Mapping Mountains – Significant Height Revisions – Yr Uchafion and 600m Twmpau

Pen y Boncyn Trefeilw (SH 962 283)

There has been a Significant Height Revision to a hill that is listed in the Yr Uchafion and 600m Twmpau, with the summit height and drop of the hill being confirmed by a Trimble GeoXH 6000 survey conducted by Myrddyn Phillips on the 16^th August 2018.  The summit of this hill was subsequently surveyed by Alan Dawson using a Leica RX1250 on the 24th November 2018, and it is this result that is given in the details for the hill at the end of this article.

Pen y Boncyn Trefeilw (SH 962 283)

The criteria for the two listings that this name change 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 this list appearing on Mapping Mountains on the 4^th November 2015.

600m Twmpau – Welsh hills at or above 600m and below 700m in height that have 30m minimum drop, with an accompanying sub category entitled the 600m Sub-Twmpau consisting of all Welsh hills at or above 600m and below 700m in height that have 20m or more and below 30m of 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 Pen y Boncyn Trefeilw, and it is adjoined to the Y Berwyn range of hills, which are situated in the south-eastern part of North Wales (Region A, Sub-Region A4), and it is positioned to the north of Llyn Efyrnwy (Lake Vyrnwy) and has the town of Y Bala towards the north north-west.

As the summit of the hill is not a part of designated open access land permission to visit should be sought, however the upper part of this hill is encircled by open access land and the summit of the hill is only a few metres from the boundary fence between this land and the grazing field where it is situated.  Therefore common sense should prevail for those who want to visit.

Prior to the survey with the Trimble GeoXH 6000 this hill was listed with a 646m summit height, which was based on the spot height that appears on the 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 644.0m (643.953m to three decimal places) (converted to OSGM15), this is not a substantial 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 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 analysis of data produced via LIDAR, 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 LIDAR analysis.  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.

The Trimble GeoXH 6000 gathering data at the summit of Pen y Boncyn Trefeilw

Therefore, this hill’s new summit height is 644.0m (converted to OSGM15) and this was produced by surveying with the Trimble GeoXH 6000, this is 2.0m lower than its previously listed height of 646m which appears as a spot height on the Ordnance Survey 1:50,000 Landranger and 1:25,000 Explorer maps.  However, this new height is in accordance with the 5m contouring on OS Maps.

The above details have now been superseded by the Leica RX1250 survey conducted by Alan Dawson, and it is this summit result that is given below. 

ills of Wales, and are reproduced below@

The full details for the hill are:

Group:  Y Berwyn

Name:  Pen y Boncyn Trefeilw

OS 1:50,000 map:  125

Summit Height (New height):  644.2m (converted to OSGM15)

Summit Grid Reference:  SH 96250 28328

  

Bwlch Height:  588.4m (converted to OSGM15)

Bwlch Grid Reference:  SH 97406 27854

Drop:  55.8m (Leica RX1250 summit and Trimble GeoXH 6000 bwlch)

For details on the survey that significantly revised the summit height of this hill

Myrddyn Phillips and Aled Williams (January 2019)