Wednesday, 27 July 2016

Mapping Mountains – Significant Height Revisions – Y Pedwarau and Y Trichant

Penmaen Mawr (SH 698 755)

There has been a Significant Height Revision initiated by a survey with the Trimble GeoXH 6000 to a hill that is listed in the Y Pedwarau and the Y Trichant, with the survey that resulted in this height revision being conducted on the 14th July 2016.

The criteria for the listings that this height revision affects are:

Y Pedwarau - Welsh hills at and above 400m and below 500m in height that have 30m minimum drop.  300m Double Sub- Pedwarau - Welsh hills at and above 390m and below 400m in height that have 20m or more and below 30m of drop.  The list is co-authored by Myrddyn Phillips and Aled Williams.

Y Trichant - Welsh hills at and above 300m and below 400m in height that have 30m minimum drop.  Sub-TrichantWelsh hills at and above 300m and below 400m in height that have 20m or more and below 30m of drop.  The list is authored by Myrddyn Phillips and the Introduction to the list and its re-naming has been published on Mapping Mountains.

The hill is named Penmaen Mawr  and it is situated in the Carneddau range of hills, and is positioned above and to the immediate south of the busy A 55 as it heads westward between the towns of Penmaenmawr and Llanfairfechan, which are situated to the hill’s east and west respectively.

The summit area of Penmaen Mawr has been quarried, and prior to the first quarry opening in 1830 and the subsequent resulting expansion of operations the hill was substantially higher, with a 1,550ft (472m) map height given it 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.

The quarrying of this hill has created two summits with a gaping hole between, the westerly summit is higher than the easterly, although the latter is more impressive due to a shear rock face leading toward its high point.  Each summit is listed by the same name of Penmaen Mawr as they are both a part of the hill that is known by this name, and their height data is given below:

Penmaen Mawr:  390.4m (converted to OSGM15, Trimble GeoXH 6000) at SH 69865 75523

Penmaen Mawr:  385.8m (LIDAR data) at SH 70292 75649

Over recent years the summit area of this hill has appeared without any uppermost contour lines as is the norm for quarried ground that is represented on Ordnance Survey maps, with the highest contour being 370m which appears on the upper western part of this hill.

Therefore this hill’s new summit height is 390.4m (converted to OSGM15) which is 20.4m higher than its uppermost contour on Ordnance Survey 1:50,000 Landranger and 1:25,000 Explorer maps.

The full details for the hill are:

Cardinal Hill:  Tal y Fan

Summit Height:  390.4m (converted to OSGM15)

Name:  Penmaen Mawr

OS 1:50,000 map:  115

Summit Grid Reference:  SH 69865 75523

Drop:  26.9m (converted to OSGM15)

The Trimble GeoXH 6000 gathering data 0.39m above the summit of Penmaen Mawr (SH 698 755) which resulted in this hill's significant height revision

Myrddyn Phillips and Aled Williams (July 2016)

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