Mapping Mountains – Significant Height Revisions – 200m Twmpau

Ffridd Rhiwlwyfen (SN 758 986) 

There has been a Significant Height Revision to a hill that is listed in the 200m 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 Ffridd Rhiwlwyfen (SN 758 986)

The criteria for the list that this height revision applies to 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.  The list is authored by Myrddyn Phillips, with the word Twmpau being an acronym standing for thirty welsh metre prominences and upward. 

200m Twmpau by Myrddyn Phillips

The name the hill is listed by is Ffridd Rhiwlwyfen, and it is adjoined to the Banc Llechwedd Mawr group of hills, which are situated in the northern part of South Wales (Region B, Sub-Region B1), and it is positioned with a minor road to its west and east, the A489 road to its north and the A487 road to its north-west, and has the town of Machynlleth towards the north north-west.

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 with a 278m summit height, based on the spot height that appears on the Ordnance Survey 1:25,000 Explorer map.

After the sub list was standardised, and interpolated heights and drop values also included the details for this hill were re-evaluated and it was listed with an estimated c 72m of drop, based on the 278m summit spot height and an estimated c 206m bwlch height, based on interpolation of 10m contouring between 200m – 210m. 

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 Ffridd Rhiwlwyfen (SN 758 986)

LIDAR analysis gives the height of this hill as 280.0m positioned at SN 75800 98654, and when compared to its originally listed summit height of 278m 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 280.0m and this was derived from LIDAR analysis, this is 2.0m higher than the originally listed summit height of 278m, which was based on the spot height that appears on the contemporary Ordnance Survey 1:25,000 Explorer map.

 

ills of Wales, and are reproduced below@

The full details for the hill are:

Group:  Banc Llechwedd Mawr 

Name:  Ffridd Rhiwlwyfen                  

OS 1:50,000 map:  135

Summit Height (New Height):  280.0m (LIDAR) 

Summit Grid Reference:  SN 75800 98654 (LIDAR) 

Bwlch Height:  205.5m (LIDAR) 

Bwlch Grid Reference:  SN 75435 97713 (LIDAR) 

Drop:  74.5m (LIDAR) 

 

Myrddyn Phillips (April 2025)