Wednesday, 2 November 2016

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

Mynydd yr Hewyrch (SH 816 150)

There has been a Significant Height Revision to a hill that is listed in the Yr Uchafion600m Twmpau and Y Trechol - The Dominant Hills of Wales, and which was initiated by a survey with the Trimble GeoXH 6000 and a survey with the Leica RX1250, with the survey that resulted in this height revision being conducted on the 2oth October 2016 in good, clear and calm conditions in the company of Alan Dawson. 

The criteria for the three listings that this height revision affects are:

Yr Uchafion - All Welsh hills at or above 500m in height that have 15m minimum drop.  The list is co-authored by Myrddyn Phillips and Aled Williams.

600 Twmpau - These are the Welsh hills at or above 600m and below 700m in height that have a minimum drop of 30m.  The word Twmpau is an acronym for thirty welsh metre prominences and upward, and the list is authored by Myrddyn Phillips. 

Y Trechol - The Dominant Hills of Wales - These are the Welsh P30 hills whose prominence  equal or exceed half that of their absolute height.  The list is authored by Myrddyn Phillips.

The name that the hill is currently listed by is Mynydd yr Hewyrch and it is situated in the hills of the Rhwng Dyfi a Dulas range which have been referred to as the Bryniau Dyfi and which are situated in the south-western part of north Wales, with the survey of this hill conducted during the late afternoon in the company of Alan Dawson, who had suggested the walk and also surveyed the hill with his Leica RX1250, and it is the result from the Leica equipment that is being used for this hill's data.

The hill is situated between the Afon Cerist and the A 470 road to its north and the conifer plantation of the Dyfi Fforest (Dovey Forest) to its south and is positioned above the small community of Dinas Mawddwy to its east, and the hill can be easily accessed from a lane leading to a path that heads for the summit via the escarpment edge above Craig Maesglase, or via the north-eastern ridge that heads straight for the summit of its adjacent top; Maen Du.

Prior to the survey with the Leica RX1250 and the Trimble GeoXH 6000 the height of the hill was listed as 676m based on the summit spot height on the Harvey 1:40,000 Britain Mountain Map.  However, the summit of Mynydd yr Hewyrch only has a 670m uppermost contour on Ordnance Survey 1:50,000 Landranger and 1:25,000 Explorer maps with the adjacent summit of Maen Du at SH 822 151 being given a 674m spot height, because of this the summit of Maen Du was previously listed as that for the hill, and the higher summit of Mynydd yr Hewyrch is therefore a recent addition and first noted by Tim Jones who reported that he believed this summit to be higher to John and Anne Nuttall in 1996.

Three points were surveyed for summit position using the Trimble GeoXH 6000 and the Leica RX1250, each position being close in height and taken to ground consisting of moorland peat hags rising above a shapely large pool which is positioned on the summit area.

The 678.5m (converted to OSGM15) height produced by the Leica RX1250 is 2.5m higher than the previously listed height of 676m, which was based on the Harvey British Mountain Map spot height, and 8.5m higher than the uppermost contour on Ordnance Survey 1:50,000 Landranger and 1:25,000 Explorer maps and therefore comes 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  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 map is being prioritised in favour of the 1:50,000 for detailing these revisions.

The summit of this hill consists of ground at the top of a moorland peat hag, and therefore this hill’s new summit height as surveyed by Alan Dawson with the Leica RX1250 is 678.5m (converted to OSGM15) which is 2.5m higher than its previously listed height of 676m which appears on the Harvey 1:40,000 British Mountain Map and 8.5m higher than the uppermost 670m contour ring on Ordnance Survey 1:50,000 Landranger and 1:25,000 Explorer maps.

The full details for the hill are:

Cardinal Hill:  Mynydd yr Hewyrch

Summit Height (New Height):  678.5m (converted to OSGM15, Leica RX1250)

Name:  Mynydd yr Hewyrch

OS 1:50,000 map:  124, 125

Summit Grid Reference:  SH 81697 15003 
Drop:  318.0m (converted to OSGM15, Trimble GeoXH 6000 bwlch)

Dominance:  46.87%

The Trimble GeoXH 6000 gathering data at one of the three positions surveyed atop Mynydd yr Hewyrch which when coupled with the Leica RX1250 survey resulted in this hill's significant height revision

Myrddyn Phillips and Aled Williams (November 2016)

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