Mapping Mountains – Hill Reclassifications – 200m Twmpau

Diosgydd (SH 775 579) – 200m Sub-Twmpau reclassified to 200m Twmpau

There has been a reclassification to the list of 200m Twmpau, with the summit height, bwlch height and their locations, the drop and status of the hill derived from detail produced by Joe Nuttall in his surface analysis programme, with subsequent LIDAR analysis conducted by the DoBIH team and independently by Myrddyn Phillips. 

LIDAR image of Diosgydd (SH 775 579)

The criteria for the list that this reclassification 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 included in the accompanying Hills to be surveyed sub list, as it was considered not to meet the criteria then used for the main P30 category.

When 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 28m of drop, based on the 233m summit spot height that appears on the Ordnance Survey 1:25,000 Explorer map and an estimated c 205m bwlch height, based on interpolation of 10m contouring between 200m – 210m. 

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 reclassification of this hill from 200m Sub-Twmpau status is due to LIDAR analysis, resulting in a 233.05m summit height and a 202.85m bwlch height, with these values giving this hill 30.2m of drop, which is sufficient for it to be classified as a 200m Twmpau. 

 

The full details for the hill are: 

Group:  Carnedd Llywelyn 

Name:  Diosgydd 

OS 1:50,000 map:  115

Summit Height:  233.05m (LIDAR) 

Summit Grid Reference:  SH 77503 57940 (LIDAR) 

Bwlch Height:  202.85m (LIDAR) 

Bwlch Grid Reference:  SH 77793 57796 (LIDAR) 

Drop:  30.2m (LIDAR) 

 

Myrddyn Phillips (October 2023)