Mapping Mountains – Significant Height Revisions – The Welsh P15s

Grinllwm (SH 772 625) 

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 Grinllwm (SH 772 625)

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 Grinllwm, and its name placement was derived from the Ordnance Survey series of Six-Inch maps, and it is adjoined to the Carnedd Llywelyn group of hills, which are situated in the north-western part of North Wales (Region A, Sub-Region A1), and it is positioned with minor roads to its north, south and east, with the B5106 road and the A470 road farther to its east, and has the town of Llanrwst towards the east south-east.

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 15m of drop, based on an estimated c 264m summit height and the 249m 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. 

LIDAR summit image of Grinllwm (SH 772 625)

The summit height produced by LIDAR analysis is 267.7m 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 267.7m and this was derived from LIDAR analysis.  This is 3.7m higher than the previous listed summit height of c 264m, which was based on interpolation of the uppermost 260m ring contour that appears on the Ordnance Survey 1:25,000 Explorer map.

 

The full details for the hill are: 

Group:  Carnedd Llywelyn 

Name:  Grinllwm 

OS 1:50,000 map:  115

Summit Height (New Height):  267.7m (LIDAR)                                                           

Summit Grid Reference:  SH 77258 62516 (LIDAR)                                                  

Bwlch Height:  249.1m (LIDAR) 

Bwlch Grid Reference:  SH 77295 62465 (LIDAR) 

Drop:  18.6m (LIDAR) 

 

Myrddyn Phillips (March 2026)