Mapping Mountains – Hill Reclassifications – The Welsh P15s

Gamallt (SN 968 714) – Welsh P15 deletion

There has been a deletion to the list of 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 Gamallt (SN 968 714)

The criteria for the list that this deletion 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 Gamallt, and this was derived from the Tithe 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, west and east, and the A470 road farther to its west, and has the town of Rhaeadr Gwy (Rhayader) towards the south.

When the listing that became known as The Welsh P15s was being compiled, this hill was included in the main list with 15m of drop, based on the 398m summit spot height that appears on the Ordnance Survey 1:25,000 Explorer map and the 383m bwlch spot height that appeared on the Ordnance Survey Vector Map Local hosted on the Geograph website and which was entitled the Interactive Coverage 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.

Therefore, the deletion of this hill from Welsh P15 status is due to LIDAR analysis, resulting in a 397.4m summit height and a 383.5m bwlch height, with these values giving this hill 13.9m 0f drop, which is insufficient for it to be classified as a Welsh P15. 

 

The full details for the hill are: 

Group:  Hirddywel 

Name:  Gamallt 

OS 1:50,000 map:  136, 147

Summit Height:  397.4m (LIDAR)                                                           

Summit Grid Reference:  SN 96811 71472 (LIDAR)                                                  

Bwlch Height:  383.5m (LIDAR) 

Bwlch Grid Reference:  SN 96756 71262 (LIDAR) 

Drop:  13.9m (LIDAR) 

 

Myrddyn Phillips (September 2025)