Friday 25 January 2019

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 of the hill being confirmed by a Leica GS15 survey conducted by John Barnard, Graham Jackson and Myrddyn Phillips on the 6th May 2013, and subsequently surveyed along with the hill’s connecting bwlch by Alan Dawson using a Leica RX 1250 and later surveyed by Myrddyn Phillips with a Trimble GeoXH 6000 on the 31st August 2018.

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 4th 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 of the hill 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.

As the summit of the hill is a part of designated open access land it can be approached from most directions, with access from Bwlch y Ddeufaen towards the west south-west being the most convenient, the hill can also be easily combined with its higher neighbour of Tal y Fan.

Prior to the survey with the Leica GS15 this hill was listed with a 603m summit height, which was based on the spot height that appears on the Ordnance Survey Interactive Coverage Map hosted on the Geograph website.

Extract from the Ordnance Survey Interactive Coverage Map hosted on the Geograph website

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

The Trimble GeoXH 6000 set-up position at the summit of Foel Lwyd

Therefore, this hill’s new summit height is 600.2m and this was produced by surveying with the Leica GS15, this is 2.8m lower than its previously listed height of 603m which appears as a spot height on the Ordnance Survey Interactive Coverage Map hosted on the Geograph website.


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:  546.8m (converted to OSGM15) (Leica RX 1250)

Bwlch Grid Reference:  SH 72653 72316 (Leica RX 1250)

Drop:  53.3m (converted to OSGM15) (Leica GS15 summit and Leica RX 1250 bwlch)



Myrddyn Phillips and Aled Williams (January 2019)



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