The Larches (ST 433 899)
There has been a Significant Height Revision to a
hill that is listed in the 30-99m 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 Larches (ST 433 899) |
The criteria for the two listings that this height revision
applies to are:
30-99m Twmpau - Welsh hills at or above 30m and below 100m in height with 30m minimum
drop, with an accompanying sub list entitled the 30-99m Sub-Twmpau with the
criteria for this sub category being all Welsh hills at or above 30m and below
100m 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.
 |
| The 30-99m Twmpau by Myrddyn Phillips |
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, with
the Introduction to 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.
 |
| Y Trechol - The Dominant Hills of Wales by Myrddyn Phillips |
The name the hill is now listed by is The Larches,
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 minor roads to its
north, west, south and east and further afield has the A48 road to its north
and the M4 motorway to its south, and has the city of Casnewydd (Newport)
towards the west.
When the original Welsh 30-99m P30 list was published on Geoff Crowder’s v-g.me website this hill was listed with a c 85m
summit height, this being the uppermost contour that appears on the Ordnance
Survey 1:25,000 Explorer map and which is positioned at ST 433 899.
 |
| Extract from the Ordnance Survey 1:25,000 Explorer map |
When this list was standardised, and interpolated
heights and drop values also included the details for this hill was re-assessed
and it was listed with an estimated c 89m summit height based on interpolation
of 5m contouring 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 summit image of The Larches |
The summit height produced by LIDAR analysis is
91.1m and 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, this hill’s new listed summit height is
91.1m and this was derived from LIDAR analysis, this is 2.1m higher than the
previous listed summit height of c 89m and 6.1m higher than the uppermost 85m
ring contour on contemporary Ordnance Survey 1:25,000 Explorer maps.
The full details for the hill are:
Group: Gwent Is Coed
Name: The Larches
OS 1:50,000 map: 171,
172
Summit Height (New Height):
91.1m (LIDAR)
Summit Grid Reference:
ST
43303 89957 (LIDAR)
Bwlch Height:
22.3m (LIDAR)
Bwlch Grid Reference:
ST 42928 90248 (LIDAR)
Drop: 68.8m (LIDAR)
Dominance: 75.54%
(LIDAR)
Myrddyn Phillips (September 2020)
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