Mapping Mountains – Significant Height Revisions – Y Trichant – The 300m Hills of Wales

Foel Lus (SH 732 761)

There has been a Significant Height Revision to a hill that is listed in the Y Trichant – The 300m Hills of Wales, with the summit height, bwlch height and their locations, the drop and status of the hill confirmed by LIDAR analysis conducted by Myrddyn Phillips.

Foel Lus (SH 732 761)

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

Y Trichant – The 300m Hills of Wales – Welsh hills at or above 300m and below 400m in height that have 30m minimum drop, with an accompanying sub list entitled the Sub-Trichant with the criteria for this sub category being all Welsh hills at or above 300m and below 400m in height with 20m or more and below 30m of drop.  The list is authored by Myrddyn Phillips, with the Introduction to the list and the renaming of it appearing on Mapping Mountains on the 13th May 2017.

The name the hill is listed by is Foel Lus and it is adjoined to the Carneddau group of hills, which are situated in the north-western part of North Wales (Region A, Sub-Region A1), and it is positioned with the A55 road to its north-west, and has the town of Penmaen-mawr towards the west and the town of Conwy towards the east north-east.  

This hill was included in the original 300m height band of Welsh P30 hills published on Geoff Crowder’s v-g.me website.  After the accompanying sub list was standardised, and interpolated heights and drop values also included the details for this hill were re-evaluated and it was listed with an estimated c 60m of drop based on the 362m summit spot height that appears on the Ordnance Survey 1:50,000 Landranger and 1:25,000 Explorer map and an estimated bwlch height of c 302m based on contouring between 300m – 310m.

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 Foel Lus

The summit height produced by LIDAR analysis in comparison to the previously listed summit height 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 summit height and Ordnance Survey or Harvey 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 to the data produced by the Trimble or by LIDAR analysis. 

Therefore, this hill’s new listed summit height is 360.0m and this was produced by LIDAR analysis.  This is not a dramatic revision in height compared to some revised heights and is 2.0m lower than the previous listed height of 362m which was derived from the summit spot height on the Ordnance Survey 1:50,000 Landranger and 1:25,000 Explorer maps.

ills of Wales, and are reproduced below@

The full details for the hill are:

Group:  Carneddau

Name:  Foel Lus

OS 1:50,000 map:  115

Summit Height (New Height):  360.0m (LIDAR)

Summit Grid Reference:  SH 73238 76184 (LIDAR)

Bwlch Height:  299.9m (LIDAR)

Bwlch Grid Reference:  SH 73386 75791 (LIDAR)

Drop:  60.1m (LIDAR)

Myrddyn Phillips (December 2019)