Mapping Mountains – Hill Reclassifications – 100m Twmpau

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 10^th 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)

For details on the summit survey of this hill

Myrddyn Phillips (March 2019)