Mapping Mountains – Hill Reclassifications – 30-99m Twmpau

Ynys y Barri (ST 120 668) – 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 Ynys y Barri (ST 120 668)

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 Ynys y Barri 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 as its name implies it is an island, which is now joined to the mainland by the A4055 road, and has the town of Y Barri (Barry) to its immediate north.

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. 

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

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 36m of drop based on the 42m summit height derived from information received from David Purchase relating to a height on the Ordnance Survey series of 1:10,000 maps and an estimated bwlch height of c 6m.

Extract from the Ordnance Survey series of 1:10,000 maps

This area was re-examined when the OS Maps website became available online.  This is the replacement for OS Get-a-map and has contours at 5m intervals.  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 Barry Island to 30-99m Twmpau status is due to LIDAR analysis, resulting in a 41.6m summit height and a 2.9m bwlch height, with these values giving this hill 38.7m 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:  Ynys y Barri

OS 1:50,000 map:  171

Summit Height:  41.6m (LIDAR)

Summit Grid Reference:  ST 12019 66818 (LIDAR)

Bwlch Height:  2.9m (LIDAR)

Bwlch Grid Reference:  ST 10943 66794 (LIDAR)

Drop:  38.7m (LIDAR)

Myrddyn Phillips (November 2019)