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

Cefn Isaf (SN 800 486) 

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 conducted by Myrddyn Phillips. 

LIDAR image of Cefn Isaf (SN 800 486)

The criteria for the list that this height revision applies to 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, and the Introduction to the Mapping Mountains publication of the list appearing on the 1^st January 2022. 

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

The name the hill is listed by is Cefn Isaf, and it is adjoined to the Drygarn Fawr group of hills, which are situated in the northern part of South Wales (Region B, Sub-Region B1), and it is positioned with a minor road to its south-east and the A483 road farther to its south-east, and has the town of Llanwrtyd (Llanwrtyd Wells) towards the east south-east.

When the original 300m height band of Welsh P30 hills were published on Geoff Crowder’s v-g.me website, this hill was included in the main P30 list with a summit height of 388m, 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. 

LIDAR summit image of Cefn Isaf (SN 800 486)

LIDAR analysis gives the summit height of this hill as 386.0m 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.

Therefore, the new listed summit height of this hill is 386.0m and this was derived from LIDAR analysis, this is 2.0m lower than the previously listed summit height of 388m which appears as a spot height on the Ordnance Survey 1:25,000 Explorer map. 

 

The full details for the hill are: 

Group:  Drygarn Fawr 

Name:  Cefn Isaf 

OS 1:50,000 map:  147

Summit Height (New Height):  386.0m (LIDAR)                                                           

Summit Grid Reference:  SN 80031 48623 (LIDAR) 

Bwlch Height:  345.5m (LIDAR) 

Bwlch Grid Reference:  SN 80361 48591 (LIDAR) 

Drop:  40.5m (LIDAR) 

 

Myrddyn Phillips (February 2023)