Mapping Mountains – Significant Height Revisions – Y Pellennig - The Remotest Hills of Wales, The Welsh Highlands - Uchafion Cymru 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 - The Remotest Hills of Wales, The Welsh Highlands - Uchafion Cymru 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 (for the summit survey) (for the bwlch survey) conducted by Myrddyn Phillips and Aled Williams respectively.

Crib y Rhiw (SH 663 248)

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

Y Pellennig –The Remotest Hills of Wales - Welsh hills whose summit is at least 2.5km from the nearest paved public road and the hill has a minimum 15m of drop.  The list is co-authored by Myrddyn Phillips and Aled Williams and is available as a downloadable e-booklet or print-booklet version on Mapping Mountains Publications with the up-to-date master list available on Mapping Mountains to download in Google Doc format. 

Y Pellennig - The Remotest Hills of Wales by Myrddyn Phillips and Aled Williams

The Welsh Highlands – Uchafion Cymru – Welsh hills at or above 500m in height with 15m minimum drop, with an accompanying sub list entitled the Welsh Highland Subs, the criteria for which is all Welsh hills at or above 500m in height with 10m or more and below 15m of drop.  This list is authored by Myrddyn Phillips and Aled Williams with the Introduction to the list published on Mapping Mountains in November 2015 and the latest update relating to the list published on Mapping Mountains in January 2023.

Welsh Highlands - Uchafion Cymru by Aled Williams and Myrddyn Phillips

600m Twmpau – Welsh hills at or above 600m and below 700m in height that have 30m minimum drop, with an accompanying sub category entitled the 600m Sub-Twmpau consisting of all Welsh hills at or above 600m and below 700m in height that have 20m or more and below 30m of drop.  With the word Twmpau being an acronym standing for thirty welsh metre prominences and upward.  The list is authored by Myrddyn Phillips and is published on Mapping Mountains in Google Doc format.

600m Twmpau by Myrddyn Phillips

The name the hill is listed by is Crib y Rhiw and it is adjoined to the Y Llethr group of hills, which are situated in the western part of North Wales (Region A, Sub-Region A2), and it is positioned with minor roads and the A496 road to its west, minor roads and the A496 road to its south and the A470 road to its east, and has the small community of Bontddu towards the south. 

When the original list that later became known as the The Welsh Highlands – Uchafion Cymru was first compiled, this hill was listed with an estimated c 27m of drop, based on an estimated c 680m summit height (with the uppermost contour on Ordnance Survey 1:25,000 Explorer maps being 670m) and an estimated c 653m bwlch height, with the former based on the estimated height difference between a northerly top that has a 2,228ft (679.1m) height given it on the Ordnance Survey Six-Inch Map and the high point of the hill which is positioned on the southerly top, with its bwlch height based on interpolation of 10m contouring between 650m - 660m, and these are the details given the hill when the 1st edition of the Y Pellennig - The Remotest Hills of Wales was published by Europeaklist in April 2015. 

Extract from the Ordnance Survey 1:25,000 Explorer map

However, it was not until a survey with GNSS equipment that an accurate summit height for this hill could be ascertained.  This was conducted by Mynddyn Phillips on the 04.06.15 using a Trimble GeoXH 6000, resulting in a 681.3m summit height positioned at SH 66368 24868, and this 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 interpolated height and Ordnance Survey, Harvey or other interactive map summit spot height has a 2m or more discrepancy when compared to the survey result produced by the Trimble GeoXH 6000 or analysis of data produced via LIDAR.  Also included are hills whose summit map data is missing an uppermost ring contour when compared with the data produced by the Trimble or by LIDAR analysis.

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

Therefore, the new listed summit height of this hill is 681.3m and this was derived from a Trimble GeoXH 6000 survey, this is 11.3m higher than the uppermost 670m ring contour that appears on the Ordnance Survey 1:25,000 Explorer map and 1.3m higher than the estimated summit height given the hill when listed in the Europeaklist booklet for the Y Pellennig - The Remotest Hills of Wales list.

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

Myrddyn Phillips and Aled Williams (September 2015)