Mapping Mountains – Summit Relocations – 30-99m Twmpau

Allt Fawr (SH 383 356)

There has been a Summit Relocation to a hill that is now listed in the 30-99m Twmpau, with the summit height and its position, drop and status of the hill confirmed by LIDAR analysis conducted by Myrddyn Phillips.

The criteria for this list are:

30-99m Twmpau – Welsh hills at and above 30m and below 100m in height that have a minimum 30m of drop, with an accompanying sub list entitled the 30-99m Sub-Twmpau with the criteria for this sub category being all Welsh hills at and above 30m and below 100m in height with 20m and 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 Allt Fawr and it is adjoined to the Pen Llŷn range of hills, which is situated in the north-western part of North Wales (Region A, Sub-Region A1), and it is positioned with the A 499 road to its west and the A 497 road to its east, and has the town of Pwllheli towards its south-west.

As the hill is not a part of designated open access land permission to visit should be sought, for those wishing to do so the proximity of the roads will give relatively easy access.

Prior to LIDAR analysis this hill was listed with c 18m of drop based on an estimated c 46m summit height with the hill given three uppermost 45m ring contours on the Ordnance Survey Interactive Coverage Map hosted on the Geograph website, and an estimated bwlch height of c 28m based on bwlch contouring at 5m intervals and between 25m – 30m on this same map.

Extract from the Ordnance Survey Interactive Coverage Map hosted on the Geograph website

Extract from OS Maps

The position of the three uppermost 45m ring contours were analysed via LIDAR with a fourth point also examined resulting in the below:

48.889m at SH 38347 35695 (new summit position)

46.569m at SH 38145 35521 (c 46m old listed summit)

44.962m at SH 38406 35898

44.944m at SH 38383 35833

The summit height produced by LIDAR analysis is 48.9m and is positioned at SH 38347 35695, this position is not given a spot height or an uppermost 45m ring contour on the Ordnance Survey Interactive Coverage Map hosted on the Geograph website, but is given a 45m ring contour on OS Maps (the recent replacement for OS Get-a-map).  The new summit position for this hill is approximately 250 metres north-eastward from the previously listed summit position.

 

LIDAR image of Allt Fawr

The full details for the hill are:

Group:  Pen Llŷn

Summit Height:  48.9m (LIDAR)

Name:  Allt Fawr

OS 1:50,000 map:  123

Summit Grid Reference (New Position):  SH 38347 35695 (LIDAR)  

Drop:  23.1m (LIDAR)

Myrddyn Phillips (April 2018)