Mapping Mountains – Significant Height Revisions – Y Pellennig, Welsh Highlands and 600m Twmpau

Crib y Rhiw (SH 663 248)

There has been a Significant Height Revision to a hill that is listed in the Y Pellennig, Welsh Highlands and the 600m Twmpau, with the summit height, bwlch height and their locations, the drop and status of the hill derived from separate Trimble GeoXH 6000 surveys conducted by Myrddyn Phillips and Aled Williams respectively.

The criteria for the listings this height revision affects appear below:

Y Pellennig –The Remotest Hills of Wales comprise all Welsh hills whose summit is 2.5km or more from the nearest paved public road and which have a minimum 15m of drop.  The list is co-authored by Myrddyn Phillips and Aled Williams.

Welsh Highlands - 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.

600m Twmpau - All Welsh hills at or above 600m and below 700m in height with 30m minimum drop.  With the 600m Sub-Twmpau being all Welsh hills at or above 600m and below 700m in height with 20m or more and below 30m of drop.  The word Twmpau is an acronym for thirty welsh metre prominences and upward and the list is authored by Myrddyn Phillips.   

The name the hill is listed by is Crib y Rhiw and it is situated in the hills of the Rhinogydd which are positioned in the western part of North Wales.  The summit survey was conducted on a warm and sunny day in the company of John Barnard and Graham Jackson, although they were not present during the 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).  Whilst the bwlch was surveyed in January 2023 by Aled Williams with Rebecca Williams in attendance. 

Crib y Rhiw is a relatively remote hill for Wales and it can be accessed on a traverse of the southern ridge of the Rhinogydd, either from Cwm Nantcol to the west or from the end of the paved road to the south which is situated above the small community of Bontddu.

The hill does not possess a summit spot height on the Ordnance Survey 1:50,000 Landranger and 1:25,000 Explorer maps, and is given an uppermost contour of 670m on both of these maps, with the height resulting from the survey with the Trimble GeoXH 6000 being 681.3m (converted to OSGM15).  Crib y Rhiw 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 Y Pellennig, Yr Uchafion and 600m Twmpau was c 680m, which was based on the estimated height difference between the position of the Six-Inch Map height 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 GeoXH 6000 is not a dramatic height revision when compared to the previously listed height of this hill 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 height 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 for detailing these revisions.

Therefore, the new listed summit height of this hill is 681.3m (converted to OSGM15), and this is 1.3m higher than its previous c 680m estimated height and 10.3m higher than its uppermost ring contour on the contemporary Ordnance Survey 1:50,000 Landranger and 1:25,000 Explorer map.

The full details for the hill are:

Cardinal Hill:  Y Llethr

Name:  Crib y Rhiw

OS 1:50,000 map:  124

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

Summit Grid Reference:  SH 66368 24868

  

Bwlch Height:  652.1m (converted to OSGM15)

Bwlch Grid Reference:  SH 66080 25302

Drop:  29.2m

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

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

Myrddyn Phillips and Aled Williams (September 2015)