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

Coed Llwyn Owen (SN 703 402) 

There has been a Significant Height Revision to a hill that used to be 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 from LIDAR analysis conducted by Myrddyn Phillips. 

LIDAR image of Coed Llwyn Owen (SN 703 402)

The criteria for the list 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

The name the hill is listed by is Coed Llwyn Owen, and it is adjoined to the Mynydd Mallaen group of hills, which are situated in the central part of South Wales (Region B, Sub-Region B1), and it is positioned with a minor road to its south and the A482 road farther to its south-west, and has the town of Llanymddyfri (Llandovery) towards the south-east.

When the original 300m height band of Welsh P30 hills were published on Geoff Crowder’s v-g.me website, this hill was not included in the accompanying Hills to be surveyed sub list, as it was considered not to meet the criteria then used for this sub 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 20m of drop, based on the 346m summit spot height that appears on the Ordnance Survey 1:25,000 Explorer map and an estimated c 326m bwlch height, based on interpolation of 10m contouring between 320m – 330m. 

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. 

LIDAR summit image of Coed Llwyn Owen (SN 703 402)

The summit height produced by LIDAR analysis is 341.5m and is positioned at SN 70382 40288, 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, Harvey or other interactive 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 with the data produced by the Trimble or by LIDAR analysis.

Therefore, the new listed summit height of this hill is 341.5m and this was derived from LIDAR analysis, this is 4.5m lower than the previously listed 346m summit height, which was based on the spot height that appears on the Ordnance Survey 1:25,000 Explorer map. 

 

The full details for the hill are: 

Group:  Mynydd Mallaen 

Name:  Coed Llwyn Owen 

OS 1:50,000 map:  146, 147, 160

Summit Height (New Height):  341.5m (LIDAR)                                                           

Summit Grid Reference:  SN 70382 40288 (LIDAR) 

Bwlch Height:  328.8m (LIDAR) 

Bwlch Grid Reference:  SN 70456 40324 (LIDAR) 

Drop:  12.7m (LIDAR) 

 

Myrddyn Phillips (December 2025)