Mapping Mountains – Summit Relocations – 100m Twmpau

Parc Llwyn Du (SN 428 188)

There has been a Summit Relocation to a hill that is listed in the 100m Twmpau, with the summit height and its location, drop and status of the hill initially confirmed by LIDAR analysis and subsequently by a summit survey with the Trimble GeoXH 6000 which were conducted by Myrddyn Phillips, with latter taking place on the 31^st July 2018.

Parc Llwyn Du (SN 428 188)

The criteria for the list that this summit relocation applies to are:

100m Twmpau - Welsh hills at or above 100m and below 200m in height with 30m minimum drop, with an accompanying sub list entitled the 100m Sub-Twmpau with the criteria for this sub category being all Welsh hills at or above 100m and below 200m in height with 20m or more and below 30m of drop, with the word Twmpau being an acronym standing for thirty welsh metre prominences and upward.

The name of the bounded land where the summit of this hill is situated is Parc Llwyn Du and this was derived from the Tithe map, and it is the name that this hill is now listed by.  The hill is adjoined to the Mynydd Sylen group of hills, which are situated in the western part of South Wales (Region C, Sub-Region C1), and is positioned with the A 48 road to its south and the B 4300 road to its north, and has the town of Caerfyrddin (Carmarthen) to the north-west.

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 the south of the summit.

When the original Welsh 100m P30 list was published on Geoff Crowder’s v-g.me website the summit location of this hill was given to the 104m spot height adjoined to a triangulation pillar that appeared on contemporary Ordnance Survey 1:50,000 Landranger and 1:25,000 Explorer maps and positioned at SN 423 186.  However, this trig pillar is placed on a covered reservoir that is considered a recent man-made construct and therefore not a part of the natural summit of the hill.

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

The details for this hill were re-examined when the Ordnance Survey Interactive Coverage Map hosted on the Geograph website became available online and this map also gave a 104m spot height positioned at SN 428 188, and which is placed on natural ground.  However, it was not until LIDAR became available that an accurate height comparison could be made between these two positions.  

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

LIDAR image of Parc Llwyn Du

The summit height produced by DTM LIDAR analysis is 104.1m at SN 42895 18829, with the summit subsequently surveyed using the Trimble GeoXH 6000 resulting in a 104.3m summit height positioned at SN 42895 18827, and its position in relation to that originally 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 the Trimble GeoXH 6000 is 104.3m and is positioned at SN 42895 18827, this position is approximately 30 metres east from where the 104m spot height appears on the Ordnance Survey Interactive Coverage Map hosted on the Geograph website, and over 560 metres east north-eastward from where the summit was originally listed at this hill’s triangulation pillar.

 

The full details for the hill are:

Group:  Mynydd Sylen

Name:  Parc Llwyn Du

OS 1:50,000 map:  159

Summit Height:  104.3m (converted to OSGM15, natural summit)

Summit Grid Reference (new position):  SN 42895 18827

Bwlch Height:  73.4m (LIDAR)

Bwlch Grid Reference:  SN 43247 18917 (LIDAR) 

Drop:  30.8m (Trimble summit and LIDAR bwlch)

The Trimble set-up position at the summit of Parc Llwyn Du

For details on the summit survey of Parc Llwyn Du

Myrddyn Phillips (January 2019)