Mapping Mountains – Significant Height Revisions – 100m Twmpau

Llety Llwydin (SN 672 923) 

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 Llety Llwydin (SN 672 923)

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 now listed by is Llety Llwydin, and this was derived from the Tithe map, and it is adjoined to the Banc Llechwedd Mawr group of hills, which are situated in the north-western part of South Wales (Region B, Sub-Region B1), and it is positioned with minor roads to its north, south and east, with the A487 road to its west, and has the town of Machynlleth towards the north-east.

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

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 19m of drop, based on an estimated c 176m summit height and an estimated c 157m 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 Llety Llwydin (SN 672 923)

The summit height produced by LIDAR analysis is 178.8m and is positioned at SN 67233 92384, 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 178.8m and this was derived from LIDAR analysis, this is 2.8m higher than the previously listed 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:  Banc Llechwedd Mawr 

Name:  Llety Llwydin 

OS 1:50,000 map:  135

Summit Height (New Height):  178.8m (LIDAR) 

Summit Grid Reference:  SN 67233 92384 (LIDAR) 

Bwlch Height:  155.5m (LIDAR) 

Bwlch Grid Reference:  SN 66961 92762 (LIDAR) 

Drop:  23.3m (LIDAR) 

 

Myrddyn Phillips (August 2025)