Mapping Mountains – Hill Reclassifications – 30-99m Twmpau

 

Ynys y Fydlyn (SH 291 917) – Double Sub-Twmpau deletion 

There has been a deletion to the list of Double Sub-Twmpau adjoined to the 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 with 30m minimum drop, this height band of hills has two accompanying sub lists, the first of which is 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, and the second sub category is entitled the Double Sub-Twmpau with the criteria for this sub category being all Welsh hills at or above 20m and below 30m 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 Ynys y Fydlyn 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-west and south-west with a series of minor roads to its east and the A5025 road farther to its south-east, and has the village of Cemaes towards the east north-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 not included in the Hills to be surveyed sub list, as it was considered not to meet the criteria then used for this sub 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 21m of drop, based on an estimated c 22m summit height which was based on interpolation of the uppermost contour on contemporary Ordnance Survey 1:25,000 Explorer maps and an estimated c 1m bwlch height. 

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

The details for this hill have been re-evaluated and it is now listed with an estimated c 15m of drop, based on the 21m summit spot height that appears on the Ordnance Survey interactive mapping hosted on the Geograph website and an estimated bwlch height of c 6m based on interpolation of 5m contouring between 5m – 10m. 

Extract from the Ordnance Survey interactive mapping hosted on the Geograph website

Therefore, the deletion of this hill from Double Sub-Twmpau status is due to detail on contemporary maps produced from Ordnance Survey data, resulting in a 21m summit height and an estimated c 6m bwlch height, with these values giving this hill an estimated c 15m of drop which is insufficient for it to be classified as a Double Sub-Twmpau. 

 

The full details for the hill are: 

Group:  Ynys Môn

Name:  Ynys y Fydlyn

OS 1:50,000 map:  114

Summit Height:  21m (spot height)

Summit Grid Reference:  SH 29153 91749 (spot height)

Bwlch Height:  c 6m (interpolation)

Bwlch Grid Reference:  SH 29202 91739 (interpolation) 

Drop:  c 15m (spot height summit and interpolated bwlch)

 

Myrddyn Phillips (April 2021)