Mapping Mountains – Hill Reclassifications – 30-99m Twmpau

 

Cromlech (SH 365 924) – 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 LIDAR analysis conducted by Myrddyn Phillips. 

LIDAR image of Cromlech (SH 365 924)

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 Cromlech 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 coast and the A5025 road to its north, and minor roads to its south and east, and has the village of Cemaes 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 21m of drop based on the 49m summit spot height and an estimated c 28m bwlch height, with the latter based on interpolation of 5m contouring between 25m – 30m that appears on the Ordnance Survey 1:25,000 Explorer map. 

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

However, it was not until LIDAR became available that the details for this hill could be accurately re-assessed.  The LIDAR (Light Detection & Ranging) technique produced highly accurate height data that is now freely available for much of England and Wales. 

Therefore, the deletion of this hill from 30-99m Sub-Twmpau status is due to LIDAR analysis, resulting in a 47.95m summit height and a 28.1m bwlch height, with these values giving this hill 19.8m 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:  Cromlech 

OS 1:50,000 map:  114

Summit Height:  47.95m (LIDAR) 

Summit Grid Reference:  SH 36568 92457 (LIDAR) 

Bwlch Height:  28.1m (LIDAR) 

Bwlch Grid Reference:  SH 36561 91044 (LIDAR) 

Drop:  19.8m (LIDAR) 

 

Myrddyn Phillips (July 2021)