Tuesday, 9 May 2017

Mapping Mountains – Significant Height Revisions – Yr Uchafion


Moel Bowydd (SH 712 470)

This is the thirty ninth post under the heading of Significant Height Revisions, with the Trimble survey that resulted in this height revision being conducted on the 8th April 2017.

The thirty ninth major height revision initiated from a survey with the Trimble GeoXH 6000 was conducted on hills adjoined to the Arenig group in the northern part of the south-west Sub-Region of North Wales (Region A, Sub-Region A3), with the town of Blaenau Ffestiniog to the south-west of the hill.

The walk that took in this hill’s summit was suggested by Aled Williams who had analysed this hill’s Uchaf qualification via LIDAR data, and the walk and survey were conducted on a beautiful sunny day with hardly a breath of breeze.

The hill can be accessed via public footpaths from its west, east and south, with Blaenau Ffestiniog being a good starting point for an ascent; an alternate and longer ascent can start from above Llan Ffestiniog using a track beside the course of the Afon Teigl.  However, the summit of this hill is not on designated open access land, therefore permission to visit should be sought, but as the summit is relatively close to a public footpath and with open access land to its immediate north, common sense should prevail and if wanting to visit the summit there is likely to be no objection to doing so.

The name of the hill is Moel Bowydd and its addition to the list of Yr Uchafion was announced on Mapping Mountains on the 03.04.17, with its addition due to LIDAR data analysed by Aled Williams.  LIDAR (Light Detection & Ranging) is highly accurate height data that is now freely available for much of England and Wales.

Aled’s analysis of LIDAR data gives the hill the following details:


Moel Bowydd

Summit Height:  500.6m

Summit Grid Reference:  SH 71276 47053

Bwlch Height:  c 485m

Bwlch Grid Reference:  SH 71586 47206

Drop:  c 16m


Prior to analysis of LIDAR data the hill was not classified as contouring on current Ordnance Survey maps implied it did not have the required height to qualify as an Uchaf.  Ordnance Survey maps give this hill an uppermost 490m ring contour and bwlch contouring between 480m – 490m.

The survey with the Trimble GeoXH 6000 produced a summit height of 500.5m (converted to OSGM15) and a bwlch height of 482.4m (converted to OSGM15) with the summit height coming 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, or a previous estimated summit height based on interpolation of contours.  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 in favour of the 1:50,000 for detailing these revisions.

The summit of this hill is easily distinguishable and is positioned at the south-western end of a grassy ridge overlooking the Maen-offeren and Llechwedd quarries.

Therefore this hill’s new summit height is 500.5m (converted to OSGM15) which is 0.1m lower than the result produced by LIDAR data and 10.5m higher than the 490m uppermost ring contour on current Ordnance Survey maps.


The full details for the hill are:

Cardinal Hill:  Manod Mawr

Summit Height (New Height):  500.5m (converted to OSGM15)

Name:  Moel Bowydd

OS 1:50,000 map:  115

Summit Grid Reference:  SH 71272 47051
         
Drop:  18.1m (converted to OSGM15)


The Trimble GeoXH 6000 gathering data at the summit of Moel Bowydd (SH 712 470) which when coupled with the analysis of LIDAR data resulted in this hill's significant height revision




Myrddyn Phillips and Aled Williams (May 2017)



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