Mapping Mountains – Hill Reclassifications – 30-99m Twmpau

 

Ynys Dafydd (SH 646 134) – 30-99m Sub-Twmpau reclassified to 30-99m Twmpau

There has been a reclassification to the list of 30-99m Twmpau, with the summit height, bwlch height and their locations, the drop and status of the hill initiated by Joe Nuttall who produced a summit analysis programme using LIDAR, and then by LIDAR analysis initially conducted by Jim Bloomer and subsequently by Myrddyn Phillips. 

LIDAR image of Ynys Dafydd (SH 646 134)

The criteria for the list that this reclassification 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 Ynys Dafydd, and it is adjoined to the Rhinogydd group of hills, which are situated in the western part of North Wales (Region A, Sub-Region A3), and it is positioned with the coast to its south-west, the Afon Dyfi (River Dovey) to its south and the A496 road to its north-west, and has the town of Abermaw (Barmouth) towards the west south-west. 

When the original 30-99m height band of Welsh P30 hills were published on Geoff Crowder’s v-g.me website this hill was included in the Hills to be surveyed sub list that accompanied the main P30 list, as it was considered not to meet the criteria then used in 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 27m of drop based on an estimated c 42m summit height and an estimated c 15m bwlch height, with both values based on interpolation of 10m contouring 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 reclassification of this hill from 30-99m Sub-Twmpau status is due to LIDAR analysis, resulting in a 52.8m summit height and a 17.1m bwlch height, with these values giving this hill 35.7m of drop, which is sufficient for it to be classified as a 30-99m Twmpau. 

 

The full details for the hill are: 

Group:  Rhinogydd 

Name:  Ynys Dafydd 

OS 1:50,000 map:  124

Summit Height:  52.8m (LIDAR) 

Summit Grid Reference:  SH 64636 17484 (LIDAR) 

Bwlch Height:  17.1m (LIDAR) 

Bwlch Grid Reference:  SH 64599 17744 (LIDAR) 

Drop:  35.7m (LIDAR) 

 

Myrddyn Phillips (September 2021)