Maes Glas Mawr (SH 768 724) – 100m
Twmpau reclassified to 100m Sub-Twmpau
There has been a reclassification to a hill that is listed in the 100m Twmpau, with the summit height, its location, the drop and status of the hill confirmed by LIDAR analysis, and a subsequent summit survey with the Trimble GeoXH 6000, both conducted by Myrddyn Phillips with the latter taking place on the 10th October 2018.
Maes Glas Mawr (SH 768 724) |
The criteria for the list that this reclassification
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.
The name of the bounded land where the summit of
this hill is situated is Maes Glas Mawr, and this was derived from local
enquiry and it is the name that this hill is now listed by, and it is adjoined
to the Carneddau group of hills, which are situated in the north-western part of North Wales (Region A,
Sub-Region A1), and it is encircled by minor
roads with the B5106 road and the Afon Conwy (River Conwy) to its east, and has
the town of Conwy towards its north.
When the original 100m 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 as its summit was thought to be higher than its adjacent
and southerly hill known as Clytiau Poethion which is positioned at SH 76337
71799, both hills have now been surveyed with Clytiau Poethion confirmed as the
higher resulting in the reclassification of Maes Glas Mawr.
When this list was standardised and interpolated
heights and drop values also included Maes Glas Mawr was listed with an
estimated c 39m of drop based on the 107m summit spot height that appears on the
Ordnance Survey 1:50,000 Landranger map and an estimated bwlch height of c 68m
based on interpolation of 10m bwlch contouring between 60m – 70m.
Extract from the Ordnance Survey 1:25,000 Explorer map |
These values were re-assessed when the Ordnance
Survey Vector Map Local hosted on the Geograph website and entitled the Interactive
Coverage Map became available online, and the listed drop value remained the
same.
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 image of Maes Glas Mawr |
Importantly LIDAR analysis gave Clytiau Poethion
(SH 76337 71799) as higher than Maes Glas Mawr and this has subsequently been
confirmed by a Trimble GeoXH 6000 survey, therefore the bylchau for each hill
is swapped as are their classifications, resulting in a Trimble GeoXH 6000 summit
height for Maes Glas Mawr of 108.5m positioned at SH 76876 72414, and a bwlch
height of 87.05m produced by LIDAR analysis and positioned at SH 76604 71933,
with these values giving this hill 21.4m of drop, which is sufficient for this
hill to be reclassified to 100m Sub-Twmpau status.
The full details for the hill are:
Group: Carneddau
Name: Maes Glas Mawr
OS 1:50,000 map: 115
Summit Height: 108.5m
(converted to OSGM15)
Summit Grid Reference:
SH
76876 72414
Bwlch Height:
87.05m (LIDAR)
Bwlch Grid Reference:
SH 76604 71933 (LIDAR)
Drop: 21.4m (LIDAR)
Myrddyn Phillips (March 2019)
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