Allt y Geifr (SN 762 284)
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 Allt y Geifr (SN 762 284) |
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 Allt y Geifr, and
it is adjoined
to the Mynydd Du group of hills, which are situated in the southern part of South Wales (Region B,
Sub-Region B5), and it is positioned encircled
by minor roads with the A4069 road farther to its north-west and south-west, and
has the village of Llangadog towards the west.
When the listing that became known as The Welsh P15s was being compiled, this
hill was included in the main P15 list with an estimated c 16m of drop, based
on the 174m summit spot height that appeared on the Ordnance Survey Vector Map
Local hosted on the Geograph website and which was entitled the Interactive
Coverage Map and an estimated c 158m bwlch height, based on interpolation of 5m
contouring between 155m – 160m that appeared on the OS Maps website.
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 Allt y Geifr (SN 762 284) |
The summit height produced by LIDAR analysis is 171.1m
and when compared to detail on the Ordnance Survey Vector Map Local hosted on
the Geograph website and which was entitled the Interactive Coverage 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 171.1m and this was derived from LIDAR analysis. This is 2.9m lower than the 174m spot height that
appeared on the Ordnance Survey Vector Map Local hosted on the Geograph website
and which was entitled the Interactive Coverage.
The full details for the hill are:
Group: Mynydd Du
Name: Allt y Geifr
OS 1:50,000 map: 146,
160
Summit Height (New Height): 171.1m (LIDAR)
Summit Grid Reference: SN 76221 28491 (LIDAR)
Bwlch Height: 156.9m (LIDAR)
Bwlch Grid Reference: SN 76558 28873 (LIDAR)
Drop: 14.2m (LIDAR)
Myrddyn Phillips (June
2024)
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