Tuesday, 2 August 2016

Mapping Mountains – Hill Reclassifications – 300m Twmpau


Penmaen Mawr (SH 702 757) - 300m Sub-Twmpau addition

THIS HILL IS NOW CLASSIFIED AS A TRICHANT AND 300m TWMPAU

The following details have been superseded by analysis of LIDAR data by Aled Williams with the result that this hill has been reclassified from a Sub-Trichant and 300m Sub-Twmpau to a Trichant and 300m Twmpau as LIDAR data gives the hill 31.3m of drop.

The below is the Hill Reclassifications post prior to LIDAR data analysis:


There has been a promotion to the listing of Twmpau (thirty welsh metre prominences and upward) hills due to a recent survey with the Trimble GeoXH 6000.  The hill is now listed in the Sub-List adjoined to the 300m height band of Twmpau hills and is situated in the northern Carneddau in north-west Wales.

The hill was not classified prior to the survey with the Trimble as the majority of the summit has been quarried and there is a lack of uppermost contour lines on current Ordnance Survey maps.  But at one stage it was a part of a relatively substantial hill that had a prominence in excess of 100m and a 1,550ft (472m) summit map height on the Ordnance Survey Popular and New Popular One-Inch maps, with this height also appearing on the Ordnance Survey Six-Inch map published in 1888, and therefore it would have met the criteria specified for inclusion to the Humps and the Y Pedwarau listings.

Because of this the hill did not appear in the Sub-List which accompanied the original Welsh P30 listings on Geoff Crowder’s v-g.me website as the Sub-List only included hills whose map details showed that when surveyed they may stand a chance of qualification to the main P30 list, with the title of the Sub-List being ‘Hills to be surveyed.’  And as this hill only has an uppermost 350m contour line in the vicinity of its summit area and also bwlch, map detail intimated that there was insufficient drop for inclusion to this Sub-List.  However, since first publication this Sub-List has been standardised and now includes all hills that have a minimum of 20m of drop and below 30m of drop.

The hill is situated to the south of the A 55 and is positioned between the towns of Llanfairfechan to its west and Penmaenmawr to its east, with the latter taking its name from the hill.  The quarrying of this hill has now left two distinct summits, the higher western summit and the lower easterly summit which this post details.   

The hill can be easily accessed from a minor road to the south of the summit that reaches over 260m in height, a public footpath heads north from just below the high point of this minor road and joins a bridlewayed track that contours around the southern slopes of the upper section of the hill.  A large section of the land above the bridleway is open access and the summit Penmaen Mawr is close to this.

The survey with the Trimble GeoXH 6000 resulted in a 384.9m (converted to OSGM15) summit height and a 355.1m (converted to OSGM15) bwlch height, giving this hill 29.8m of drop and therefore it qualifies as a 300m Sub-Twmpau.


The full details for the hill are:

Cardinal Hill:  Carnedd Llywelyn

Summit Height:  384.9m (converted to OSGM15)

Name:  Penmaen Mawr

OS 1:50,000 map:  115

Summit Grid Reference:  SH 70200 75727  

Drop:  29.8m (converted to OSGM15)


Penmaen Mawr (SH 702 757) now classified as a 300m Sub-Twmpau



Myrddyn Phillips (August 2016)















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