Mapping Mountains – Hill Reclassifications – 30-99m Twmpau

Yr Orsedd (SH 299 270) – 30–99m Sub-Twmpau reclassified to 30-99m Twmpau

There has been confirmation of a Hill Reclassification to the list of 30-99m Twmpau, 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 list that this reclassification 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.

When the original Welsh P30 hills were published on Geoff Crowder’s v-g.me website this hill appeared in the accompanying Hills to be surveyed sub list as it did not meet the criteria then used for the main P30 list; however this sub list has now been standardised with drop values and interpolated heights also included in the main P30 and the accompanying sub list.

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 positioned at SH 295 266 on the Ordnance Survey 1:25,000 Explorer map and an estimated bwlch height of c 33m based on interpolation of 10m bwlch contouring.

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

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 locally known name of the higher summit is Yr Orsedd, whilst the lower south-westerly summit is known locally as Bryn Tŷ Newydd, 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.

Bryn Tŷ Newydd, the old listed summit to the south-west

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.

The confirmation of the reclassification of Yr Orsedd to 30-99m Twmpau status is due to LIDAR analysis and a summit survey with the Trimble GeoXH 6000 conducted by Myrddyn Phillips, resulting in a 67.5m summit height and a 32.0m bwlch height, with these values giving this hill 35.4m of drop which is sufficient for it to be classified as a 30-99m Twmpau.

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:  SH 29952 27038

Bwlch Height:  32.0m (LIDAR)

Bwlch Grid Reference:  SH 29808 27337 (LIDAR)

 

Drop:  35.4m (Trimble summit and LIDAR bwlch)

For details on the summit survey of this hill

Myrddyn Phillips (February 2019)