Mapping Mountains – Hill Reclassifications – 30-99m Twmpau

Caerau (ST 134 748) – 30-99m Sub-Twmpau addition

There has been an addition 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 Caerau (ST 134 748)

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 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 Caerau 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 positioned with the A48 road to its north-west and the A4232 road to its immediate south and east, and has the district of Ely in the western fringes of the city of Caerdydd (Cardiff) towards the 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 not 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 this sub 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 an c 81m interpolated summit height and an estimated bwlch height of c 53m.

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

This reassessment was reliant upon the Ordnance Survey Vector Map Local hosted on the Geograph website and which is entitled the Interactive Coverage Map.  This mapping has many more spot heights compared to other publicly available Ordnance Survey maps, and the OS Maps website which 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.

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. 

Close up LIDAR image of Caerau

Therefore, the addition of Caerau to 30-99m Sub-Twmpau status is due to LIDAR analysis, resulting in an 80.9m summit height positioned at ST 13476 74874 and a 51.4m bwlch height positioned at ST 13446 74765, with these values giving this hill 29.5m of drop, which is sufficient for it to be classified as a 30-99m Sub-Twmpau.

The full details for the hill are:

Group:  Bro Morgannwg

Name:  Caerau

OS 1:50,000 map:  171

Summit Height:  80.9m (LIDAR)

Summit Grid Reference:  ST 13476 74874 (LIDAR)

Bwlch Height:  51.4m (LIDAR)

Bwlch Grid Reference:  ST 13446 74765 (LIDAR)

Drop:  29.5m (LIDAR)

Myrddyn Phillips (November 2019)