Mapping Mountains – Summit Relocations – 30-99m Twmpau and Y Trechol – The Dominant Hills of Wales

Yr Orsedd (SH 299 270)

There has been confirmation of a Summit Relocation to a hill that is listed in the 30-99m Twmpau and Y Trechol – The Dominant Hills of Wales, with the summit height, its location, the drop and status of the hill confirmed by LIDAR analysis, and a subsequent summit survey with the Trimble GeoXH 6000 conducted by Myrddyn Phillips, with the latter taking place on the 3^rd October 2018.

Yr Orsedd (SH 299 270)

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

30-99m Twmpau - Welsh hills at or above 30m and below 100m in height with 30m minimum drop, with an accompanying sub list entitled the 30-99m Sub-Twmpau with the criteria for this sub category being all Welsh hills at or above 30m and below 100m 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.

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, with the Introduction to the Mapping Mountains publication of this list appearing on the 3^rd December 2015.

The name of the hill is Yr Orsedd, and this was derived from local enquiry, and it is adjoined to the Pen Llŷn group of hills, which are situated in the western part of North Wales (Region A, Sub-Region A1), and the hill is encircled by minor roads with the coast to its south-west and east, and has the village of Abersoch towards its north-east.

As the summit of this hill is not a part of designated open access land permission to visit should be sought, for those wishing to do so the easiest approach is via the east using gates to access the summit.

This hill was first listed in the Hills to be surveyed sub-list that accompanied the original Welsh P30 list published on Geoff Crowder’s v-g.me website, with a 64m summit height which is positioned at SH 295 266 and appeared as a spot height on the Ordnance Survey 1:25,000 Explorer map. 

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

When this list was standardised with interpolated heights and drop values included this hill was listed with c 31m of drop based on the 64m summit spot height and an estimated bwlch height of c 33m based on interpolation of 10m bwlch contouring, and the locally known name for this hill is Bryn Tŷ Newydd.

Bryn Tŷ Newydd (SH 295 265)

The details for this hill were reassessed when the Ordnance Survey Interactive Coverage Map hosted on the Geograph website became available online, and this map has a 66m spot height at SH 299 270 which is adjoined to the hill to the north-east, which only has an uppermost 60m ring contour on the 1:25,000 Explorer map.  Therefore the summit position of the listed hill was relocated and its drop value increased accordingly, with the Interactive Coverage Map also giving the south-westerly hill a second 64m map heighted summit.

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

Prior to the Trimble GeoXH 6000 summit survey this hill and its adjoining south-westerly 64m twin map heighted hill were analysed via LIDAR, and this confirmed that the north-easterly summit is the higher.

The height produced by the Trimble GeoXH 6000 survey to the summit of this hill is 67.5m and is positioned at SH 29952 27038, as opposed to the 64.2m LIDAR height for the higher of the two 64m Geograph summits to the south-west and which is positioned at SH 29571 26573.  This confirms the hill’s summit position compared to where previously listed, and it 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.

Therefore, the summit height produced by the Trimble GeoXH 6000 survey is 67.5m and is positioned at SH 29952 27038, this position is given a 66m spot height on the Ordnance Survey Interactive  Coverage Map hosted on the Geograph website and is approximately 550 metres north-east from where the previously listed summit is positioned.

The Trimble GeoXH 6000 gathering data at the summit of Yr Orsedd

The full details for the hill are:

Group:  Pen Llŷn

Name:  Yr Orsedd

OS 1:50,000 map:  123

Summit Height:  67.5m (converted to OSGM15)

Summit Grid Reference (new position):  SH 29952 27038

Bwlch Height:  32.0m (LIDAR)

Bwlch Grid Reference:  SH 29808 27337 (LIDAR)

 

Drop:  35.4m (Trimble summit and LIDAR bwlch)

Dominance:  52.52% (Trimble summit and LIDAR bwlch)

For details on the summit survey of this hill

Myrddyn Phillips (February 2019)