Mapping Mountains – Significant Height Revisions – The Welsh P15s

Y Boncyn (SH 532 398) 

There has been a Significant Height Revision to a hill that is listed in The Welsh P15s, 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 Y Boncyn (SH 532 398)

The criteria for the list that this height revision applies to are:

The Welsh P15s – Welsh hills with 15m minimum drop, irrespective of their height, with an accompanying sub list entitled the Welsh Sub-P15s, with the criteria for this sub category being all Welsh hills with 14m or more and below 15m of drop.  The list is authored by Myrddyn Phillips, with the Introduction to the list appearing on Mapping Mountains on the 10^th May 2019. 

The Welsh P15s by Myrddyn Phillips

The name the hill is listed by is Y Boncyn, and it is adjoined to the Moel Hebog group of hills, which are situated in the north-western part of North Wales (Region A, Sub-Region A1), and it is positioned with the A487 road to its north and the A497 road to its south, and has the town of Porthmadog towards the east south-east.

When the listing that became known as The Welsh P15s was being compiled, this hill was not included in the main P15 or Sub P14 list, as with no significant contours of note on the Ordnance Survey 1:50,000 Landranger and 1:25,000 Explorer map, it was difficult to know whether any hill of note existed. 

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 Y Boncyn (SH 532 398)

The summit height produced by LIDAR analysis is 66.3m and when compared to detail on the Ordnance Survey 1:50,000 Landranger and 1:25,000 Explorer map, 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 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 66.3m and this was derived from LIDAR analysis.  This is 16.3m higher than the uppermost 50m contour that appears on the Ordnance Survey 1:50,000 Landranger and 1:25,000 Explorer map. 

 ills of Wales, and are reproduced below@

The full details for the hill are: 

Group:  Moel Hebog 

Name:  Y Boncyn 

OS 1:50,000 map:  124

Summit Height (New Height):  66.3m (LIDAR)                                                           

Summit Grid Reference:  SH 53275 39885 (LIDAR) 

Bwlch Height:  51.0m (LIDAR) 

Bwlch Grid Reference:  SH 53321 39934 (LIDAR) 

Drop:  15.3m (LIDAR) 

 

Myrddyn Phillips (September 2024)