Mapping Mountains – Significant Height Revisions – Yr Uchafion and 600m Twmpau

Foel Lwyd (SH 720 723)

There has been a Significant Height Revision to a hill that is listed in the Yr Uchafion and 600m Twmpau, with the summit height, bwlch height and their locations, the drop and status of the hill derived from a Leica GS15 summit survey conducted by John Barnard, Graham Jackson and Myrddyn Phillips and LIDAR bwlch analysis conducted by Aled Williams.

Foel Lwyd (SH 720 723)

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

Yr Uchafion – Welsh hills at or above 500m in height that have 15m minimum drop.  The list is co-authored by Myrddyn Phillips and Aled Williams, with the Introduction to this list appearing on Mapping Mountains on the 4^th November 2015.

600m Twmpau – Welsh hills at or above 600m and below 700m in height that have 30m minimum drop, with an accompanying sub category entitled the 600m Sub-Twmpau consisting of all Welsh hills at or above 600m and below 700m in height that have 20m or more and below 30m of drop.  With the word Twmpau being an acronym standing for thirty welsh metre prominences and upward.  The list is authored by Myrddyn Phillips.

The name the hill is listed as is Foel Lwyd, and it is adjoined to the Carneddau range of hills, which are situated in the north-western part of North Wales (Region A, Sub-Region A1), and it is positioned with the A 55 road to its north and the Afon Conwy and the A 470 road to its east, and has the town of Penmaenmawr towards the north.

Prior to the survey with the Leica GS15 this hill was listed with an estimated c 54m of drop, based on the 603m summit spot height that appears on the Ordnance Survey 1:25,000 Explorer map and the Ordnance Survey Vector Map Local hosted on the Geograph website and which is entitled the Interactive Coverage Map, and an estimated c 549m bwlch height based on interpolation of 10m contouring between 540m - 550m.

Extract from the Ordnance Survey Vector Map local hosted on the Geograph website and which is entitled the Interactive Coverage Map

The summit height produced by the Leica GS15 survey is 600.2m, this is not a substantial height revision when compared to some revised heights, but it does come 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 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 any other GNSS receiver, 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.  As heights on different scaled Ordnance Survey maps are not consistent the height given on the 1:25,000 Explorer map is being prioritised in favour of the 1:50,000 Landranger map for detailing these revisions.

The Leica GS15 set-up position at the summit of Foel Lwyd

Therefore, this new listed summit height of this hill is 600.2m and this was derived by a Leica GS15 survey, this is 2.8m lower than its previously listed height of 603m which appears as a spot height on the Ordnance Survey 1:25,000 Explorer map and the Ordnance Survey Vector Map Local hosted on the Geograph website and which is entitled the Interactive Coverage Map.

ills of Wales, and are reproduced below@

The full details for the hill are:

Group:  Carneddau

Name:  Foel Lwyd

OS 1:50,000 map:  115

Summit Height (New height):  600.2m (converted to OSGM15, Leica GS15)

Summit Grid Reference:  SH 72036 72323 (Leica GS15) 

 

Bwlch Height:  547.2m (LIDAR)

Bwlch Grid Reference:  SH 72647 72325 (LIDAR)

Drop:  53.0m (Leica GS15 summit and LIDAR bwlch)

Myrddyn Phillips and Aled Williams (January 2019)