Mapping Mountains – Hill Reclassifications – 100m Twmpau

Mynydd Mwyn (SH 409 823) – 100m Sub-Twmpau addition 

There has been an addition 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 addition 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 listed by is Mynydd Mwyn and this was derived from the Tithe map, 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 B5112 road to its north-west and has a minor road immediately to its east, and has the village of Llannerch-y-medd towards the north north-east. 

When the original 100m 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. 

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

After 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 22m of drop, based on the 101m summit spot height and an estimated c 79m bwlch height based on interpolation of 5m contouring between 75m – 80m that appear on the Ordnance Survey 1:25,000 Explorer map. 

Therefore, the addition of this hill to 100m Sub-Twmpau status is due to detail on contemporary maps produced from Ordnance Survey data, resulting in a 101m summit height and an estimated c 79m bwlch height, with these values giving this hill an estimated c 22m of drop which is sufficient for it to be classified as a 100m Sub-Twmpau. 

 

The full details for the hill are: 

Group:  Ynys Môn 

Name:  Mynydd Mwyn 

OS 1:50,000 map:  114, 115

Summit Height:  101m (spot height) 

Summit Grid Reference:  SH 40983 82338 (spot height) 

Bwlch Height:  c 79m (interpolation) 

Bwlch Grid Reference:  SH 40939 83375 (interpolation) 

Drop:  c 22m (spot height summit and interpolated bwlch) 

 

Myrddyn Phillips (March 2021)