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

Five Acres (SS 537 932) – Lesser Dominant deletion

There has been a deletion to the listing of Y Trechol – The Dominant Hills of Wales due to LIDAR analysis conducted by Myrddyn Phillips, with the criteria for this list being:

Y Trechol – The Dominant Hills of Wales - Welsh P30 hills whose prominence equal or exceed half that of their absolute height.  With an accompanying sub list entitled Y Trechol - The Lesser Dominant Hills of Wales, with the criteria for this sub category being those additional Welsh P30 hills whose prominence is 33% or more and below 50% of their absolute height.

Prior to LIDAR analysis this hill was listed with 33.86% dominance based on an estimated c 43m drop value, with an estimated c 127m summit height based on an uppermost 125m ring contour and an estimated c 84m bwlch height based on interpolation of 5m contouring between 80m – 85m on the Ordnance Survey 1:25,000 Explorer map. 

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

The bounded land where the summit of this hill is situated is named Five Acres on the Tithe map and this is the name the hill is now listed by.  The hill is adjoined to the Gŵyr group of hills, which are situated in the western part of South Wales (Region C, Sub-Region C1), and is positioned between the B roads of the 4295 to the north-west and 4271 to the south, and has the village of Pen-clawdd towards the north.

If wanting to visit the hill permission to do so should be sought as the summit area is not a part of designated open access land, for those wishing to do so the nearest public footpath is to its south, with other possibilities via gated fields to the east.

The deletion of Five Acres from Lesser Dominant status is due to LIDAR analysis conducted by Myrddyn Phillips.   The LIDAR (Light Detection & Ranging) technique produced highly accurate height data that is now freely available for much of England and Wales. 

LIDAR image of the summit of Five Acres with the natural summit shown and the field boundary's being prominent on the image

LIDAR image of Five Acres

The 1m DTM LIDAR analysis gives the hill the following details:

Name:  Five Acres

Summit Height:  125.1m (natural summit)

Summit Grid Reference:  SS 53779 93281 and SS 53779 93291 and SS 53774 93296 and SS 53780 93297 (natural summit) (LIDAR gives a 125.2m high raised field boundary positioned at SS 53791 93318 which is considered a relatively recent man-made construct)

Bwlch Height:  84.3m

Bwlch Grid Reference:  SS 56307 93757

Drop:  40.8m

Dominance:  32.59%

Therefore, the 125.1m LIDAR analysis for the summit gives four positions of equal height at SS 53779 93281 and SS 53779 93291 and SS 53774 93296 and SS 53780 93297 and the 84.3m LIDAR analysis for the bwlch position at SN 56307 93757 gives this hill 40.8m of drop and 32.59% dominance, with is insufficient for Lesser Dominant status.

The full details for the hill are:

Group:  Gŵyr

Name:  Five Acres

Dominance:  32.59% (LIDAR)

OS 1:50,000 map:  159

Summit Grid Reference:  SS 53779 93281 and SS 53779 93291 and SS 53774 93296 and SS 53780 93297 (LIDAR) 

Summit Height:  125.1m (LIDAR)

Bwlch Grid Reference:  SS 56307 93757 (LIDAR)

Drop Summit to Bwlch:  40.8m (LIDAR)

Drop Bwlch to ODN:  84.3m (LIDAR)

Myrddyn Phillips (November 2018)