Mapping Mountains – Hill Reclassifications – Y Trechol – The Dominant Hills of Wales

Cae Rosser (SO 388 005) – Lesser Dominant reclassified to Dominant

There has been a reclassification to the listing of Y Trechol – The Dominant Hills of Wales, with the summit height, bwlch height and their locations, the drop, dominance and status of the hill confirmed by LIDAR analysis conducted by Myrddyn Phillips.

LIDAR image of Cae Rosser (SO 388 005)

The criteria for the list that this reclassification applies to are:

Y Trechol – The Dominant Hills of Wales – Welsh P30 hills whose prominence equal or exceed half that of their absolute height.  With the criteria for Lesser Dominant status being those additional Welsh P30 hills whose prominence is between one third and half that of their absolute height.  The list is authored by Myrddyn Phillips with the Introduction to the start of the Mapping Mountains publication of this list appearing on the 3^rd December 2015, and the list is now available in its entirety on Mapping Mountains in Google Doc format.

The name the hill is listed by is Cae Rosser and this was derived from the Tithe map and it is adjoined to the Gwent Is Coed group of hills which are situated in the south-eastern part of South Wales (Region C, Sub-Region C3), and it is positioned with the A472 road to its north and the A449 road to its east and the Afon Wysg (River Usk) to the west, and has the town of Brynbuga (Usk) towards the north north-west.

When the original 30-99m height band of Welsh P30 hills published on Geoff Crowder’s v-g.me website were standardised and interpolated heights and drop values also included, this hill was listed with c 33m of drop, based on an estimated c 70m summit height and an estimated c 37m bwlch height, with each based on interpolation of 10m contouring that appears on the Ordnance Survey 1:50,000 Landranger and 1:25,000 Explorer map.

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

The details for this hill were re-assessed 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 had many spot heights not on other publicly available Ordnance Survey maps and gave a 73m summit spot height for this hill, which increased its estimated drop value to c 36m and a dominance of 49.32%.

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 Lesser Dominant status is due to LIDAR analysis, resulting in a 72.7m summit height and a 32.2m bwlch height, with these values giving this hill 40.5m of drop and 55.68% dominance, which is sufficient for it to be classified as a Dominant hill.

The full details for the hill are:

Group:  Gwent Is Coed

Name:  Cae Rosser

OS 1:50,000 map:  171

Summit Grid Reference:  SO 38857 00521 (LIDAR)

Summit Height:  72.7m (LIDAR)

Bwlch Grid Reference:  SO 39355 00008 (LIDAR)

Drop Summit to Bwlch:  40.5m (LIDAR)

Drop Bwlch to ODN:  32.2m (LIDAR)

Dominance:  55.68% (LIDAR)

Myrddyn Phillips (July 2020)