Mapping Mountains – Significant Height Revisions – Y Trichant – The 300m Hills of Wales and Y Trechol – The Dominant Hills of Wales

 

Rhos Ymryson (SN 460 499) 

There has been a Significant Height Revision to a hill that is listed in the Y Trichant – The 300m Hills of Wales and Y Trechol – The Dominant Hills of Wales, with the summit height, bwlch height and their locations, the drop and status of the hill derived from LIDAR analysis conducted by Myrddyn Phillips. 

LIDAR image of Rhos Ymryson (SN 460 499)

The criteria for the two listings that this height revision applies to are:

Y Trichant – The 300m Hills of Wales – Welsh hills at or above 300m and below 400m in height that have 30m minimum drop, with an accompanying sub list entitled the Sub-Trichant, with the criteria for this sub category being all Welsh hills at or above 300m and below 400m in height with 20m or more and below 30m of drop.  The list is authored by Myrddyn Phillips with the Introduction to the list and the renaming of it appearing on Mapping Mountains on the 13^th May 2017, and the Introduction to the Mapping Mountains publication of the list appearing on the 1^st January 2022. 

Y Trichant - The 300m Hills of Wales 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 Rhos Ymryson, and it is adjoined to the Mynydd Bach group of hills, which are situated in the western part of South Wales (Region B, Sub-Region B1), and it is positioned with the B4338 road to its north-east, a minor road to its north-west and the A475 road to its south, and has the village of Llanwenog towards the south-east. 

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

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

When the original 300m 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 327m summit height, based on the spot height that appears on the contemporary Ordnance Survey 1:25,000 Explorer map and which is positioned at SN 459 500.  However, this height is given to the top of a covered reservoir, whilst the contemporary Ordnance Survey 1:50,000 Landranger map gives a 324m summit spot height, which conforms with the rounded up 1062ft (323.7m) height shown on the Ordnance Surveys series of Six-Inch maps. 

Extract from the Ordnance Survey series of Six-Inch maps

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 summit image of Rhos Ymryson (SN 460 499)

The summit height produced by LIDAR analysis to the highest remaining natural ground on this hill is 323.3m and is positioned at SN 46032 49996, and this comes within the parameters of the Significant Height Revisions used within this page heading, these parameters are:

The term Significant Height Revisions applies to any listed hill whose interpolated height and Ordnance Survey or Harvey map summit spot height has a 2m or more discrepancy when compared to the survey result produced by the Trimble GeoXH 6000 or analysis of data produced via LIDAR, also included are hills whose summit map data is missing an uppermost ring contour when compared to the data produced by the Trimble or by LIDAR analysis.

Therefore, the new listed summit height of this hill is 323.3m and this was derived from LIDAR analysis, this is 3.7m lower than the originally listed summit height of 327m, which was based on the spot height that appears on the contemporary Ordnance Survey 1:25,000 Explorer map. 

 

The full details for the hill are: 

Group:  Mynydd Bach 

Name:  Rhos Ymryson 

OS 1:50,000 map:  146

Summit Height (New Height):  323.3m (LIDAR)                                                           

Summit Grid Reference:  SN 46032 49996 (LIDAR) 

Bwlch Height:  162.6m (LIDAR) 

Bwlch Grid Reference:  SN 61745 57992 (LIDAR) 

Drop:  160.8m (LIDAR) 

Dominance:  49.72% (LIDAR) 

 

Myrddyn Phillips (December 2023)