Mapping Mountains – Hill Reclassifications – 30-99m Twmpau

 

Dinas Fach (SM 825 226) – 30-99m 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 obtained from the summit analysis programme that uses LIDAR produced by Joe Nuttall, and then by LIDAR analysis initially conducted by Jim Bloomer and subsequently by Myrddyn Phillips. 

LIDAR image of Dinas Fach (SM 825 226)

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 Dinas Fach, and it is adjoined to the Carn Llidi group of hills which are situated in the south-western part of Mid and West Wales (Region B, Sub-Region B4), and it is a tidal island with its connection to the mainland at low tide positioned to the north-east of its summit, and has the A487 road to its north and the village of Solfach (Solva) towards the north-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 not included in the main P30 list or the accompanying Hills to be surveyed sub list, as it was considered not to meet the criteria then used in either category. 

Extract from the Ordnance Survey 1:50,000 Landranger map

After the sub list was standardised, and interpolated heights and drop values also included the details for this hill were re-evaluated and it remained unlisted, due to a single uppermost ring contour being shown on the Ordnance Survey 1:50,000 Landranger map and intermittent ring contours being shown on the 1:25,000 Explorer map. 

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

Just prior to analysing this hill via LIDAR its details were re-examined against online interactive mapping and it was listed with an estimated c 29m of drop, based on the 30m summit spot height that appears on the Magic Maps website and an estimated c 1m bwlch height, based on interpolation of 5m contouring between 0 – 5m that appears on the Ordnance Survey 1:25,000 Explorer map. 

Extract from the Magic Maps website

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 30-99m Twmpau status is due to LIDAR analysis, resulting in a 30.45m summit height and a 0.3m bwlch height, with these values giving this hill 30.15m of drop, which is sufficient for it to be classified as a 30-99m Twmpau. 

 

The full details for the hill are: 

Group:  Carn Llidi 

Name:  Dinas Fach 

OS 1:50,000 map:  157

Summit Height:  30.45m (LIDAR) 

Summit Grid Reference:  SM 82559 22659 (LIDAR) 

Bwlch Height:  0.3m (LIDAR) 

Bwlch Grid Reference:  SM 82605 22745 (LIDAR) 

Drop:  30.15m (LIDAR) 

 

Myrddyn Phillips (September 2021)