Mapping Mountains – Significant Height Revisions – Y Trichant – The 300m Hills of Wales

Top Field (SO 110 879) 

There has been a Significant Height Revision to a hill that is listed in the Y Trichant – The 300m Hills of Wales, with the summit height, bwlch height and their locations, the drop and status of the hill derived from LIDAR analysis and a subsequent Trimble GeoXH 6000 survey conducted by Myrddyn Phillips. 

Top Field (SO 110 879)

The criteria for the list this height revision affects are:

Y Trichant – The 300m Hills of Wales – Welsh hills at or above 300m and below 400m in height that have 30m minimum drop, with an accompanying sub list entitled the Sub-Trichant with the criteria for this sub category being all Welsh hills at or above 300m and below 400m in height with 20m or more and below 30m of drop.  The list is authored by Myrddyn Phillips, with the Introduction to the list and the renaming of it appearing on Mapping Mountains on the 13^th May 2017. 

Y Trichant - The 300m Hills of Wales by Myrddyn Phillips

The name the hill is listed by is Top Field and this was derived from the Tithe map, and it is adjoined to the Cilfaesty group of hills, which are situated in Mid and West Wales (Region B, Sub-Region B1), and it is positioned with minor roads to its north, west and east and the A483 road farther to its south-west, and has the town of Y Drenewydd (Newtown) towards the north. 

When the original 300m height band of Welsh P30 hills was published on Geoff Crowder’s v-g.me website, this hill was included in the Hills to be surveyed sub list with a summit height of 371m, based on the spot height that appears on the Ordnance Survey 1:25,000 Explorer map, with an accompanying note stating; 374m on 1986 1:50,000 map.  On the contemporary Ordnance Survey 1:50,000 Landranger map, the summit spot height is now given as 371m. 

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 Top Field (SO 110 879)

LIDAR analysis gives the summit height of this hill as 373.6m, and as the summit has now been surveyed with the Trimble GeoXH 6000 it is this result that is being prioritised for listing purposes 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 Top Field

Therefore, the new listed summit height for this hill is 373.8m and this was derived from a Trimble GeoXH 6000 survey, this is 2.8m higher than the previously listed summit height of 371m which appears as a spot height on the contemporary Ordnance Survey 1:25,000 Explorer map and now also appears on the 1:50,000 Landranger map. 

 

The full details for the hill are: 

Group:  Cilfaesty 

Name:  Top Field 

OS 1:50,000 map:  136

Summit Height (New Height):  373.8m (converted to OSGM15, Trimble GeoXH 6000)

Summit Grid Reference:  SO 11015 87958 (Trimble GeoXH 6000) 

Bwlch Height:  346.1m (LIDAR) 

Bwlch Grid Reference:  SO 11279 88639 (LIDAR) 

Drop:  27.7m (Trimble GeoXH 6000 summit and LIDAR bwlch) 

 

Myrddyn Phillips (October 2021)