Mapping Mountains – Significant Height Revisions – The 500-Metre Tops of England and Wales – Deweys

 

Yr Eifl (SH 364 447) 

There has been a Significant Height Revision to a hill that is listed in the The 500-Metre Tops of England and Wales, with the summit height, bwlch height and their locations, the drop and status of the hill derived from a Leica GS15 survey conducted by John Barnard, Graham Jackson and Myrddyn Phillips, and LIDAR analysis conducted by Aled Williams. 

Yr Eifl (SH 364 447)

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

The 500-Metre Tops of England and Wales are affectionately known after their hill list compiler; Michael Dewey.  This list mixes metric and imperial height in its criteria to bookend up to the 2000ft height band and takes in all hills in England, Isle of Man and Wales that are 500m and above and below 2000ft (609.6m) in height that have 30m minimum drop. 

Mountain tables by Michael Dewey

Michael Dewey died on the 5^th November 2022.  His wife; Gillian wished for this list to be carried on and made provision for it to be co-authored by Myrddyn Phillips.  Michael will always be the compiler and originator, it’s just that he’s now got a co-author to safeguard and carry this list on.

The name the hill is listed by is Yr Eifl and it is adjoined to its own group of hills, which are situated in the north-western part of North Wales, and it is positioned with minor roads to its north-east, the B4417 road to its south and the A499 road to its east, and has the villages of Trefor towards the north north-east and Llithfaen towards the south south-west.

When the original list of Deweys appeared in the Mountain tables book published by Constable in 1995, this hill was listed with a summit height of 564m, based on the spot height adjoined to a triangulation pillar that appears on the Ordnance Survey 1:50,000 Landranger and 1:25,000 Explorer map. 

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

However, it was not until the survey with the Leica GS15 that the details for this hill could be accurately re-assessed.  Although the trig pillar is positioned on a relatively stable base it is positioned on the top of a substantial ancient cairn which consists of a multitude of individual stones and rocks and as this is considered as not being permanent in nature, it is the highest remaining natural ground on the periphery of the ancient cairn that is taken as both its listed summit height and summit position, and when compared to its originally listed summit height of 564m this comes within the parameters of the Significant Height Revisions used within this page heading, 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 other GNSS equipment 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. 

Gathering data with the Leica GS15 at the listed summit of Yr Eifl (SH 364 447)

Therefore, the new listed summit height of this hill is 560.7m and this was derived from a Leica GS15 survey, this is 3.3m lower than the originally listed summit height of 564m, which was based on the spot height adjoined to a triangulation pillar that appears on the contemporary Ordnance Survey 1:50,000 Landranger and 1:25,000 Explorer map. 

 

The full details for the hill are:

Group:  Yr Eifl

Name:  Yr Eifl

OS 1:50,000 map:  123

Summit Height (New Height):  560.7m (converted to OSGM15, Leica GS15)

Summit Grid Reference:  SH 36482 44734 (Leica GS15)  

Bwlch Height:  129.4m (LIDAR)

Bwlch Grid Reference:  SH 45347 48678 (LIDAR)

Drop:  431.3m (Leica GS15 summit and LIDAR bwlch)

 

Myrddyn Phillips (August 2025)