Mapping Mountains – Significant Height Revisions – 200m Twmpau

 

Pt. 277.7m (SO 106 406) 

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 Pt. 277.7m (SO 106 406)

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, with the word Twmpau being an acronym standing for thirty welsh metre prominences and upward. 

200m Twmpau by Myrddyn Phillips

The hill is being listed by the point (Pt. 277.7m) notation, and it is adjoined to the Mynydd Epynt group of hills, which are situated in the central part of South Wales (Region B, Sub-Region B1), and it is positioned with a minor road to its west and the A470 road to its east, and has the small community of Erwyd (Erwood) 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 not included in the accompanying 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-evaluated and it was listed with an estimated c 18m of drop, based on the 275m summit spot height that appears on the contemporary Ordnance Survey 1:25,000 Explorer map and an estimated c 257m bwlch height, based on interpolation of 10m contouring between 250m – 260m. 

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 Pt. 277.7m (SO 106 406)

LIDAR analysis gives the summit of hill as 277.7m positioned at SO 10666 40689, and when compared to its originally listed summit height of 275m 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 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, the new listed summit height of this hill is 277.7m and this was derived from LIDAR analysis, this is 2.7m higher than the originally listed summit height of 275m, 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:  Mynydd Epynt 

Name:  Pt. 277.7m 

OS 1:50,000 map:  148, 161

Summit Height (New Height):  277.7m (LIDAR) 

Summit Grid Reference:  SO 10666 40689 (LIDAR) 

Bwlch Height:  255.1m (LIDAR) 

Bwlch Grid Reference:  SO 10264 40290 (LIDAR) 

Drop:  22.6m (LIDAR) 

 

Myrddyn Phillips (August 2024)