Mapping Mountains – Significant Height Revisions – The Welsh P15s

 

Allt y Gaer (SO 052 904) 

There has been a Significant Height Revision to a hill that was 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 Allt y Gaer (SO 052 904)

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 listed by is Allt y Gaer, 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 the A489 road to its north and minor roads to its west and east, and has the town of Y Drenewydd (Newtown) towards the east. 

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

When the listing that became known as The Welsh P15s was being compiled, this hill was included in the main list with an estimated c 18m of drop, based on the 306m summit spot height that appears on the Ordnance Survey 1:50,000 Landranger map and an estimated c 288m bwlch height, based on interpolation of 5m contouring between 285m – 290m. 

Extract from the Ordnance Survey 1:50,000 Landranger 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 Allt y Gaer (SO 052 904)

The summit height produced by LIDAR analysis is 308.5m and when compared to the previously 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 308.5m and this was derived from LIDAR analysis.  This is 2.5m higher than the previous listed summit height, which was based on the 306m spot height that appears on the Ordnance Survey 1:50,000 Landranger map. 

 ills of Wales, and are reproduced below@

The full details for the hill are: 

Group:  Hirddywel 

Name:  Allt y Gaer 

OS 1:50,000 map:  136

Summit Height (New Height):  308.5m (LIDAR)                                                           

Summit Grid Reference:  SO 05235 90407 (LIDAR)                                                  

Bwlch Height:  290.5m (LIDAR) 

Bwlch Grid Reference:  SO 05208 90307 (LIDAR) 

Drop:  18.0m (LIDAR) 

 

Myrddyn Phillips (August 2025)