Mapping Mountains – Hill Reclassifications – 200m Twmpau

Diosgydd (SH 777 583) – 200m Sub-Twmpau addition

There has been an addition to the list of 200m 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 Diosgydd (SH 777 583)

The criteria for the list that this addition applies to are:

200m Twmpau – Welsh hills at or above 200m and below 300m in height that have 30m minimum drop, with an accompanying sub list entitled the 200m Sub-Twmpau, with the criteria for this sub category being all Welsh hills at or above 200m and below 300m 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 200m Twmpau by Myrddyn Phillips

The name the hill is now listed by is Diosgydd and this was derived from the Tithe map, and it is adjoined to the Carnedd Llywelyn group of hills, which are situated in the north-western part of North Wales (Region A, Sub-Region A1), and it is positioned encircled by minor roads, with the A5 road farther to its south-west, and has the town of Betws-y-coed towards the south-east.

When the original 200m height band of Welsh P30 hills were published on Geoff Crowder’s v-g.me website, this hill was not included in the accompanying 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 14m of drop, based on an estimated c 221m summit height and an estimated c 207m bwlch height, with both heights based on interpolation of 10m contouring that appears on the contemporary 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 addition of this hill to 200m Sub-Twmpau status is due to LIDAR analysis, resulting in a 220.85m summit height and a 200.3m bwlch height, with these values giving this hill 20.6m of drop, which is sufficient for it to be classified as a 200m Sub-Twmpau. 

 

The full details for the hill are: 

Group:  Carnedd Llywelyn 

Name:  Diosgydd 

OS 1:50,000 map:  115

Summit Height:  220.85m (LIDAR) 

Summit Grid Reference:  SH 77763 58341 (LIDAR) 

Bwlch Height:  200.3m (LIDAR) 

Bwlch Grid Reference:  SH 77620 58363 & SH 77888 58272 (LIDAR) 

Drop:  20.6m (LIDAR) 

 

Myrddyn Phillips (December 2023)