Mapping Mountains – Hill Reclassifications – Y Trechol – The Dominant Hills of Wales

Pt. 63.3m (SH 491 760) – Lesser Dominant deletion

There has been a deletion to the listing of Y Trechol – The Dominant Hills of Wales, with the bwlch height and its location confirmed by LIDAR analysis, and a subsequent summit survey with the Trimble GeoXH 6000 confirming its drop and status, both conducted by Myrddyn Phillips with the latter taking place on the 22^nd October 2018.

LIDAR image of Pt. 63.3m (SH 491 760)

The criteria for the list that this deletion 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.  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.

The hill is now being listed by the point (Pt. 63.3m) notation as I do not know an appropriate name for it either from historic research or local enquiry.  The hill is adjoined to the Ynys Môn group of hills, which are situated in the north-western part of North Wales (Region A, Sub-Region A1), and it is encircled by minor roads with the B5420 to its south-west and the B5109 to its north-west, and has the town of Llangefni towards its west.

When the original Welsh P30 hills were published on Geoff Crowder’s v-g.me website this hill was included in the Hills to be surveyed sub list, as it did not at that stage meet the criteria used for the main P30 list. 

When the sub list was standardised, and interpolated heights and drop values also included for the P30 and sub list this hill was listed with an estimated c 30m of drop, based on the 64m summit spot height that appears on the Ordnance Survey 1:25,000 Explorer map and an estimated bwlch height of c 34m based on interpolation of 5m bwlch contouring between 30m – 35m that appears on the Ordnance Survey 1:25,000 Explorer map.

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

When the original Dominants list was compiled this hill was listed as a Lesser Dominant with 46.875% dominance based on the 64m summit spot height and the estimated drop of c 30m.

The drop value for this hill was next re-assessed when the Ordnance Survey Vector Map Local hosted on the Geograph website became available online; this mapping is entitled the Interactive Coverage Map, and the estimated drop value of c 30m remained the same.

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 bwlch image of Pt. 63.3m (SH 491 760)

The deletion of Pt. 63.3m from Lesser Dominant status is due to LIDAR bwlch analysis and a subsequent summit survey with the Trimble GeoXH 6000 conducted by Myrddyn Phillips, resulting in a 63.3m summit height and a 35.1m bwlch height, with these values giving this hill 28.2m of drop which is insufficient for it to retain its Lesser Dominant status.

The Trimble GeoXH 6000 gathering data at the summit of Pt. 63.3m (SH 491 760)

The full details for the hill are:

Group:  Ynys Môn

Name:  Pt. 63.3m

OS 1:50,000 map:  114, 115

Summit Grid Reference:  SH 49157 76080

Summit Height:  63.3m (converted to OSGM15)

Bwlch Grid Reference:  SH 49043 76701 (LIDAR)

Drop Summit to Bwlch:  28.2m (Trimble summit and LIDAR bwlch)

 

Drop Bwlch to ODN:  35.1m (LIDAR)

Dominance:  N/A, insufficient drop (LIDAR)

For details on the summit survey of this hill

Myrddyn Phillips (May 2019)