Mapping Mountains – Hill Reclassifications – 500m Twmpau

Cefn Cyfarwydd (SH 752 630) – 500m Sub-Twmpau reclassified to 500m Twmpau

There has been a reclassification of a hill that is listed in the 500m Twmpau, with the summit height, bwlch height and their locations, the drop and status of the hill confirmed by a survey with the Trimble GeoXH 6000 conducted by Myrddyn Phillips and Aled Williams which took place on the 17^th February 2019.

Cefn Cyfarwydd (SH 752 630)

The criteria for the list that this reclassification applies to are:

500m Twmpau - Welsh hills at or above 500m and below 600m in height with 30m minimum drop, with an accompanying sub list entitled the 500m Sub-Twmpau with the criteria for this sub category being all Welsh hills at or above 500m and below 600m in height with 20m or more and below 30m of drop, with the word Twmpau being an acronym standing for thirty welsh metre prominences and upward.

The name of the hill is Cefn Cyfarwydd, and it is adjoined to the Carneddau group of hills, which are situated in the north-western part of North Wales (Region A, Sub-Region A1), and it is positioned with the A5 road to its south and the B5106 and the A470 roads and the Afon Conwy to its east, and has the villages of Trefriw and Llanrwst towards the east.  

Prior to the survey with the Trimble GeoXH 6000 this hill was listed with 26m of drop based on the 503m summit and 477m bwlch spot heights that appear on the Ordnance Survey 1:25,000 Explorer map.

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

The details for this hill were re-assessed when the Ordnance Survey Vector Map Local hosted on the Geograph website became available online; this mapping is entitled the Interactive Coverage Map.  As this mapping is larger in format it enables a better judgement of interpolated heights and it occasionally has contours at 5m intervals compared to the 10m contour intervals normally shown on the 1:25,000 Explorer map.  However, this mapping has the same spot heights and 10m contouring as the 1:25,000 Explorer map and therefore the drop value for this hill remained listed as 26m.

The area taking in this hill was re-examined when the OS Maps website became available online.  This is the replacement for OS Get-a-map and has contours at 5m intervals.  This mapping shows the 477m bwlch spot height to be positioned on a separate 475m contour ring implying that the height of this hill’s bwlch is substantially lower than 477m, with interpolation based on this contouring giving the hill c 30m of drop.  But it was not until the survey with the Trimble GeoXH 6000 that the increased drop value was confirmed.

Extract from the OS Maps website

The reclassification of Cefn Cyfarwydd to 500m Twmpau status is due to a survey with the Trimble GeoXH 6000, resulting in a 501.7m summit height and a 471.0m bwlch height, with these values giving this hill 30.7m of drop which is sufficient for it to be classified as a 500m Twmpau.

The Trimble GeoXH 6000 gathering data at the summit of Cefn Cyfarwydd

The full details for the hill are:

Group:  Carneddau

Name:  Cefn Cyfarwydd

OS 1:50,000 map:  115

Summit Height:  501.7m (converted to OSGM15)

Summit Grid Reference:  SH 75201 63067

Bwlch Height:  471.0m (converted to OSGM15)

Bwlch Grid Reference:  SH 74708 62751

Drop:  30.7m

For details on the survey of Cefn Cyfarwydd

Myrddyn Phillips (July 2019)