Mapping Mountains – Significant Height Revisions – The Welsh P15s

Llan Ucha Castell Madog (SO 027 385) 

There has been a Significant Height Revision to a hill that is listed in The Welsh P15s, 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 Llan Ucha Castell Madog (SO 027 385)

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

The Welsh P15s – Welsh hills with 15m minimum drop, irrespective of their height, with an accompanying sub list entitled the Welsh Sub-P15s, with the criteria for this sub category being all Welsh hills with 14m or more and below 15m of drop.  The list is authored by Myrddyn Phillips, with the Introduction to the list appearing on Mapping Mountains on the 10^th May 2019. 

The Welsh P15s by Myrddyn Phillips

The name the hill is now listed by is Llan Ucha Castell Madog and this was derived from the Tithe map, and it is adjoined to the Mynydd Epynt group of hills, which are situated in the central part of South Wales (Region B, Sub-Region B1), and it is positioned with the B4520 road to its west and a minor road to its east, and has the small community of Capel Uchaf (Upper Chapel) towards the north-west.

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 21m of drop, based on the 399m summit spot height that appeared on the Ordnance Survey Vector Map Local hosted on the Geograph website which was entitled the Interactive Coverage Map and an estimated c 378m bwlch height, based on interpolation of 10m contouring between 370m – 380m. 

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

One of the resources recently available online is the mapping on the OS Maps website and the details for this hill were subsequently re-assessed against this mapping.  This is the replacement for OS Get-a-map and until recent times had contours at 5m intervals which were proving consistently more accurate compared to the 5m contours that sometimes appear on the Ordnance Survey 1:25,000 Explorer map and used to appear on the online Vector Map Local.  This mapping had an uppermost 400m ring contour and bwlch contouring between 380m – 385m, with interpolation giving an estimated c 18m of drop.  Through discussion it was decided to wait for full LIDAR coverage before reassessing the status of this hill.

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 Llan Ucha Castell Madog (SO 027 385)

The summit height produced by LIDAR analysis is 400.7m and when compared to its originally listed summit height, this comes within the parameters of the Significant Height Revisions used within this page heading, and 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 400.7m and this was derived from LIDAR analysis.  This is 1.7m higher than the original listed summit height of 399m which was derived from the Ordnance Survey Vector Map Local hosted on the Geograph website which was entitled the Interactive Coverage Map. 

 ills of Wales, and are reproduced below@

The full details for the hill are:

Group:  Mynydd Epynt 

Name:  Llan Ucha Castell Madog 

OS 1:50,000 map:  160

Summit Height (New Height):  400.7m (LIDAR)                                                           

Summit Grid Reference:  SO 02707 38531 (LIDAR) 

Bwlch Height:  381.6m (LIDAR) 

Bwlch Grid Reference:  SO 02779 38800 & SO 02780 38801 (LIDAR) 

Drop:  19.1m (LIDAR) 

 

Myrddyn Phillips (May 2025)