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

 

Ynys Fach (SN 668 951) – Dominant addition

There has been an addition to the list of the Y Trechol – The Dominant Hills of Wales, with the summit height, bwlch height and their locations, the drop and status of the hill derived by Joe Nuttall who produced a summit analysis programme using LIDAR, and then by LIDAR analysis initially conducted by Jim Bloomer and subsequently by Myrddyn Phillips. 

LIDAR image of Ynys Fach (SN 668 951)

The criteria for the list that this addition 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 inits entirety on Mapping Mountains in Google Doc format. 

Y Trechol - The Dominant Hills of Wales by Myrddyn Phillips

The name the hill is now listed by is Ynys Fach, and it is adjoined to the Pumlumon group of hills which are situated in the north-western part of Mid and West Wales (Region B, Sub-Region B2), and it has the A487 road to its south-east and the town of Machynlleth 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 Hills to be surveyed sub list that accompanied the main P30 list, as it was considered not to meet the criteria then used in the main P30 category. 

After 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 27m of drop based on an estimated c 34m summit height and an estimated c 7m bwlch height, with both values based on interpolation of 10m contouring that appears on the Ordnance Survey 1:25,000 Explorer map. 

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. 

Therefore, the reclassification of this hill to Dominant status is due to LIDAR analysis, resulting in a 43.5m summit height and a 2.1m bwlch height, with these values giving this hill 41.3m of drop and 95.10% dominance which is sufficient for Ynys Fach to be classified as a Dominant hill. 

 

The full details for the hill are:

Group:  Pumlumon

Name:  Ynys Fach

OS 1:50,000 map:  135

Summit Grid Reference:  SN 66845 95141 (LIDAR)

Summit Height:  43.5m (LIDAR)

Bwlch Grid Reference:  SN 67483 95096 (LIDAR)

Drop Summit to Bwlch:  41.3m (LIDAR)

Drop Bwlch to ODN:  2.1m (LIDAR)

Dominance:  95.10%

 

Myrddyn Phillips (March 2021)