Mapping Mountains – Significant Height Revisions – 100m Twmpau

 

Bryn Llwyn (SN 672 998) 

There has been a Significant Height Revision to a hill that is listed in the 100m Twmpau, with the summit height, bwlch height and their locations, the drop and status of the hill derived from LIDAR analysis conducted by Myrddyn Phillips. 

LIDAR image of Bryn Llwyn (SN 672 998)

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

100m Twmpau – Welsh hills at or above 100m and below 200m in height that have 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.  The list is authored by Myrddyn Phillips, with the word Twmpau being an acronym standing for thirty welsh metre prominences and upward. 

100m Twmpau by Myrddyn Phillips

The name the hill is listed by is Bryn Llwyn, and it is adjoined to the Tarren y Gesail group of hills, which are situated in the south-western part of North Wales (Region A, Sub-Region A3), and it is positioned with a minor road to its north and the A493 road to its south-east, and has the village of Cwrt towards the east.

When the original 100m height band of Welsh P30 hills were published on Geoff Crowder’s v-g.me website, this hill was included in the Hills to be surveyed sub list, as it was considered not to meet the criteria then used for the main P30 list.

After the sub list was standardised, and interpolated heights and drop values also included the details for this hill were re-assessed and it was listed with an estimated c 27m of drop, based on an estimated c 173m summit height and an estimated c 146m bwlch height, with both heights based on interpolation of 10m contouring that appears on the Ordnance Survey 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 summit image of Bryn Llwyn (SN 672 998)

The summit height produced by LIDAR analysis is 175.4m and is positioned at SN 67202 99820, 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 height and Ordnance Survey, Harvey or other interactive 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 with the data produced by the Trimble or by LIDAR analysis.

Therefore, the new listed summit height of this hill is 175.4m and this was derived from LIDAR analysis, this is 2.4m higher than the previously listed c 173m summit height which was estimated from interpolation of the uppermost 170m ring contour that appears on the Ordnance Survey 1:25,000 Explorer map.

 

ills of Wales, and are reproduced below@

The full details for the hill are: 

Group:  Tarren y Gesail 

Name:  Bryn Llwyn 

OS 1:50,000 map:  135

Summit Height (New Height):  175.4m (LIDAR) 

Summit Grid Reference:  SN 67202 99820 (LIDAR) 

Bwlch Height:  145.4m (LIDAR) 

Bwlch Grid Reference:  SN 66942 99795 (LIDAR) 

Drop:  30.0m (LIDAR) 

 

Myrddyn Phillips (November 2025)