Mapping Mountains – Hill Reclassifications – 30-99m Twmpau

 

Yr Ogof (SH 356 888) – 30-99m Sub-Twmpau addition

There has been an addition 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 addition 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 Yr Ogof 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 coast to its west, the A5025 road to its north-west and minor roads to its west, south and east, and has the village of Llanfechell towards the north 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. 

After 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 24m of drop based on the 92m summit spot height that appears on the Ordnance Survey 1:25,000 Explorer map and an estimated c 68m bwlch height, based on interpolation of 5m contouring between 65m – 70m. 

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

Therefore, the addition of this hill to 30-99m Sub-Twmpau status is due to detail on contemporary maps produced from Ordnance Survey data, resulting in a 92m summit height and an estimated c 68m bwlch height, with these values giving this hill an estimated c 24m of drop, which is sufficient for it to be classified as a 30-99m Sub-Twmpau. 

 

The full details for the hill are: 

Group:  Ynys Môn 

Name:  Yr Ogof 

OS 1:50,000 map:  114

Summit Height:  92m (spot height) 

Summit Grid Reference:  SH 35650 8879 (spot height) 

Bwlch Height:  c 68m (interpolation) 

Bwlch Grid Reference:  SH 37450 88739 (interpolation) 

Drop:  c 24m (spot height summit and interpolated bwlch) 

 

Myrddyn Phillips (July 2021)