Mapping Mountains – Significant Height Revisions – 100m Twmpau

Carn Gelli (SM 923 379) 

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 derived from detail on contemporary maps produced from Ordnance Survey data and LIDAR analysis conducted by Myrddyn Phillips. 

LIDAR image of Carn Gelli (SM 923 379)

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 that have 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 100m Twmpau by Myrddyn Phillips

The name the hill is listed by is Carn Gelli, and it is adjoined to the Garn Fawr group of hills, which are situated in the south-western part of South Wales (Region B, Sub-Region B1), and it is positioned encircled by minor roads with the A487 road farther to its south-east, and has the town of Abergwaun (Fishguard) towards the east south-east. 

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

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 summit height of 188m, based on the spot height that appears on the Ordnance Survey 1:25,000 Explorer map, with an accompanying note stating; 191m on 2002 1:50,000 map. 

Extract from the 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 Carn Gelli (SM 923 379)

The height produced by LIDAR analysis to the summit of this hill is 190.7m positioned at SM 92313 37943, 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 190.7m and this was derived from LIDAR analysis, this is 2.7m higher than the originally listed summit height of 188m, 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:  Garn Fawr 

Name:  Carn Gelli 

OS 1:50,000 map:  157

Summit Height (New Height):  190.7m (LIDAR) 

Summit Grid Reference:  SM 92313 37943 (LIDAR) 

Bwlch Height:  128m (spot height) 

Bwlch Grid Reference:  SM 91953 38745 (spot height) 

Drop:  63m (LIDAR summit and spot height bwlch) 

 

Myrddyn Phillips (April 2023)