Saturday, 19 September 2015

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

Crib y Rhiw (SH 663 248)

This is the fifteenth post under the heading of Significant Height Revisions, and the following details are retrospective as the Trimble survey that resulted in this height revision was conducted on 4th June 2015.

The fifteenth major height revision initiated from a survey with the Trimble GeoXH 6000 was conducted in the hills of the Rhinogydd in the west of North Wales on a warm and sunny day in the company of John Barnard and Graham Jackson, although they were not present during the accrual survey as John was inspecting the northerly top of this hill and Graham remained at the southerly connecting bwlch with our Leica GS15 that was gathering data to determine the drop value of Diffwys (SH 661 234).

The hill is relatively remote for Wales but it can be accessed during a traverse of the southern Rhinogydd ridge, either from Cwm Nantcol to the west or from the end of the paved road to the south which is above the small community of Bontddu.

The name of the hill is Crib y Rhiw and the height given the hill on current Ordnance Survey maps is c 670m, with the height resulting from the survey with the Trimble being 681.3m (converted to OSGM15).  This hill has a northerly top that has a 2,228ft (679.1m) height given it on the Ordnance Survey Six Inch Map, and its previously listed height in the 600m Twmpau list was c 680m based on the estimated height difference between this Six Inch Map height and its position on the northerly top and the high point of the hill which is positioned on the southerly top.

The 681.3m (converted to OSGM15) height produced by the Trimble is not a dramatic height revision when compared to this hill’s previously listed height of c 680m, but it does come 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 for detailing these revisions.

Therefore this hill’s new summit height is 681.3m (converted to OSGM15) which is 1.3m higher than its previously estimated height and 10.3m higher than its uppermost ring contour on current Ordnance Survey maps.

The full details for the hill are:

Cardinal Hill:  Y Llethr

Summit Height (New Height):  681.3m (converted to OSGM15)

Name:  Crib y Rhiw

OS 1:50,000 map:  124

Summit Grid Reference:  SH 66368 24868
Drop:  c 28m

Gathering data from the summit of Crib y Rhiw which resulted in this hill's significant height revision

For details on the survey that significantly revised the summit height of this hill please click {here}

Myrddyn Phillips and Aled Williams (June 2014)

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