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

Trostrey Hill (SO 369 051) – Dominant reclassified to Lesser Dominant

There has been a reclassification to the list of the 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 Trostrey Hill (SO 369 051)

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, with the Introduction to 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.

Y Trechol - The Dominant Hills of Wales by Myrddyn Phillips

The name the hill is listed by is Trostrey Hill and it is adjoined to the Mynyddoedd Duon group of hills which are situated in the eastern part of South Wales (Region C, Sub-Region C3), and it is positioned with the Afon Wysg (River Usk) and the B4598 road to its west, the A40 road to its north, the A472 road to its south and the A449 road to its east, and has the village of Raglan towards the east north-east and the town of Brynbuga (Usk) towards the south.

When the original Welsh 100m P30 list was published on Geoff Crowder’s v-g.me website this hill was listed with a 199m summit height based on the spot height that appears on the Ordnance Survey 1:50,000 Landranger and 1:25,000 Explorer map and which is adjoined to a triangulation pillar positioned at SO 36917 05185.

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

When the P30 lists were 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 136m of drop based on the 199m summit height and an estimated bwlch height of c 63m.

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 gives a 62m spot height on the area of this hill’s bwlch positioned at SO 389 089, and when coupled with this hill’s 199m map summit height gave this hill 137m of drop.  These details were transferred to the Dominants list when first compiled, with these values giving this hill 68.84% dominance.

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 Dominant to Lesser Dominant status is due to LIDAR analysis, resulting in a 199.3m summit height and a 129.1m bwlch height, with these values giving this hill 70.2m of drop and 35.23% dominance.  With the 199.3m summit height taken to remaining natural ground with LIDAR also giving a 199.5m height positioned at SO 36920 05184 to a raised field boundary that is excluded from the height of this hill as it is considered a relatively recent man-made construct. 

The full details for the hill are:

Group:  Mynyddoedd Duon

Name:  Trostrey Hill

OS 1:50,000 map:  161

Summit Height:  199.3m (LIDAR)

Summit Grid Reference:  SO 36920 05188 (LIDAR)

Bwlch Height:  129.1m (LIDAR)

Bwlch Grid Reference:  SO 37462 05935 (LIDAR) 

Drop:  70.2m (LIDAR)

Dominance:  35.23% (LIDAR) 

Myrddyn Phillips (March 2020)