Mapping Mountains – Summit Relocations – Y Trechol – The Dominant Hills of Wales

Clytiau Poethion (SH 763 717)

There has been a Summit Relocation to a hill that is listed in the 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 10^th October 2018.

LIDAR image of Maes Glas Mawr and Clytiau Poethion

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

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 bounded land where the summit of this hill is situated is Clytiau Poethion, and this was derived from the Tithe map and it is the name that this hill is now listed by, and it is adjoined to the Carneddau group of hills, which are situated in the north-western part of North Wales (Region A, Sub-Region A1), and the hill is encircled by minor roads with the B5106 and the Afon Conwy (River Conwy) to its east, and has the town of Conwy to its north.

When the original 100m height band of Welsh P30 hills were published on Geoff Crowder’s v-g.me website the adjacent northerly hill known as Maes Glas Mawr (SH 76876 72414) was included in the main P30 list as its summit was thought to be higher than that of Clytiau Poethion (SH 76337 71799), based on a 107m summit spot height that appears on the Ordnance Survey 1:50,000 Landranger map as opposed to an upper 100m contour ring given to Clytiau Poethion.

Extract from the Ordnance Survey 1:50,000 Landranger map

Therefore, when the Dominant list was first compiled it was the hill now listed as Maes Glas Mawr that was included in the Lesser Dominant list with 36.45% dominance.

However, it was not until LIDAR became available that the details for each 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 analysis gives Clytiau Poethion as higher than Maes Glas Mawr and this has subsequently been confirmed by a Trimble GeoXH 6000 survey, therefore the bylchau for each hill is swapped as are their classifications, resulting in a Trimble GeoXH 6000 summit height for Clytiau Poethion of 109.1m positioned at SH 76337 71799, as opposed to the 108.5m summit height for Maes Glas Mawr positioned at SH 76876 72414.  This confirms the qualifying 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.

LIDAR bwlch image for Clytiau Poethion

Therefore, the summit height produced by the Trimble GeoXH 6000 survey is 109.1m and is positioned at SH 76337 71799, this position is not given a spot height on contemporary Ordnance Survey maps and is approximately 800 metres south-westward from where the previously listed summit is positioned.

The full details for the hill are:

Group:  Carneddau

Name:  Clytiau Poethion

OS 1:50,000 map:  115

Summit Height:  109.1m (converted to OSGM15)

Summit Grid Reference (new position):  SH 76337 71799

Bwlch Height:  68.7m (LIDAR)

Bwlch Grid Reference:  SH 77496 72493 (LIDAR)

 

Drop:  40.35m (Trimble summit and LIDAR bwlch)

Dominance:  37.00% (Trimble summit and LIDAR bwlch)

For details on the summit survey of this hill

Myrddyn Phillips (April 2019)