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, with the summit height, bwlch height and their locations, the drop, dominance and status of the hill derived from a Trimble GeoXH 6000 summit survey and LIDAR bwlch analysis conducted by Aled Williams and Myrddyn Phillips.

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 that have 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.  The list is authored by Myrddyn Phillips, with the word Twmpau being an acronym standing for thirty welsh metre prominences and upward.

30-99m Twmpau by Myrddyn Phillips

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.  The list is authored by Myrddyn Phillips with the Introduction to the start of the Mapping Mountains publication of this list appearing on the 3^rd December 2015, and the list is now available in its entirety on Mapping Mountains in Google Doc format. 

Y Trechol - The Dominant Hills of Wales by Myrddyn Phillips

The name the hill is listed by is Penrhos Garnedd, and it is adjoined to the Gyder 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 A55 road to its south, the A487 road to its north-west and the A4087 road to its east, and has the city of Bangor towards the north-east.

When the original 30-99m height band of Welsh P30 hills were published on Geoff Crowder’s v-g.me website, this hill was included in the main P30 list with a non interpolated 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 subsequently 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 55873 70069 that appears in it.

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

However, it was not until LIDAR became available that the details for this hill could be accurately re-assessed.  The LIDAR (Light Detection & Ranging) technique produced highly accurate height data that is now freely available for much of England and Wales. 

LIDAR image of Penrhos Garnedd (SH 560 702)

Initial LIDAR analysis was conducted by Aled Williams, who forwarded the details to me and suggested that the hill should be surveyed with the Trimble GeoXH 6000.  I then analysed this hill via LIDAR and consulted summit log reports on the Hill Bagging website.  This resulted in three points noted to be surveyed, with a fourth point also surveyed when on the hill.

LIDAR summit image of Penrhos Garnedd (SH 560 702)

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)

LIDAR analysis gives the highest ground on this hill as 94.8m positioned at SH 56022 70229, with other disturbed ground to its south-west.  However, this is to the top of a raised hedge bank and protocols dictate that as this is deemed a relatively recent man-made construct such ground is discounted from the height of a hill. 

The height produced by the Trimble GeoXH 6000 to the summit of this hill is 94.0m and is positioned at SH 56027 70234, and when compared to the originally listed summit position and the raised hedge bank this comes within the parameters of the Summit Relocations used within this page heading, these parameters are: 

The term Summit Relocations applies when the high point of the hill is found to be positioned; in a different field, to a different feature such as in a conifer plantation,  within a different map contour either on Ordnance Survey maps or interactive mapping, to a different point where a number of potential summit positions are within close proximity, when natural ground or the natural and intact summit of a hill is confirmed compared to a higher point such as a raised field boundary or covered reservoir that is considered a relatively recent man-made construct, 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.

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 the contemporary Ordnance Survey 1:50,000 Landranger and 1:25,000 Explorer map, and is approximately 200 metres north-eastward from where the 95m spot height appears on the Ordnance Survey 1:25,000 Explorer map and approximately 5 metres north-eastward from the high point of the raised hedge bank .

 

The full details for the hill are:

Group:  Glyder Fawr

Name:  Penrhos Garnedd

OS 1:50,000 map:  114, 115

Summit Height:  94.0m (converted to OSGM15, Trimble GeoXH 6000)

Summit Grid Reference (New Position):  SH 56027 70234 (Trimble GeoXH 6000)

Bwlch Height:  40.5m (LIDAR)

Bwlch Grid Reference:  SH 56305 70028 & SH 56306 70030 & SH 56307 70031 (LIDAR)

 

Drop:  53.5m (Trimble GeoXH 6000 summit and LIDAR bwlch)

Dominance:  56.90% (Trimble GeoXH 6000 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)