Mapping Mountains – Significant Height Revisions – 100m Twmpau

Pt. 170.7m (SJ 193 018) 

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

Pt. 170.7m (SJ 193 018)

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

The 100m Twmpau by Myrddyn Phillips

The hill is being listed by the point (Pt. 170.7m) 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 a minor road to its north, the B4385 road to its west, the B4390 road to its south and the A483 road to its east, and has the village of Aberriw (Berriew) towards the south-west. 

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 main P30 list, with a 169m summit height based on the spot height 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 image of Pt. 170.7m (SJ 193 018)

The summit height produced by LIDAR analysis is 170.6m, but as the summit of this hill has now been surveyed with the Trimble GeoXH 6000, resulting in 170.7m at SJ 19323 01898, it is this result that is being prioritised for listing purposes and this height 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. 170.7m (SJ 193 018)

Therefore, this hill’s new listed summit height is 170.7m and this was derived from a Trimble GeoXH 6000 survey, this is positioned at SJ 19323 01898 and is 1.7m higher than the previously listed summit height of 169m 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:  Carnedd Wen

Name:  Pt. 170.7m

OS 1:50,000 map:  136

Summit Height (New Height):  170.7m (converted to OSGM15, Trimble GeoXH 6000)

Summit Grid Reference:  SJ 19323 01898 (Trimble GeoXH 6000)

Bwlch Height:  132.5m (LIDAR)

Bwlch Grid Reference:  SJ 18115 01656 (LIDAR)

Drop:  38.2m (Trimble GeoXH 6000 summit and LIDAR bwlch)

 

Myrddyn Phillips (June 2021)