Mapping Mountains – Hill Reclassifications – 30-99m Twmpau

Dinas Powys (ST 148 722) – 30-99m Sub-Twmpau reclassified to 30-99m Twmpau

There has been confirmation of a reclassification of a hill to the listing of the 30-99m Twmpau, with the summit height, bwlch height and their locations, the drop and status of the hill confirmed by LIDAR analysis conducted by Myrddyn Phillips.

LIDAR image of Dinas Powys (ST 148 722)

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 with 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 name the hill is listed by is Dinas Powys and it is adjoined to the Bro Morgannwg group of hills, which are situated in the southern part of South Wales (Region C, Sub-Region C2), and it is encircled by a number of A roads, with the A4232 to its north, the A4050 to its west, the A4231 towards its south and the A4055 to its east, and has the town of Y Barri (Barry) to the south-west and the town of Penarth towards the 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 included in the Hills to be surveyed sub list that accompanied the main P30 list, as it did not meet the criteria then used in the main P30 list. 

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 29m of drop based on a c 62m interpolated summit height and the 33m spot height that appears at the bwlch on Ordnance Survey 1:25,000 Explorer maps which for this area has contours at 5m intervals.

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

The details for this hill were re-examined when the Ordnance Survey Vector Map Local hosted on the Geograph website and which is entitled the Interactive Coverage Map became available online.  This mapping has many more spot heights compared to other publicly available Ordnance Survey maps.

Extract from the Ordnance Survey Vector Map Local

The details for this hill were also re-assessed when the OS Maps website became available online.  This is the replacement for OS Get-a-map and has contours at 5m intervals which are proving consistently more accurate compared to the 5m contours that sometimes appear on Ordnance Survey 1:25,000 Explorer maps and the online Vector Map Local.  These re-assessments resulted in the hill being listed with c 30m of drop with the interpolated summit height increasing from c 62m to c 63m.

Extract from the OS 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. 

The confirmation of the reclassification of Dinas Powys to 30-99m Twmpau status is due to LIDAR analysis, resulting in a 69.4m summit height positioned at ST 14823 72225 and a 33.6m bwlch height positioned at ST 14757 72333, with these values giving this hill 35.8m of drop, which is sufficient for this hill to be classified as a 30-99m Twmpau.

The full details for the hill are:

Group:  Bro Morgannwg

Name:  Dinas Powys

OS 1:50,000 map:  171

Summit Height:  69.4m (LIDAR)

Summit Grid Reference:  ST 14823 72225 (LIDAR)

Bwlch Height:  33.6m (LIDAR)

Bwlch Grid Reference:  ST 14757 72333 (LIDAR)

Drop:  35.8m (LIDAR)

Myrddyn Phillips (November 2019)