Mapping Mountains – Significant Height Revisions – 200m Twmpau

Sixteen Acres (SO 120 987)

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. 

Sixteen Acres (SO 120 987)

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 name the hill is now listed by is Sixteen Acres and this was derived from local enquiry, 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 the B4390 road to its north, the B4389 road to its west and the A483 road further to its south-east, and has the village of Tregynon towards the 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-assessed and it was listed with an estimated c 20m of drop, based on an estimated c 216m summit height and the 196m bwlch spot height that appeared on the Ordnance Survey Vector Map Local hosted on the Geograph website and which was named the Interactive Coverage 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 Sixteen Acres

The result produced by LIDAR analysis gives this hill a 219.2m summit height, and as the summit has now been surveyed with the Trimble GeoXH 6000 it is this result that is being prioritised for listing purposes. 

The Trimble GeoXH 6000 gathering data at the summit of Sixteen Acres

The summit height and position produced by the Trimble GeoXH 6000 survey is 219.2m at SO 12005 98756, 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 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. 

Therefore, this hill’s new listed summit height is 219.2m and this was derived from a Trimble GeoXH 6000 survey, this is positioned at SO 12005 98756 and is 3.2m higher than its previously listed height of c 216m which was estimated from interpolation of its uppermost ring contour that appears on the Ordnance Survey 1:25,000 Explorer map. 

 

The full details for the hill are:

Group:  Carnedd Wen

Name:  Sixteen Acres

OS 1:50,000 map:  136

Summit Height (new height):  219.2m (converted to OSGM15, Trimble GeoXH 6000)

Summit Grid Reference:  SO 12005 98756 (Trimble GeoXH 6000) 

Bwlch Height:  199.1m (LIDAR)

Bwlch Grid Reference:  SO 11953 98547 (LIDAR)

Drop:  20.1m (Trimble GeoXH 6000 summit and LIDAR bwlch)

 

Myrddyn Phillips (January 2021)