Friday, 2 June 2017

Mapping Mountains – Significant Height Revisions – Yr Uchafion

Bwlan (SH 946 255)

This is the fortieth post under the heading of Significant Height Revisions, with the Trimble survey that resulted in this height revision being conducted on the 4th May 2017.

The fortieth major height revision initiated from a survey with the Trimble GeoXH 6000 was conducted on a hill in the Y Berwyn group in the south-eastern Sub-Region of North Wales (Region A, Sub-Region A4) between the stream valleys of the Afon Nadroedd and the Afon Eiddew, with both tumbling down in to Llyn Efyrnwy (Lake Vyrnwy) which is situated to the hill’s south-east.

The walk that took in the survey of this hill’s summit was conducted on a beautiful sunny day with a brisk easterly breeze, and was done in the company of Aled Williams who had analysed this hill’s previous status as an Uchaf via LIDAR data.

The hill can be accessed from its north via a track that emerges from a conifer plantation and makes its way up to the bwlch, which is just north-west of the summit, however by far the best approach is from the south-east following a path either beside or above Pistyll Rhyd-y-meinciau, this is a series of waterfalls which are a part of the Afon Eiddew.

The name of the hill is Bwlan and its addition to the list of Yr Uchafion was due to a basic levelling survey (BLS) conducted by Myrddyn Phillips on the 10.05.05, resulting in a 51ft 6˝ (15.7m) drop value, subsequently the hill has been deleted from the ranks of Yr Uchafion 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:


Summit Height:  526.1m

Summit Grid Reference:  SH 94690 25541

Bwlch Height:  512.6m

Bwlch Grid Reference:  SH 94180 25715

Drop:  13.55m

Current Ordnance Survey 1:50,000 Landranger and 1:25,000 Explorer maps give this hill a 528m summit spot height with the Ordnance Survey enlarged mapping hosted on the Geograph website giving a 514m spot height on the area of this hill’s bwlch.

The survey with the Trimble GeoXH 6000 produced a summit height of 525.9m (converted to OSGM15) and a bwlch height of 512.1m (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 ground comprising the summit of this hill is relatively easily distinguishable and comprises a mixture of grass and reed grass amongst a good sprinkling of moorland heather.

Therefore this hill’s new summit height is 525.9m (converted to OSGM15) which is 0.2m lower than the result produced by LIDAR data and 2.1m lower than the 528m spot height on the Ordnance Survey 1:50,000 Landranger and 1:25,000 Explorer maps.

The full details for the hill are:

Cardinal Hill:  Foel y Geifr

Summit Height (New Height):  525.9m (converted to OSGM15)

Name:  Bwlan

OS 1:50,000 map:  125

Summit Grid Reference:  SH 94692 25540 
Drop:  13.8m (converted to OSGM15)

The Trimble GeoXH 6000 gathering data at the summit of Bwlan (SH 946 255) which resulted in this hill's significant height revision

Myrddyn Phillips and Aled Williams (June 2017)

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