Saturday, 6 August 2016

Mapping Mountains – Significant Height Revisions – Y Pedwarau


Moelfre (SH 717 744)

There has been a Significant Height Revision initiated by a survey with the Trimble GeoXH 6000 to a hill listed in the Y Pedwarau, and which was subsequently confirmed by LIDAR data analysed by Aled Williams, with the survey that resulted in this height revision being conducted on the 4th July 2016.

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

Y Pedwarau - Welsh hills at and above 400m and below 500m in height that have 30m minimum drop.

400m Sub- Pedwarau - Welsh hills at and above 400m and below 500m in height that have 20m or more and below 30m of drop.

The name of the hill is Moelfre and it is situated in the Carneddau range of hills which are in north-west Wales.  This hill range comprises some of the highest mountains in the country but the evening walk when the survey was conducted concentrated on some of its 300m and 400m lower hills.

Moelfre 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.

Prior to the survey with the Trimble GeoXH 6000 the height of the hill was listed as 435m, with this height taken from the Ordnance Survey 1:50,000 Landranger map, Ordnance Survey 1:25,000 Explorer map and the Ordnance Survey enlarged mapping hosted on the Geograph website.  The 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 survey result produced by the Trimble GeoXH 6000, 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 identified 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 hosted 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)








No comments: