Mapping Mountains – Significant Height Revisions – 200m Twmpau

Gronglwyd (SN 598 558) 

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 derived from LIDAR analysis conducted by Myrddyn Phillips. 

LIDAR image of Gronglwyd (SN 598 558)

The criteria for the list that this height revision applies to 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 Gronglwyd and this was derived from the Tithe map, and it is adjoined to the Mynydd Bach group of hills, which are situated in the western part of South Wales (Region B, Sub-Region B1), and it is positioned with minor roads to its north and west, and the A485 road to its south-east, and has the town of Tregaron towards the east north-east. 

Extract from the Ordnance Survey 1:25,000 Explorer map

When the original 200m height band of Welsh P30 hills were published on Geoff Crowder’s v-g.me website, this hill was listed with a summit height of 239m, based on the spot height that appeared on the 1985 Ordnance Survey 1:50,000 Landranger map (latterly updated with a 240m spot height). 

Extract from the contemporary Ordnance Survey 1:50,000 Landranger 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 summit image of Gronglwyd (SN 598 558)

The height produced by LIDAR analysis to the summit of this hill is 240.3m positioned at SN 59868 55899, 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 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, the new listed summit height of this hill is 240.3m and this was derived from LIDAR analysis, this is 1.3m higher than the originally listed summit height of 239m, which was based on the spot height that appears on the 1985 Ordnance Survey 1:50,000 Landranger map.

 

ills of Wales, and are reproduced below@

The full details for the hill are:

Group:  Mynydd Bach 

Name:  Gronglwyd 

OS 1:50,000 map:  146

Summit Height (New Height):  240.3m (LIDAR) 

Summit Grid Reference:  SN 59868 55899 (LIDAR) 

Bwlch Height:  208.1m (LIDAR) 

Bwlch Grid Reference:  SN 60305 55837 (LIDAR) 

Drop:  32.2m (LIDAR) 

 

Myrddyn Phillips (November 2022)