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

Brandy Hill (SN 213 133) and Castell Meherin (SN 146 117) – Dominant re-twinning

Approaching the summit of Brandy Hill

The summit of Castell Meherin

There has been a re-twinning of two hills in the Y Trechol – The Dominant Hills of Wales list, with their summit height, their location and drop initially confirmed by LIDAR analysis and subsequently by summit surveys with the Trimble GeoXH 6000 which were conducted by Myrddyn Phillips, with the latter taking place on the 1^st August 2018.

The criteria for the listing that this re-twinning 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 addition Welsh P30 hills whose prominence is 33.33% or more and below 50% of their absolute height, with the Introduction to the Mapping Mountains publication of this list appearing on the 3^rd December 2015.

When the Y Trechol - The Dominant Hills of Wales list was first compiled these two hills were listed as twin summits based on their 205m summit heights on Ordnance Survey 1:50,000 Landranger and 1:25,000 Explorer maps and which are both adjoined to triangulation pillars.

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

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

The two summits are a part of the Brandy Hill group of hills, which are situated in the south-western part of Mid and West Wales (Region B, Sub-Region B4), and are positioned with the A 477 road to their south, and the A 40 road to their north, with the B 4328 nearer and to the summit of Brandy Hill to its north-west and the B 4314 nearer and to the summit of Castell Meherin to its north.

These two hills were then separated and de-twinned based on the flush bracket heights adjoined to their trig pillars, resulting in Brandy Hill being listed as the higher hill and therefore the hill included in the Dominants list.  The flush bracket height for each hill is given below:

Brandy Hill:  206.047m at SN 21342 13381

Castell Meherin:  205.258m at SN 14646 11773

However, the trig pillar on Brandy Hill is positioned on a raised embankment that forms part of a field boundary.  Whereas the trig pillar on Castell Meherin is positioned on the raised embankment of an ancient hill fort.  As raised field boundaries are considered a recent man-made construct and not a part of the natural height of a hill and ancient hill forts are considered as permanent in nature and now a part of the hill, the highest natural ground at the base of the field boundary on Brandy Hill was taken for the height of this hill, and this is also where LIDAR analysis places this hill’s summit, whereas ground at the base of the trig pillar on Castell Meherin was taken as the height of this hill’s summit.  The results of each Trimble survey and their measurement offsets are given below:

Brandy Hill:  205.477 – 0.40m offset = 205.077m

Castell Meherin:  206.257m – 1.18m offset = 205.077m

Remarkably the Trimble GeoXH 6000 gave the height of each hill as equal to the nearest millimetre.  However, although these hills are now considered as Dominant twin summits, Brandy Hill is being prioritised for the drop value given in the 200m Twmpau list and the values associated with this are also given below.      

Gathering data at the summit of Brandy Hill

Gathering data at the summit of Castell Meherin

Therefore, as the Trimble GeoXH 6000 survey resulted in each summit having the same 205.077m height Brandy Hill and Castell Meherin are now re-twinned.

The full details for each hill are:

Group:  Brandy Hill

Name:  Brandy Hill

OS 1:50,000 map:  158

Summit Grid Reference:  SN 21352 13359

Summit Height:  205.1m (converted to OSGM15)

Bwlch Grid Reference:  SN 14049 17973 (LIDAR)

Drop Summit to Bwlch:  151.6m (Trimble summit and LIDAR bwlch)

Drop Bwlch to ODN:  53.5m (LIDAR)

Dominance:  73.92% (Trimble summit and LIDAR bwlch)

Group:  Brandy Hill

Name:  Castell Meherin

OS 1:50,000 map:  158

Summit Grid Reference:  SN 14646 11773

Summit Height:  205.1m (converted to OSGM15)

Bwlch Grid Reference:  SN 18517 12805 (LIDAR, natural bwlch)

Drop Summit to Bwlch:  42.8m (Trimble summit and LIDAR bwlch)

Drop Bwlch to ODN:  162.2m (LIDAR, natural bwlch)

Dominance:  20.89% (Trimble summit and LIDAR bwlch)

For details on the survey of Brandy Hill

For details on the survey of Castell Meherin

Myrddyn Phillips (January 2019)