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

The Rookery (SN 611 227) – 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 derived from LIDAR analysis conducted by Myrddyn Phillips. 

LIDAR image of The Rookery (SN 611 227)

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. 

Y Trechol - The Dominant Hills of Wales by Myrddyn Phillips

The name the hill is listed by is The Rookery, and it is adjoined to the Mynydd Mallaen group of hills, which are situated in the central part of South Wales (Region B, Sub-Region B1), and it is positioned with the A40 road to its north, a minor road to its west, the Afon Tywi to its south and the A483 road to its east, and has the town of Llandeilo towards the east.

When the original 100m 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 the details for this hill were re-evaluated, and it was listed with an estimated c 49m drop and 48.04% dominance, based on the 102m summit spot height that appears on the Ordnance Survey 1:25,000 Explorer map and an estimated c 53m bwlch height based on interpolation of 10m contouring between 50m – 60m. 

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

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. 

LIDAR summit image of The Rookery (SN 611 227)

Therefore, the reclassification of this hill from Lesser Dominant status is due to LIDAR analysis, resulting in a 102.6m summit height and a 50.3m bwlch height, with these values giving this hill 52.2m drop and 50.94% dominance, which is insufficient for it to be classified as a Dominant hill. 

 

The full details for the hill are:

Group:  Mynydd Mallaen

Name:  The Rookery 

OS 1:50,000 map:  159

Summit Grid Reference:  SN 61191 22707 (LIDAR)

Summit Height:  102.6m (LIDAR)

Bwlch Grid Reference:  SN 61577 23807 (LIDAR)

Drop Summit to Bwlch:  52.2m (LIDAR)

Drop Bwlch to ODN:  50.3m (LIDAR)

Dominance:  50.94% (LIDAR)

 

Myrddyn Phillips (April 2022)