Saturday, 6 August 2016

Mapping Mountains – Significant Height Revisions – Y Pedwarau

Moelfre (SH 717 744)

This is the thirty third post under the heading of Significant Height Revisions, and the Trimble survey that resulted in this height revision was conducted on the 14th July 2016 in clear albeit late evening conditions in the company of Aled Williams.

The thirty third major height revision initiated from a survey with the Trimble GeoXH 6000 was conducted 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 hill is positioned above the towns of Penmaenmawr which is to its north and Llanfairfechan which is to its west and it 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 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 as being 433.0m (converted to OSGM15) high.  This is not a dramatic height revision when compared to some revised heights, but it does come within the parameters of the Significant Height Revisions used within this page heading, these parameters are:

The term ‘significant height revision’ applies to any listed hill whose Ordnance Survey summit spot height has a 2m or more discrepancy when compared to the surveyed Trimble height, also included are hills whose summit map data is missing an uppermost ring contour when compared to the data produced by the Trimble.  As heights on different scaled Ordnance Survey maps are not consistent the height given on the 1:25,000 map is being prioritised for detailing these revisions.

The summit of the hill is easily distinguishable and consists of an ancient cairn that is now an oval mound, with the centre of the mound being slightly hollowed, and therefore this hill’s new summit height as surveyed by the Trimble GeoXH 6000 is 433.0m (converted to OSGM15) which is 2.0m lower than its previously listed height of 435m which appears on the Ordnance Survey enlarged mapping on the Geograph website and the 1:25,000 Explorer and 1:50,000 Landranger maps.

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 (bwlch LIDAR data)

The Trimble GeoXH 6000 gathering data at the summit of Moelfre (SH 717 744) which resulted in this hill's significant height revision

Myrddyn Phillips and Aled Williams (August 2016)

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