Mapping Mountains – Summit Relocations – Y Trechol – The Dominant Hills of Wales

Town Hill (SS 639 939)

There has been a Summit Relocation to a hill that is listed in the Y Trechol – The Dominant Hills of Wales, with the summit height, drop and status of the hill being confirmed by LIDAR analysis conducted by Myrddyn Phillips.

LIDAR image of Town Hill

The criteria for the list that this summit relocation 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.

The name of the hill is Town Hill, and it is adjoined to the Mynydd Penlle’rcastell group of hills, which are situated in the western part of South Wales (Region C, Sub-Region C1), and it is encircled by A roads with the A 4216 to its west, the A 483 to its north and the A 4118 to its east and south, and it forms a part of the city of Abertawe (Swansea).

As the summit of this hill is positioned on a grass verge beside a road it can be approached from most directions, however be careful of the traffic!

Prior to LIDAR analysis the summit position of the Dominant hill was listed as SS 615 940, the hill at this position remains unnamed on Ordnance Survey maps and therefore it is now listed by the point (Pt. c 175m) notation.  This hill was listed with a c 175m summit height and a 92m bwlch height, with these values giving this hill c 83m of drop and 47.43% dominance.

Extract from the Ordnance Survey Interactive Coverage Map hosted on the Geograph website

However, an adjoining hill named Town Hill on Ordnance Survey maps was also listed with a c 175m summit height.  The summit of this hill is positioned at SS 639 939, and it was listed with c 47m of drop based on a c 128m bwlch height.

Extract from the Ordnance Survey Interactive Coverage Map hosted on the Geograph website

Since the original compilation of the Dominants list the summit heights of these two hills have been examined on the Ordnance Survey series of Six-Inch maps resulting in:

Pt. c 175m at SS 615 940:  574.0ft surface height

Town Hill at SS 639 939:  571ft levelled height

The above heights are relative to the Liverpool datum point and each have a loss of 0.4ft relative to the Newlyn Datum point.  Therefore these heights are:

Pt. c 175m at SS 615 940:  573.6ft surface height (174.8m)

Town Hill at SS 639 939:  570.6ft levelled height (173.9m)

Extract from the Ordnance Survey series of Six-Inch maps

Extract from the Ordnance Survey series of Six-Inch maps

These values imply that the summit positioned at SS 615 940 is higher than the summit of Town Hill positioned at SS 639 939.  However, LIDAR analysis gives a 175.6m height to Town Hill with this positioned at SS 63915 93921.  LIDAR also gives a higher point on Town Hill; 175.8m at SS 63895 93893, this is in a back garden of a house and as aerial images suggest that this ground has been terra-formed it is being discounted from that of the height of the hill.

LIDAR does not cover the summit positioned at SS 615 940, therefore the best available evidence gives the following:

Pt. c 175m at SS 615 940:  174.8m (based on imperial surface height)

Town Hill at SS 639 939:  175.6m (based on LIDAR analysis)

The summit height produced by LIDAR analysis is 175.6m at SS 63915 93921 and its position in relation to that previously given comes within the parameters of the Summit Relocations used within this page heading, these parameters are:

The term Summit Relocations applies to any listed hill whose summit meets the following criteria; where there are a number of potential summit positions within close proximity and the highest point is not where previously given, or a relocation of approximately 100 metres or more in distance from either the position of a map spot height or from where the summit of the hill was previously thought to exist, or when the summit of the hill is in a different field compared to where previously given, or when the natural and intact summit of a hill is confirmed compared to a higher point such as a raised field boundary that is judged to be a relatively recent man-made construct.  As heights on different scaled Ordnance Survey maps are not consistent the height given on the 1:25,000 Explorer map is being prioritised in favour of the 1:50,000 Landranger map for detailing these relocations.

The summit height produced by LIDAR analysis is 175.6m and is positioned at SS 63915 93921, this position is not given a spot height on contemporary Ordnance Survey maps but is positioned within an uppermost 175m contour ring.  This position is approximately 2.4 km east from where the 174.8m surface height appears on the Ordnance Survey Six-Inch map and where the old summit of the Dominant hill was once listed to.   

 

The full details for the hill are:

Group:  Mynydd Penlle’rcastell

Name:  Town Hill

OS 1:50,000 map:  159

Summit Height:  175.6m (LIDAR)

Summit Grid Reference (new position):  SS 63915 93921 (LIDAR)

Bwlch Height:  91.5m (LIDAR)

Bwlch Grid Reference:  SS 65517 99306 (LIDAR) 

 

Drop:  84.1m (LIDAR)

Dominance:  47.88% (LIDAR)

Myrddyn Phillips (January 2019)