Mapping Mountains – Significant Height Revisions – The Welsh P15s

 

Ynys Greigiog (SN 672 946) 

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 Ynys Greigiog (SN 672 946)

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 Ynys Greigiog, and this was derived from the Tithe map, and it is adjoined to the Banc Llechwedd Mawr group of hills, which are situated in the north-western part of South Wales (Region B, Sub-Region B1), and it is positioned with the A487 road to its south-east, and has the town of Machynlleth towards the north-east.

When the listing that became known as The Welsh P15s was being compiled, this hill was not included in either the main list or the accompanying sub list, and was subsequently documented with an estimated c 5m of drop, based on an estimated c 19m summit height and an estimated c 14m bwlch height, with both heights based on interpolation of 10m contouring that appears on the Ordnance Survey 1:25,000 Explorer 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 Ynys Greigiog (SN 672 946)

The summit height produced by LIDAR analysis is 25.2m 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 25.2m and this was derived from LIDAR analysis.  This is 6.2m higher than the previous listed summit height of c 19m, which was based on interpolation of the uppermost 10m ring contour that appears on the Ordnance Survey 1:25,000 Explorer map.

 

The full details for the hill are: 

Group:  Banc Llechwedd Mawr 

Name:  Ynys Greigiog 

OS 1:50,000 map:  135

Summit Height (New Height):  25.2m (LIDAR)                                                           

Summit Grid Reference:  SN 67268 94660 (LIDAR)                                                  

Bwlch Height:  9.5m (LIDAR) 

Bwlch Grid Reference:  SN 67379 94463 (LIDAR) 

Drop:  15.7m (LIDAR) 

 

Myrddyn Phillips (September 2025)

 

ills of Wales, and are reproduced below@