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 10th
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.
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)
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