Mapping Mountains – Significant Height Revisions – 100m Twmpau

Garden Field (SO 180 383)

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 conducted by Myrddyn Phillips.

LIDAR image of Garden Field (SO 180 383)

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 name the hill is now listed by is Garden Field and this derived from the Tithe map, and it is adjoined to the Mynyddoedd Duon group of hills which are situated in the eastern part of South Wales (Region C, Sub-Region C3), and it is encircled by minor roads with the Afon Gwy (River Wye) and the A438 road further to its north-west, and the village of Aberllynfi (Three Cocks) 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 listed with a 168m 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. 

The height produced by LIDAR analysis is 170.3m and is positioned at SO 18079 38331,  this is not a substantial revision compared to some revised heights, but it does come 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 170.3m and this was derived from LIDAR analysis, this is 2.3m higher than the previously listed summit height of 168m which was based on the spot height that appears on the Ordnance Survey 1:25,000 Explorer map.

ills of Wales, and are reproduced below@

The full details for the hill are:

Group:  Mynyddoedd Duon

Name:  Garden Field

OS 1:50,000 map:  161

Summit Height (New Height):  170.3m (LIDAR)

Summit Grid Reference:  SO 18079 38331 (LIDAR)

Bwlch Height:  141.3m (LIDAR)

Bwlch Grid Reference:  SO 18629 38227 (LIDAR) 

Drop:  28.9m (LIDAR)

Myrddyn Phillips (March 2020)