Mapping Mountains – Significant Height Revisions – 200m Twmpau

 

Pt. 218.8m (SO 122 978)

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 confirmed by LIDAR analysis and a subsequent Trimble GeoXH 6000 survey conducted by Myrddyn Phillips. 

LIDAR image of Pt. 218.8m (SO 122 978)

The criteria for the list this height revision affects 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. 

The 200m Twmpau by Myrddyn Phillips

The hill is being listed by the point (Pt. 218.8m) notation as an appropriate name for it either through local enquiry and / or historic research has not been found by the author, and it is adjoined to the Carnedd Wen group of hills which are situated in the south-eastern part of North Wales (Region A, Sub-Region A4), and it is positioned with minor roads to its south-east and north, and has the B4389 road to its south-west, and has the small community of Betws Cedewain towards the south. 

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 accompanying Hills to be surveyed sub list, and listed with an estimated c 230m summit height. 

When 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 231m summit height and a 204m bwlch height, with the latter based on the spot height that appears on the Ordnance Survey 1:25,000 Explorer map.  An anomaly was also noted with the heights given the contours on the upper part of this hill on the 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. 

The result produced by LIDAR analysis gives this hill a 218.8m summit height and 13.6m of drop, confirming that the uppermost contour on the 1:25,000 Explorer map is not 230m, and as the summit has now been surveyed with the Trimble GeoXH 6000 it is this result that is being prioritised for listing purposes. 

LIDAR summit image of Pt. 218.8m (SO 122 978)

The summit height produced by the Trimble GeoXH 6000 survey is 218.8m, this is a dramatic height revision when compared to its previously listed height and 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 summit 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. 

The Trimble GeoXH 6000 gathering data at the summit of Pt. 218.8m

Therefore, this hill’s new listed summit as surveyed with the Trimble GeoXH 6000 is 218.8m, this is positioned at SO 12216 97806 and is 12.2m lower than its previously listed height of c 231m which was based on what was presumed to be an uppermost 230m ring contour on the Ordnance Survey 1:25,000 Explorer map. 

 

The full details for the hill are:

Group:  Carnedd Wen

Name:  Pt. 218.8m 

OS 1:50,000 map:  136

Summit Height (New Height):  218.8m (converted to OSGM15, Trimble GeoXH 6000) 

Summit Grid Reference:  SO 12216 97806 (Trimble GeoXH 6000) 

Bwlch Height:  205.3m (LIDAR) 

Bwlch Grid Reference:  SO 12899 97950 (LIDAR) 

Drop:  13.5m (Trimble GeoXH 6000 summit and LIDAR bwlch)

 

Myrddyn Phillips (February 2021)