Tuesday, 13 February 2018

Mapping Mountains – Summit Relocations – 30-99m Twmpau

Pt. 47.8m (SH 604 438)

There has been a Summit Relocation to a hill that now appears in the list of 30-99m Twmpau, with the hill’s height and drop confirmed by analysis conducted by Aled Williams of data produced via LIDAR.  With the title of Twmpau being an acronym for thirty welsh metre prominences and upward.

Accompanying each 100m height band within the Twmpau list is a sub list entitled Sub-Twmpau, the criteria for the height band of 30-99m Twmapu are:

30-99m Twmpau – All Welsh hills at and above 30m and below 100m in height that have a minimum 30m of drop.

30-99m Sub-Twmpau – All Welsh hills at and above 30m and below 100m in height with 20m or more and below 30m of drop.

As I do not know a separate and independent name for this hill it is being listed by the point (Pt. 47.8m) notation, and it is situated in the Moelwynion range of hills and is placed in the Region of North Wales (Region A, Sub-Region A1).  The hill is positioned with the Welsh Highland Railway and the A 498 road to its west and the A 4085 road to its east, and has the village of Beddgelert to its north north-west and the town of Porthmadog towards its south-west.

As the hill is not a part of designated open access land permission to visit should be sought, however the hill has a well-used path leading through the deciduous wood that takes in its summit indicating that people have visited this area for many years.

Prior to analysis of data produced via LIDAR this hill was not listed in the Hills to be surveyed sub list that accompanied the Welsh P30 lists published on Geoff Crowder’s v-g.me website.  When these lists were re-evaluated after they were standardised and also drop values added, this hill was listed with c 20m of drop based on the 46m summit spot height positioned at SH 60384 43736 that appears on the Ordnance Survey 1:25,000 Explorer map and an estimated bwlch height of c 26m based on 10m contour intervals between 20m – 30m.  However, since these lists were re-evaluated OS Maps (the recent replacement for OS Get-a-map) are now available and show 5m contour intervals with the uppermost 45m contour ring matching the location of where LIDAR gives the 47.8m summit.  

Extract from the Ordnance Survey Interactive Coverage Map hosted on the Geograph website indicating the position of the LIDAR summit in relation to where the 46m spot height appears

The uppermost 45m ring contour shown on the OS Maps website

The summit height produced by data via LIDAR is 47.8m and is positioned at SH 60404 43809, this position is not given a spot height on Ordnance Survey maps and is approximately 80 metres northward from the previously listed summit position.

The full details for the hill are:

Group:  Moelwynion

Summit Height:  47.8m (data via LIDAR)

Name:  Pt. 47.8m

OS 1:50,000 map:  124

Summit Grid Reference (New Position):  SH 60404 43809 
Drop:  23.8m (data via LIDAR)

My thanks to Aled Williams for sending the details of this hill to me

Myrddyn Phillips (February 2018)

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