Mapping Mountains – Hill Reclassifications – 800m Twmpau

Mynydd Perfedd (SH 623 618) – 800m Sub-Twmpau addition

There has been a reclassification to the listing of 800m Twmpau, with the summit height, bwlch height and their locations, the drop and status of the hill derived from a Trimble GeoXH 6000 survey conducted by Myrddyn Phillips and Aled Williams which took place on the 6^th July 2019.

Mynydd Perfedd (SH 623 618)

The criteria for the list that this reclassification applies to are:

800m Twmpau - Welsh hills at or above 800m and below 900m in height with 30m minimum drop, with an accompanying sub list entitled the 800m Sub-Twmpau with the criteria for this sub category being all Welsh hills at or above 800m and below 900m 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 list is authored by Myrddyn Phillips and the details to it were published on Mapping Mountains on the 15^th March 2015.

The name the hill is listed by is Mynydd Perfedd and it is adjoined to the Glyder Fawr group of hills, which are situated in the north-western part of North Wales (Region A, Sub-Region A1), and it is positioned with the Afon Ogwen and the A5 road to its north-east and Llyn Padarn, Llyn Peris and the A4086 road towards its south-west, and has the town of Bethesda towards the north and the village of Llanberis towards the west south-west.

When the 800m height band of hills that later became known as the Twmpau were originally compiled this hill was listed with an estimated c 19m of drop, based on the 812m summit spot height that appears on the Ordnance Survey 1:50,000 Landranger and 1:25,000 Explorer map and an estimated c 793m bwlch height, based on interpolation of 10m contouring between 790m – 800m.

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

The details for this hill were re-assessed when the Ordnance Survey Vector Map Local hosted on the Geograph website and which is entitled the Interactive Coverage Map became available online.  This mapping has additional spot heights not available on any other Ordnance Survey publicly available map and in the case of this hill it has an 813m summit and 793m bwlch spot height, with these values giving this hill 20m of drop.  However, as there is inconsistency between the summit heights on the various scaled Ordnance Survey maps the drop value was left as c 19m until an accurate survey could take place.

Extract from the Ordnance Survey Vector Map Local hosted on the Geograph website

Therefore, the reclassification of this hill to 800m Sub-Twmpau status is due to a Trimble GeoXH 6000 survey, resulting in an 812.8m summit height and a 792.4m bwlch height, with these values giving this hill 20.4m of drop, which is sufficient for 800m Sub-Twmpau status.

 

The Trimble GeoXH 6000 gathering data at the summit of Mynydd Perfedd

The Trimble GeoXH 6000 gathering data at the bwlch of Mynydd Perfedd

The full details for the hill are:

Group:  Glyderau

Name:  Mynydd Perfedd

OS 1:50,000 map:  115

Summit Height:  812.8m (converted to OSGM15)

Summit Grid Reference:  SH 62319 61877

Bwlch Height:  792.4m (converted to OSGM15)

Bwlch Grid Reference:  SH 62267 62382

Drop:  20.4m

For details on the survey of this hill

Myrddyn Phillips (October 2019)