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

Penrhos Garnedd (SH 560 702)

There has been a Summit Relocation to a hill that is listed in the 30-99m Twmpau and Y Trechol – The Dominant Hills of Wales, due to LIDAR analysis initially conducted by Aled Williams, with subsequent LIDAR analysis and a Trimble GeoXH 6000 survey conducted by Myrddyn Phillips, with the latter taking place on the 30^th September 2018.

The summit of Penrhos Garnedd (SH 560 702)

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 Penrhos Garnedd, and it is adjoined to the Gyderau 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 A55 to its south, the A487 to its north-west and the A4087 road to its east, and has the city of Bangor to 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 a public footpath approaches the summit from its north-west.

This hill was first listed in the original Welsh P30 list published on Geoff Crowder’s v-g.me website with a c 95m summit height positioned at SH 559 703.  With the summit height given as c 90m in the initial draft compilation with an accompanying note stating Three points of same height, this refers to the hill having three uppermost 90m ring contours on the Ordnance Survey 1:25,000 Outdoor Leisure map number 17 of the day.  The height of this hill was amended when the Ordnance Survey 1:25,000 Explorer map number 263 became available and listed as c 95m with the grid reference centred in this ring contour as opposed to using the 95m spot height positioned at SH 55874 70070 that appears in it.

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

Prior to the Trimble GeoXH 6000 survey this hill’s summit and bwlch contours were analysed with LIDAR by Aled Williams, who kindly forwarded these details to me and suggested that the hill should be surveyed with the Trimble.  I then analysed this hill via LIDAR and consulted logged reports on the Hill Bagging website of past visits.  This resulted in three points noted to be surveyed, with a fourth point also surveyed when on the hill.

LIDAR image of Penrhos Garnedd

Therefore, four data sets were taken with the Trimble GeoXH 6000 from the summit area of this hill, resulting in:

1^st survey:  93.388m at SH 55902 70421 (Bryn Ogwen road)

2^nd survey:  93.077m at SH 55908 70429 (Bryn Ogwen road)

3^rd survey:  94.045m at SH 56027 70234 (summit)

4^th survey:  92.540m at SH 55892 70474 (crossroads at Ffordd Penrhos and Coed y Maes)

The height produced by the Trimble GeoXH 6000 survey to the summit of this hill is 94.0m and is positioned at SH 56027 70234, this is not a dramatic difference in position compared to where previously listed, but it does come 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 Trimble GeoXH 6000 set-up position at the summit of Penrhos Garnedd

Therefore, the summit height produced by the Trimble GeoXH 6000 survey is 94.0m and is positioned at SH 56027 70234, this position is not given a spot height on contemporary Ordnance Survey maps and is approximately 200 metres north-east from where the 95m spot height appears on the Ordnance Survey 1:25,000 Explorer map.

 

Extract from the Ordnance Survey Interactive Coverage Map hosted on the Geograph website with the circled position indicating the new summit position

The full details for the hill are:

Group:  Glyderau

Name:  Penrhos Garnedd

OS 1:50,000 map:  114, 115

Summit Height:  94.0m (converted to OSGM15)

Summit Grid Reference (new position):  SH 56027 70234

Bwlch Height:  40.4m (LIDAR)

Bwlch Grid Reference:  SH 56305 70029 (LIDAR)

 

Drop:  53.7m (Trimble summit and LIDAR bwlch)

Dominance:  57.05% (Trimble summit and LIDAR bwlch)

My thanks to Aled Williams for his initial LIDAR analysis

For details on the survey that relocated the summit position of this hill

Myrddyn Phillips (February 2019)