Mapping Mountains – Significant Height Revisions – 100m Twmpau and Y Trechol – the Dominant Hills of Wales

Llys y Gwynt (SH 779 728)

There has been a Significant Height Revision to a hill that is listed in the 100m Twmpau and Y Trechol – The Dominant Hills of Wales, with the summit height and its location, and the drop of the hill confirmed by LIDAR analysis, and a subsequent summit survey with the Trimble GeoXH 6000, both conducted by Myrddyn Phillips with the latter taking place on the 10^th October 2018.

Llys y Gwynt (SH 779 728)

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

100m Twmpau - Welsh hills at or above 100m and below 200m in height with 30m minimum drop, with an accompanying sub list entitled the 100m Sub-Twmpau with the criteria for this sub category being all Welsh hills at or above 100m and below 200m in height with 20m or more and below 30m of drop, with the word Twmpau being an acronym standing for thirty welsh metre prominences and upward.

Y Trechol – The Dominant Hills of Wales – Welsh P30 hills whose prominence equal or exceed half that of their absolute height.  With the criteria for Lesser Dominant status being those additional Welsh P30 hills whose prominence is between one third and half that of their absolute height.  The list is authored by Myrddyn Phillips with the Introduction to the start of the Mapping Mountains publication of this list appearing on the 3^rd December 2015.

The name of the hill is Llys y Gwynt, 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 is positioned with the B5106 to its west, the B5279 to its south and the Afon Conwy (River Conwy) to its east, and has the town of Conwy to its north.

When interpolated heights and drop values were added to the original Welsh 100m P30 list that was published on Geoff Crowder’s v-g.me website, this hill was listed with an estimated c 116m summit height based on interpolation of its uppermost 110m contour ring 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 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 analysis for this hill was closely followed by the survey of its summit with the Trimble GeoXH 6000. 

LIDAR image of Llys y Gwynt

The summit height produced by the Trimble GeoXH 6000 survey is 120.6m,  this is a substantial revision when compared to some revised heights, and it 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 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.  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.

Therefore, this hill’s new listed summit height is 120.6m and this was produced by surveying with the Trimble GeoXH 6000, this is 4.6m higher than its previously listed height of c 116m which was based on interpolation of its uppermost 110m ring contour that appears on the Ordnance Survey 1:50,000 Landranger and 1:25,000 Explorer map, however this new height is in accordance with the 5m contouring on OS Maps.

Extract from OSMaps showing an uppermost 120m ring contour

ills of Wales, and are reproduced below@

The full details for the hill are:

Group:  Carneddau

Name:  Llys y Gwynt

OS 1:50,000 map:  115

Summit Height (New height):  120.6m (converted to OSGM15)

Summit Grid Reference:  SH 77982 72884  

Bwlch Height:  50.7m (LIDAR)

Bwlch Grid Reference:  SH 76137 71943 (LIDAR)

Drop:  70.0m (Trimble summit and LIDAR bwlch)

Dominance:  58.00% (Trimble summit and LIDAR bwlch)

For details on the summit survey of this hill

Myrddyn Phillips (March 2019)