 65m contour.  In the original hand-written list the height given is c 80, however when this list was published it was decided to use the 1:25,000 map height but at the same time retain the hill as a sub. 

Extract from the Ordnance Survey 1:50,000 Landranger map

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 15m of drop, based on an estimated c 67m summit height and an estimated c 52m bwlch height, with both heights based on interpolation of 5m contouring that appear on the Ordnance Survey 1:25,000 Explorer map. 

Since publication of the P30 lists on Geoff Crowder’s v-g.me website there have been a number of Ordnance Survey 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 are current and digitally updated such as the old Ordnance Survey Vector Map Local hosted on the Geograph website and which was entitled the Interactive Coverage 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 maps and for this hill it had a 67m summit spot height and a 51m bwlch spot height, with these values giving this hill 16m of drop. 

Therefore, the deletion of this hill from 30-99m Sub-Twmpau status is due to detail on the Ordnance Survey Vector Map Local hosted on the Geograph website and which was entitled the Interactive Coverage Map, resulting in a 67m summit height and a 51m bwlch height, with these values giving this hill 16m of drop, which is insufficient for it to be classified as a 30-99m Sub-Twmpau. 

 

The full details for the hill are: 

Group:  Ynys Môn 

Name:  Mynydd Pant y Gaseg 

OS 1:50,000 map:  114

Summit Height:  67m (spot height) 

Summit Grid Reference:  SH 41410 94459 (spot height)               

Bwlch Height:  51m (spot height) 

Bwlch Grid Reference:  SH 41246 94369 (spot height) 

Drop:  16m (spot height summit and bwlch) 

 

Myrddyn Phillips (September 2021)