Mapping Mountains – Significant Height Revisions – 30-99m Twmpau and Y Trechol – The Dominant Hills of Wales

Broughton Burrows (SS 413 927)

There has been a Significant Height Revision to a hill that is listed in the 30-99m Twmpau and Y Trechol – The Dominant Hills of Wales, with the hill’s height, drop and dominance confirmed by LIDAR analysis conducted by Myrddyn Phillips.

LIDAR image of Broughton Burrows

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

30-99m Twmpau – Welsh hills at or above 30m and below 100m in height that have a minimum 30m of drop, with an accompanying sub list entitled the 30-99m Sub-Twmpau with the criteria for this sub category being all Welsh hills at or above 30m and below 100m 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 addition Welsh P30 hills whose prominence is 33.33% or more and below 50% 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 Broughton Burrows, and it is adjoined to the Gŵyr group of hills, which are situated in the western part of South Wales (Region C, Sub-Region C1), and it is positioned at the western end of the Gŵyr, and has the small community of Llangynydd towards the south-east.

Although the summit of this hill is not a part of open access land, and as such permission to visit should be sought, there is general acceptance for people to visit, with the nearest public footpath encircling the summit of the hill which comprises a dune system.

When the original Welsh 30-99m P30 list was published on Geoff Crowder’s v-g.me website, this hill was listed with a 49m summit height which appears as a spot height on the Ordnance Survey 1:25,000 Explorer map.

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

When interpolated height and also drop values were added to these lists, this hill was listed with c 32m of drop based on the 49m spot height and an interpolated bwlch height of c 17m based on what was thought to be bwlch contouring between 15m – 20m as indicated on the 1:25,000 Explorer map.  However, as this hill is a part of a dune system this map did not show continuous contours on either the summit or bwlch area of this hill.

Prior to LIDAR analysis this hill was analysed using the 5m contouring on OS Maps, which is the recent replacement for OS Get-a-map.  The bwlch height was confirmed as being between 15m – 20m and an interpolated height of c 18m the listed.  However, this mapping showed a number of 60m continuous ring contours for the summit area of this hill and therefore LIDAR was examined, with the summit height produced being 61.9m and this comes within the parameters of the Significant Height Revisions used within this page heading, these parameters are:

Extract from OS Maps

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 summit height is 61.9m and this was produced from LIDAR analysis, this is 12.9m higher than its previously listed height of 49m which appears as a spot height on the Ordnance Survey 1:25,000 Explorer map, however this new height is in accordance with the 5m contouring on OS Maps.

LIDAR summit image of Broughton Burrows

ills of Wales, and are reproduced below@

Close up LIDAR summit image of Broughton Burrows

The full details for the hill are:

Group:  Gŵyr

Name:  Broughton Burrows

OS 1:50,000 map:  159

Summit Height (New height):  61.9m (LIDAR)

Summit Grid Reference:  SS 41391 92781 (LIDAR)

Bwlch Height:  17.4m (LIDAR)

Bwlch Grid Reference:  SS 41739 92663 (LIDAR) 

 

Drop:  44.5m (LIDAR)

Dominance:  71.95% (LIDAR)

Myrddyn Phillips (December 2018)