Mapping Mountains – Significant Height Revisions – 200m Twmpau

 

Cow Pasture (SO 110 977)

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. 

Cow Pasture (SO 110 977)

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 Cow Pasture and this was derived from the Tithe map, 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 B4389 road to its south-west and minor roads to its north and east, and has the village of Tregynon towards the 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 included in the accompanying Hills to be surveyed sub list with a summit height of 213m, which was taken from 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 Cow Pasture

The result produced by LIDAR analysis gives this hill a 215.5m 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 summit height and position produced by the Trimble GeoXH 6000 survey is 215.5m at SO 11094 97769, 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. 

The Trimble GeoXH 6000 gathering data at the summit of Cow Pasture

Therefore, this hill’s new listed summit height is 215.5m and this was derived from a Trimble GeoXH 6000 survey, this is positioned at SO 11094 97769 and is 2.3m higher than its previously listed height of 213m which appears as a spot height on the Ordnance Survey 1:25,000 Explorer map. 

 

The full details for the hill are:

Group:  Carnedd Wen

Name:  Cow Pasture

OS 1:50,000 map:  136     

Summit Height (new height):  215.5m (converted to OSGM15, Trimble GeoXH 6000)

Summit Grid Reference:  SO 11094 97769 (Trimble GeoXH 6000) 

Bwlch Height:  190.2m (LIDAR)

Bwlch Grid Reference:  SO 11043 97985 (LIDAR)

Drop:  25.3m (Trimble GeoXH 6000 summit and LIDAR bwlch)

 

Myrddyn Phillips (February 2021)