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

Castell Cefnllys (SO 088 613) 

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 from LIDAR analysis conducted by Myrddyn Phillips. 

LIDAR image of Castell Cefnllys (SO 088 613)

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 now listed by is Castell Cefnllys, and this was derived from the Ordnance Survey Draft Surveyors Map, and it is adjoined to the Hirddywel group of hills, which are situated in the northern part of South Wales (Region B, Sub-Region B2), and it is positioned with minor roads to its north north-east, west and south, and the A483 road farther to its west, and has the town of Llandrindod towards the west.

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 304m summit height, based on the spot height 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

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 67m of drop, based on the 304m summit spot height and an estimated c 237m bwlch height, based on interpolation of 10m contouring between 230m – 240m.

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 Castell Cefnllys (SO 088 613)

The summit height produced by LIDAR analysis is 310.45m and is positioned at SO 08841 61344, 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 310.45m and this was derived from LIDAR analysis, this is 6.45m higher than the previously listed 304m summit height, which was based on the spot height that appears on the Ordnance Survey 1:50,000 Landranger and 1:25,000 Explorer map. 

 

The full details for the hill are: 

Group:  Hirddywel 

Name:  Castell Cefnllys 

OS 1:50,000 map:  147

Summit Height (New Height):  310.45m (LIDAR)                                                           

Summit Grid Reference:  SO 08841 61344 (LIDAR) 

Bwlch Height:  233.3m (LIDAR) 

Bwlch Grid Reference:  SO 10270 62837 (LIDAR) 

Drop:  77.1m (LIDAR) 

 

Myrddyn Phillips (April 2025)