The Knoll (ST 424 904)
There has been a Significant Height Revision to a
hill that is listed in the 100m Twmpau
and Y Trechol – The Dominant Hills of Wales, with the summit height, bwlch height
and their locations, the drop, dominance and status of the hill confirmed by
LIDAR analysis conducted by Myrddyn Phillips.
 |
| LIDAR image of The Knoll (ST 424 904) |
The criteria for the two listings that this height revision
applies to are:
100m Twmpau - Welsh hills at or above 100m and below 200m in height with 30m
minimum drop, with an accompanying sub list entitled the 100m Sub-Twmpau with
the criteria for this sub category being all Welsh hills at or above 100m and
below 200m in height with 20m or more and below 30m of drop, with the word
Twmpau being an acronym standing for thirty
welsh metre prominences and upward.
Y Trechol - The Dominant Hills of
Wales - Welsh P30 hills whose prominence
equal or exceed half that of their absolute height. With the
criteria for Lesser Dominant status being those additional Welsh P30 hills whose
prominence is between one third and half that of their absolute height. The list is authored by Myrddyn Phillips with
the Introduction to the start of the Mapping Mountains publication of this list
appearing on the 3rd December 2015, and the list is now available in its entirety on Mapping Mountains in Google Doc format.
The name the hill is listed by is
The Knoll, and it is adjoined
to the Gwent Is Coed group of hills which are situated
in the south-eastern part of South Wales (Region C, Sub-Region C3), and it is
positioned with the A48 road to its north and the M4 motorway to its south, and
has the city of Casnewydd (Newport) towards the west south-west.
When the original 30-99m 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 c 90m summit height, based on its uppermost contour
that appears on the Ordnance Survey 1:50,000 Landranger and 1:25,000 Explorer
map.
 |
| Extract from the Ordnance Survey 1:25,000 Explorer map |
When the sub list was standardised, and
interpolated heights and drop values also included the details for this hill
were re-assessed and it was listed with c 51m of drop, based on an estimated c
94m summit height based on interpolation of 5m contouring and a 43m bwlch spot height
that appears on 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 close up image of The Knoll |
The summit height produced by LIDAR analysis is 101.9m
positioned at ST 42467 90424, this is a substantial height revision compared to
some revised heights and it 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, this hill’s new listed summit height is
101.9m and this was produced by LIDAR analysis, this is 7.9m higher than its previously
listed height of c 94m which was based on interpolation of its uppermost 90m
ring contour on the Ordnance Survey 1:25,000 Explorer map.
The full details for the hill are:
Group: Gwent Is Coed
Name: The Knoll
OS 1:50,000 map: 171,
172
Summit Height (New Height):
101.9m (LIDAR)
Summit Grid Reference:
ST
42467 90424 (LIDAR)
Bwlch Height:
43.1m (LIDAR)
Bwlch Grid Reference:
ST 42135 90345 (LIDAR)
Drop: 58.7m (LIDAR)
Dominance: 57.66%
(LIDAR)
Myrddyn Phillips (July 2020)
No comments:
Post a Comment