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

Boncyn Bwlch Glas (SJ 031 176) 

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 Boncyn Bwlch Glas (SJ 031 176)

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 Boncyn Bwlch Glas, and it is adjoined to the Foel Cedig group of hills, which are situated in the southern part of North Wales (Region A, Sub-Region A3), and it is positioned with a minor road to its north, west and south, the B4395 road to its south-east and the B4393 road to its north-east, and has the village of Llanwddyn towards the north-west.

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 Hills to be surveyed sub list with a summit height of c 320m with an accompanying note stating; Two points of same height.

After the sub list was standardised, and interpolated heights and drop values also included the details for this hill were re-evaluated and it was listed with an estimated c 321m summit height, based on interpolation of the uppermost 320m ring contour 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 analysis gives the summit height of this hill as 323.5m 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 323.5m and this was derived from LIDAR analysis, this is 2.5m higher than the previously listed summit height of c 321m which was derived from interpolation of the uppermost 320m ring contour that appears on the Ordnance Survey 1:25,000 Explorer map. 

 

The full details for the hill are: 

Group:  Foel Cedig 

Name:  Boncyn Bwlch Glas 

OS 1:50,000 map:  125

Summit Height (New Height):  323.5m (LIDAR)                                                           

Summit Grid Reference:  SJ 03107 17613 (LIDAR) 

Bwlch Height:  293.8m (LIDAR) 

Bwlch Grid Reference:  SJ 03272 17643 (LIDAR) 

Drop:  29.7m (LIDAR) 

 

Myrddyn Phillips (July 2022)