Sunday, 26 November 2017

Mapping Mountains – Significant Height Revisions – Yr Uchafion and 700m Twmpau

Twyn Llech (SO 255 353)

There has been a Significant Height Revision to a hill that appears in the Yr Uchafion and 700m Twmpau lists, and which was initiated by a survey with the Trimble GeoXH 6000, with the survey that resulted in this height revision being conducted on the 28th September 2017 in good, clear conditions with just a breath of breeze.

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

Yr Uchafion – All Welsh hills at and 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.

700m Twmpau – All Welsh hills at and above 700m and below 800m 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 Twyn Llech and it is a part of the Mynyddoedd Duon (Black Mountains) range, which is an extensive group of hills in the south-eastern part of South Wales, and it is situated above and between the upper part of the Olchon Valley to its south-east and the Dyffryn Ewias (Vale of Ewyas) to its south.  The hill is also situated between the small communities of Craswall towards its east and Capel-y-ffin towards its south, and the hill can be accessed from a car park just below the high point of Bwlch yr Efengyl (Gospel Pass) via the summit of Hay Bluff, or it can be approached from the south-east via paths leading to its southerly ridge from either the Olchon Valley and the Dyffryn Ewias (Vale of Ewyas).

Prior to the survey with the Trimble GeoXH 6000 the height of the hill was listed as 706m based on the summit spot height which is positioned at approximately SO 25545 35372 (taken by using a ruler) on the Harvey 1:40,000 Offa’s Dyke South map.  Whilst Ordnance Survey 1:50,000 Landranger and 1:25,000 Explorer maps give a summit spot height of 703m positioned at SO 25586 35010.

The Harvey maps 706m summit spot height

The Ordnance Survey 703m summit spot height

Two points were surveyed for summit position using the Trimble GeoXH 6000, one on the southerly point where the Ordnance Survey give their 703m spot height and one on the northerly point where Harvey maps give their 706m spot height.  The summit area of this hill has two rises which complement these spot height positions and which have a slight dip in between.

The results produced by the Trimble GeoXH 6000 appear below:

Northerly high point:  703.639m (converted to OSGM15) at SO 25520 35383

Southerly high point:  702.721m (converted to OSGM15) at SO 25572 35032

The above result 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 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, 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 Explorer map is being prioritised in favour of the 1:50,000 Landranger map for detailing these revisions.

As the Harvey summit position was the one used for listing, it is the 706m spot height that this Significant Height Revisions post applies to.

The summit of this hill consists of tussock grass close to the main and now well maintained paved path that crosses this hill on a south – north direction, and therefore this hill’s new summit height as surveyed with the Trimble GeoXH 6000 is 703. 6m (converted to OSGM15) which is 2.4m lower than its previously listed height of 706m which appears on the Harvey 1:40,000 Offa’s Dyke South map.

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

Cardinal Hill:  Twyn Llech

Summit Height (New height):  703.6m (converted to OSGM15)

Name:  Twyn Llech

OS 1:50,000 map:  161

Summit Grid Reference:  SO 25520 35383 
Drop:  154.5m (converted to OSGM15)

The Trimble GeoXH 6000 gathering data at the summit of Twyn Llech (SO 255 353) which resulted in this hill's significant height revision

Myrddyn Phillips and Aled Williams (November 2017)

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