Mapping Mountains – Significant Height Revisions – 200m Twmpau and Y Trechol – The Dominant Hills of Wales

Mynydd Llwyd (ST 433 935)

There has been a Significant Height Revision to a hill that is listed in the 200m Twmpau and Y Trechol – The Dominant Hills of Wales, with the summit height, bwlch height, their locations, the drop, dominance and status of the hill derived from detail on contemporary and historic Ordnance Survey maps.

The criteria for the two listings this height revision affects are:

200m Twmpau – Welsh hills at or above 200m and below 300m in height that have 30m minimum drop, with an accompanying sub list entitled the 200m Sub-Twmpau with the criteria for this sub category being all Welsh hills at or above 200m and below 300m in height with 20m or more and below 30m of drop.

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, and the list is now available in its entirety on Mapping Mountains in Google Doc format.

The name the hill is listed by is Mynydd Llwyd and it is adjoined to the Gwent Is Coed group of hills which are situated in the south-eastern part of South Wales (Region C, Sub-Region C3), and it is positioned with the A449 road to its west, the A48 road to its south and the B4235 road to its north-east, and has the city of Casnewydd (Newport) towards the west south-west.

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

When the original 200m height band of Welsh P30 hills were published on Geoff Crowder’s v-g.me website this hill was included in the main P30 list under the name of Gray Hill and listed with a 273m summit height, based on the spot height that appears on the Ordnance Survey 1:25,000 Explorer map, with an accompanying note stating; 275m on 1986 1:50000 map.

Extract from the Ordnance Survey 1:50,000 Landranger map

Since publication of these P30 lists on Geoff Crowder’s v-g.me website there have been a number of Ordnance Survey maps made available online, some of these are historic such as the series of Six-Inch maps on the National Library of Scotland website, and it is this scale of mapping that shows a 901.8ft (274.9m) Surface Height on the summit area of this hill.  It is this height that has been rounded up and appears as the 275m spot height on contemporary Ordnance Survey 1:50,000 Landranger maps.

Extract from the Ordnance Survey series of Six-Inch maps

As the Surface Height is more accurate than the 273m spot height which has been ascertained from photogrammetry, the 275m height is now used for this hill, and 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 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 275m and this was derived from the spot height that appears on the Ordnance Survey 1:50,000 Landranger map, which in itself has been derived from the Surface Height on the Ordnance Surveys series of Six-Inch maps.

The full details for the hill are:

Group:  Gwent Is Coed

Name:  Mynydd Llwyd

OS 1:50,000 map:  171, 172

Summit Height (New Height):  275m (spot height)

Summit Grid Reference:  ST 43399 93559 (hand-held GPS via DoBIH)

Bwlch Height:  c 183m (interpolation)

Bwlch Grid Reference:  ST 42894 93984 (interpolation)

Drop:  c 92m (spot height summit and interpolated bwlch)

Dominance:  33.45% (spot height summit and interpolated bwlch)

Myrddyn Phillips (May 2020)