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

Geirth Mawr (SH 535 393) - Dominant deletion

There has been confirmation of a deletion from the list of the 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 detail on contemporary maps produced from Ordnance Survey data and confirmed from subsequent LIDAR analysis conducted by Myrddyn Phillips. 

LIDAR image of Geirth Mawr (SH 535 393)

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, 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 Geirth Mawr, and it is adjoined to the Moel Hebog 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 A497 road to its north and minor roads to its west and east, and has the town of Porthmadog towards the 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 Hills to be surveyed sub list, as it was considered not to meet the criteria then used for the main P30 category.

When the sub list was standardised, and interpolated heights and drop values also included the details for this hill were re-evaluated and it was listed with an estimated c 30m of drop, based on an estimated c 58m summit height, based on interpolation of 10m contouring and the 28m bwlch spot height that appears on the Ordnance Survey 1:25,000 Explorer map, resulting in a dominance value of 51.72%, which was sufficient for Dominant status.

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

Since the original publication of the Welsh P30 lists on Geoff Crowder’s v-g.me website there have been a number of maps made available online.  Some of these are historic such as the series of Six-Inch maps on the National Library of Scotland website.  Whilst others were digitally updated such as the Ordnance Survey Vector Map Local that was hosted on the Geograph website and which was entitled the Interactive Coverage Map, whilst others are current and digitally updated such as the interactive mapping on the Magic Maps and WalkLakes websites.

The details for this hill were re-assessed when the Ordnance Survey Vector Map Local hosted on the Geograph website and which was entitled the Interactive Coverage Map became available online.  This mapping had many spot heights not on other publicly available Ordnance Survey maps and for this hill it had a 56m summit spot height, resulting in its drop value being amended to 28m, which is insufficient for continued Dominant status.

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. 

Therefore, the confirmation of the deletion of this hill from Dominant status is due to LIDAR analysis, resulting in a 57.4m summit height and a 27.9m bwlch height, with these values giving this hill 29.5m of drop, which is insufficient for Dominant status.  

 

The full details for the hill are:

Group:  Moel Hebog

Name:  Geirth Mawr

OS 1:50,000 map:  124

Summit Height:  57.4m (LIDAR)

Summit Grid Reference:  SH 53511 39363 (LIDAR)

Bwlch Height:  27.9m (LIDAR)

Bwlch Grid Reference:  SH 53302 39651 & SH 53311 39649 (LIDAR)

Drop:  29.5m (LIDAR)

Dominance:  N/A (insufficient drop)

Myrddyn Phillips (February 2017)