Wednesday, 2 November 2016

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

Mynydd yr Hewyrch (SH 816 150)

This is the thirty fifth post under the heading of Significant Height Revisions, and the Leica RX1250 and Trimble GeoXH 6000 survey that resulted in the height of this hill being revised was conducted on the 2oth October 2016 in good, clear conditions.

The thirty fifth major height revision initiated from a survey with the Trimble GeoXH 6000 with the result quoted being that from the Leica RX1250 was conducted in the hills of the Bryniau Dyfi in the south-western part of north Wales during the late afternoon in the company of Alan Dawson, who had suggested the walk and also surveyed the hill with his Leica RX1250.  The Bryniau Dyfi are referred to in English as the Dovey Hills and are positioned between the Aran to their north and Cadair Idris to their west.

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.

The name of the hill is Mynydd yr Hewyrch and 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 current Ordnance Survey 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 by 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 (converte 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 current Ordnance Survey 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 summit spot height has a 2m or more discrepancy when compared to the surveyed Trimble height, 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 is relatively easily distinguishable and 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 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)

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)

No comments: