Y Grongaer (SN 573 215
& SN 574 215)
There has been a Summit Relocation 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 derived from LIDAR analysis conducted by Myrddyn
Phillips.
 |
| LIDAR image of Y Grongaer (SN 573 215 & SN 574 215) |
The criteria for the two listings that this summit
relocation applies to are:
100m Twmpau - Welsh hills at or above 100m and below 200m in height that have 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.
 |
| The 100m Twmpau by Myrddyn Phillips |
 |
| Y Trechol - The Dominant Hills of Wales by Myrddyn Phillips |
The name the hill is listed by is Y Grongaer, and it is adjoined
to the Mynydd Mallaen group of hills, which are situated in the central part of South Wales (Region B,
Sub-Region B1), and it is positioned with the
A40 road to its north, the B4297 road to its west, the B4300 road to its south
and a minor road to its east, and has the town of Llandeilo towards the east.
When the original Welsh 100m P30 list was published on Geoff Crowder’s v-g.me website, this hill was listed with a 143m
summit height, based on the spot height that is positioned at SN 57326 21596 and
appears on the Ordnance Survey 1:25,000 Explorer map.
 |
| 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 Grongaer (SN 573 215 & SN 574 215) |
The summit height produced by LIDAR analysis is 140.7m
and is positioned at SN 57399 21574 and SN 57400 21575, and this comes within the parameters of the
Summit Relocations used within this page heading, these parameters are:
The term Summit Relocations applies to when the
high point is positioned in a different field, to a different feature such as a
conifer plantation, within a different map contour, a different point where a
number of potential summit positions are within close proximity, when natural
ground or the natural and intact summit is confirmed compared to a higher point
such as a raised field boundary or covered reservoir that is considered a
relatively recent man-made construct, or a relocation of approximately 100
metres or more in distance from either the position of a map spot height or
from where the summit of the hill was previously thought to exist.
Therefore, the summit height produced by LIDAR
analysis is 140.7m and is positioned at SN 57399 21574 and SN 57400 21575, this position is not
given a spot height on the contemporary Ordnance Survey 1:50,000 Landranger and
1:25,000 Explorer map, and is approximately 75 metres eastward and placed in a
different ring contour from where the previously listed summit is
positioned.
The full details for the
hill are:
Group: Mynydd Mallaen
Name: Y Grongaer
OS 1:50,000 map: 159
Summit Height: 140.7m (LIDAR)
Summit Grid Reference
(New Position): SN 57399 21574 & SN 57400
21575 (LIDAR)
Bwlch Height: 53.05m (LIDAR)
Bwlch Grid
Reference: SN 57923 22123 (LIDAR)
Drop: 87.6m (LIDAR)
Dominance: 62.29% (LIDAR)
Myrddyn Phillips (April
2022)
