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

Y Dduallt (ST 041 961) 

There has been a Significant Height Revision to a hill that is listed in the Y Trichant – The 300m 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 Y Dduallt (ST 041 961)

The criteria for the list this height revision affects 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. 

Y Trichant - The 300m Hills of Wales by Myrddyn Phillips

The name the hill is listed by is Y Dduallt, and it is adjoined to the Cymoedd Morgannwg group of hills, which are situated in the central part of South Wales (Region C, Sub-Region C2), and it is encircled by minor roads with the A4233 road further to its west and the A4059 road and the A470 road further to its east, and has the town of Pontypridd towards the south-east. 

When the original 300m height band of Welsh P30 hills was published on Geoff Crowder’s v-g.me website, this hill was not 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 for this sub category. 

After the P30 lists were standardised, and interpolated heights and drop values also included, this hill was listed with 21m of drop based on the 354m summit spot height positioned at ST 04088 96112 and the 333m bwlch spot height that appear on the Ordnance Survey 1:25,000 Explorer map.  With its adjacent hill positioned at ST 036 958 listed with an estimated c 360m summit height based on the uppermost ring contour that appears on the Ordnance Survey 1:50,000 Landranger and 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. 

The summit height produced by LIDAR analysis is 363.0m and this is positioned at ST 04179 96153, this is a relatively dramatic increase in height compared to some revised heights, and it 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 summit 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, this hill’s new listed summit height is 363.0m and this was derived from LIDAR analysis, this is positioned at ST 04179 96153 and is 9.0m higher than its previously listed height of 354m which appears as a spot height on the Ordnance Survey 1:25,000 Explorer map. 

 

The full details for the hill are:

Group:  Cymoedd Morgannwg 

Name:  Y Dduallt 

OS 1:50,000 map:  170

Summit Height (New Height):  363.0m (LIDAR)                                                           

Summit Grid Reference:  ST 04179 96153 (LIDAR) 

Bwlch Height:  312.2m (LIDAR) 

Bwlch Grid Reference:  ST 03283 95733 (LIDAR) 

Drop:  50.8m (LIDAR)

 

Myrddyn Phillips (February 2021)