Tuesday, 10 July 2018

Mapping Mountains – Significant Height Revisions – Yr Uchafion and 500m Twmpau

Banc y Foel (SN 725 186)

There has been a Significant Height Revision to a hill that appears in the Yr Uchafion and 500m Twmpau lists, and which was initiated by a survey with the Trimble GeoXH 6000 which was conducted on the 6th May 2018 in good, clear conditions with hardly a breath of breeze.

The criteria for the two listings that this height revision applies to are:

Yr Uchafion – Welsh hills at or above 500m in height that have 15m minimum drop.  The list is co-authored by Myrddyn Phillips and Aled Williams, with the Introduction to the list appearing on Mapping Mountains on the 4th November 2015.

500m Twmpau – Welsh hills at or above 500m and below 600m in height that have 30m minimum drop, with the word Twmpau being an acronym standing for thirty welsh metre prominences and upward.  The list is authored by Myrddyn Phillips.

The name of the hill is Banc y Foel and it is adjoined to the Mynydd Du range of hills, which are positioned in the western part of South Wales (Region C, Sub-Region C1).  The hill is positioned with the A 4069 road to its east, and has the village of Llangadog towards the north and Brynaman towards the south.

Banc y Foel (SN 725 186)

Prior to the survey with the Trimble GeoXH 6000 the height of the hill was estimated as c 536m based on a relatively large 530m uppermost contour ring on Ordnance Survey 1:50,000 Landranger and 1:25,000 Explorer maps.

Extract from the Ordnance Survey 1:25,000 Explorer map

The summit height produced by The Trimble GeoXH 6000 survey is 531.6m (converted to OSGM15), this is a substantial height revision when compared to some revised heights, and comes within the parameters of the Significant Height Revisions used within this page heading, these parameters are:

The term Significant Height Revisions applies to any listed hill whose interpolated height and Ordnance Survey or Harvey map summit spot height has a 2m or more discrepancy when compared to the survey result produced by the Trimble GeoXH 6000 or by LIDAR analysis, also included are hills whose summit map data is missing an uppermost ring contour when compared to the data produced by the Trimble or by analysis of LIDAR.  As heights on different scaled Ordnance Survey maps are not consistent the height given on the 1:25,000 Explorer map is being prioritised in favour of the 1:50,000 Landranger map for detailing these revisions.

Therefore, this hill’s new summit height is 531.6m (converted to OSGM15) and this was produced by surveying with the Trimble GeoXH 6000, this is 4.4m lower than its previously listed height of c 536m which was based on interpolation of the hill’s uppermost 530m ring contour on the Ordnance Survey 1:50,000 Landranger and 1:25,000 Explorer map.

ills of Wales, and are reproduced below@
The full details for the hill are:

Group:  Mynydd Du

Name:  Banc y Foel

OS 1:50,000 map:  160

Summit Grid Reference:  SN 72536 18670 
Summit Height (New height):  531.6m (converted to OSGM15)

Drop:  38.9m (converted to OSGM15)

The Trimble GeoXH 6000 gathering data at the summit of Banc y Foel (SN 725 186) which resulted in this hill's significant height revision

Myrddyn Phillips and Aled Williams (July 2018)

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