Chepstow Hill (ST 355
905)
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 Chepstow Hill (ST 355 905) |
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 Chepstow Hill
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 Afon Wysg (River Usk) to its north-west, the M4 motorway to its south
and the A449 road to its east, and has the city of Casnewydd (Newport) towards
the south-west.
When the original 100m height band of Welsh P30 hills were published on Geoff Crowder’s v-g.me website this hill was listed
with a 111m summit height based on the spot height that appears on the Ordnance
Survey 1:25,000 Explorer map.
 |
| Extract from the Ordnance Survey 1:25,000 Explorer map |
The details for this hill were re-assessed when
the Ordnance Survey Vector Map Local hosted on the Geograph website and which
is entitled the Interactive Coverage Map became available online. This mapping had many spot heights not on
other publicly available Ordnance Survey maps and gives a 114m spot height on
the summit area of this hill and positioned at ST 356 905. This 114m spot height also appears on
Ordnance Survey data available on the Magic Maps website.
 |
| Extract from the Magic Maps website |
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.
The summit height produced by LIDAR analysis is 116.6m
positioned at ST 35547 90567, this is a substantial height revision compared to
the original 111m listed summit height, 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
116.6m and this was produced by LIDAR analysis, this is 5.6m higher than its original
listed height of 111m that appears as a spot height on the Ordnance Survey
1:25,000 Explorer map, and 2.6m higher than that of the online Vector Map Local.
The full details for the hill are:
Group: Gwent Is Coed
Name: Chepstow Hill
OS 1:50,000 map: 171
Summit Height (New Height):
116.6m (LIDAR)
Summit Grid Reference:
ST
35547 90567 (LIDAR)
Bwlch Height:
63.2m (LIDAR)
Bwlch Grid Reference:
ST 36979 90843 (LIDAR)
Drop: 53.4m (LIDAR)
Dominance: 45.76%
(LIDAR)
Myrddyn Phillips (June 2020)
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