Monday, 5 December 2016

Mapping Mountains – Hill Reclassifications – Y Pedwarau

Moelfre (SH 717 744) - 400m Sub-Pedwar deletion

There has been a deletion of a 400m Sub-Pedwar by analysis of LIDAR data coupled with a previous survey with the Trimble GeoXH 6000.  The hill continued to be listed as a 400m Sub-Pedwar based on a 433.0m (converted to OSGM15) surveyed summit height and an on-site visual inspection of one of two potential positions for this hill’s critical bwlch, this option for the bwlch position was considered lower than the second option for the bwlch position at SH 71909 74471 which was surveyed as being 416.9m (converted to OSGM15) high.  Because of this the hill was retained as a 400m Sub-Pedwar with an estimated drop of c 20m.  The 400m Sub-Pedwarau are the Welsh hills at or above 400m and below 500m in height with a drop between 20m and below 30m.

LIDAR image of Moelfre (SH 717 744)

The hill is positioned above the towns of Penmaenmawr to its north and Llanfairfechan to its west and can be easily accessed from a green track that continues from the end of a minor lane to the hill’s west, or from its north-west where public footpaths and bridleways lead close to its summit. 

The hill’s summit and one prospective position for its bwlch was surveyed in the late evening with Aled Williams and the hill is positioned in the northern Carneddau in north-west Wales.  This hill range comprises some of the highest mountains in the country but the evening walk concentrated on some of its 300m and 400m lower hills.

The name of the hill is Moelfre and prior to the survey with the Trimble GeoXH 6000 the height of the hill was listed as 435m.  This height appears on the Ordnance Survey 1:25,000 Explorer and 1:50,000 Landranger maps and the Ordnance Survey enlarged map on the Geograph website.  This 435m height seems to be a recent addition as the series of Ordnance Survey Six-Inch maps from 1888 - 1946 have a 1,422ft (433.4m) height, whilst the Ordnance Survey Seventh Series One-Inch map and the Historical 1:25,000 map has 1,423ft (433.7m).

Moelfre was surveyed with the Trimble GeoXH 6000 as being 433.0m (converted to OSGM15) high and one of its two bwlch positional options at SH 71909 74471 was surveyed as being 416.9m (converted to OSGM15) high, with the second bwlch positional option at SH 72253 74153 estimated as being c 413m high, which gave this hill c 20m of drop and therefore it retained its status as a 400m Sub-Pedwar.

However, since the survey with the Trimble, Aled has analysed the bwlch position via LIDAR data, whose height we had previously estimated from a visual inspection.  LIDAR (Light Detection & Ranging) is highly accurate height data that is now freely available for much of England and Wales.  Analysing LIDAR data gave the following result:

Summit Height:  433.2m

Summit Grid Reference:  SH 71735 74416

Bwlch Height:  414.7m

Bwlch Grid Reference:  SH 72254 74153

Drop:  18.5m

The summit data produced by LIDAR is comparable to that produced by the Trimble GeoXH 6000, these are detailed below:

433.2m LIDAR summit height at SH 71735 74416

433.0m (converted to OSGM15) Trimble summit height at SH 71733 74418

Therefore, the 414.7m LIDAR data produced for the bwlch position at SH 72254 74153 that had previously been visually inspected is considered accurate and when coupled with the 433.0m (converted to OSGM15) summit height produced by the Trimble GeoXH 6000 gives this hill 18.3m of drop, which is insufficient for the hill to retain its 400m Sub-Pedwar status, and therefore Moelfre is deleted from the listing of 400m Sub-Pedwar hills and the list of the Y Pedwarau will be updated accordingly.  The list of Pedwar hills is available from the Haroldstreet website.

The full details for the hill are:

Cardinal Hill:  Tal y Fan

Summit Height:  433.0m (converted to OSGM15)

Name:  Moelfre

OS 1:50,000 map:  115

Summit Grid Reference:  SH 71733 74418 

Drop:  18.3m (Trimble summit and LIDAR bwlch)

The list of additions, reclassifications and deletions from the 400m Sub-Pedwar list since the 1st edition of the Y Pedwarau was published by Europeaklist are as follows:




Moelfre (SH 717 744) the deleted 400m Sub-Pedwar

Myrddyn Phillips and Aled Williams (December 2016)

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