Mapping Mountains – Hill Reclassifications – 500m Twmpau

Pt. 511.7m (SH 693 485) – 500m Sub-Twmpau addition

There has been an addition to the listing of the 500m Twmpau, with the summit height, bwlch height and their locations, the drop and status of the hill derived from LIDAR analysis conducted by Aled Williams. 

LIDAR image of Pt. 511.7m (SH 693 485)

The criteria for the list that this addition 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 list is authored by Myrddyn Phillips and is available to download in Google Doc format from Mapping Mountains. 

The 500m Twmpau by Myrddyn Phillips

The hill is being listed by the point (Pt. 511.7m) notation and it is adjoined to the Moelwyn Mawr group of hills, which are situated in the north-western part of North Wales (Region A, Sub-Region A2), and it is positioned with the A470 road to its east, and has the town of Blaenau Ffestiniog towards the south.

When the list that later became known as the 500m Twmpau was first compiled, this hill was not included in the accompanying sub list due to a basic levelling survey conducted by Myrddyn Phillips on the 13^th July 2004 that resulted in 65ft (19.8m) of drop, and it was subsequently listed with an estimated c 512m summit height and an estimated c 492m bwlch height, with both heights based on interpolation of 10m contouring that appear on the Ordnance Survey 1:25,000 Explorer map. 

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

Since the original compilation of this list there have been a number of maps made available online.  Some of these are historic such as the series of Six-Inch maps on the National Library of Scotland website.  Whilst others were digitally updated such as the Ordnance Survey Vector Map Local that was hosted on the Geograph website and which was entitled the Interactive Coverage Map, whilst others are current and digitally updated such as the interactive mapping on the Magic Maps and WalkLakes websites.

One of the mapping resources now available online is the WalkLakes website which hosts an interactive map originated from the Ordnance Survey Open Data programme.  This map has many spot heights not on other publicly available maps and for this hill it has a 512m summit spot height which enabled the numerical detail to be updated, but with 19.8m of drop it was still not included as a 500m Sub-Twmpau. 

Extract from the interactive mapping hosted on the WalkLakes website

However, it was not until LIDAR became available that the details for this hill could be accurately re-assessed.  The LIDAR (Light Detection & Ranging) technique produced highly accurate height data that is now freely available for much of England and Wales.

Therefore, the addition of this hill to 500m Sub-Twmpau status is due to LIDAR analysis, resulting in a 511.7m summit height and a 491.7m bwlch height, with these values giving this hill 20.0m of drop, which is sufficient for it to be classified as a 500m Sub-Twmpau. 

 

The full details for the hill are:

Group:  Moelwyn Mawr

Name:  Pt. 511.7m

OS 1:50,000 map:  115

Summit Height:  511.7m (LIDAR) 

Summit Grid Reference:  SH 69325 48552 (LIDAR) 

Bwlch Height:  491.7m (LIDAR) 

Bwlch Grid Reference:  SH 69254 48542 (LIDAR) 

Drop:  20.0m (LIDAR)

 

Myrddyn Phillips (July 2023)