Mapping Mountains – Hill Reclassifications – 500m Twmpau

 

Pt. 551.8m (SH 665 322) – 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. 551.8m (SH 665 322)

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. 551.8m) notation and it is adjoined to the Y Llethr 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 B4573 road to its west, the A470 road to its east, and has a minor road to its west south-west, and has the village of Trawsfynydd towards the north-east.

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 31^st August 2004 that resulted in 63ft (19.2m) of drop, and it was originally listed with an estimated c 552m summit height and an estimated c 533m bwlch height, with both heights based on interpolation of 10m contouring. 

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.

The details for this hill were re-assessed when the Ordnance Survey Vector Map Local hosted on the Geograph website and which was entitled the Interactive Coverage Map became available online.  This mapping had many spot heights not on other publicly available Ordnance Survey maps and for this hill it had a 551m summit spot height positioned at SH 66591 32219.

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 two 512m summit spot heights which enabled the numerical detail to be updated, but with 19.2m of listed 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 551.8m summit height and a 531.7m bwlch height, with these values giving this hill 20.1m of drop, which is sufficient for it to be classified as a 500m Sub-Twmpau.

 

The full details for the hill are:

Group:  Y Llethr

Name:  Pt. 551.8m

OS 1:50,000 map:  124

Summit Height:  551.8m (LIDAR) 

Summit Grid Reference:  SH 66588 32216 (LIDAR) 

Bwlch Height:  531.7m (LIDAR) 

Bwlch Grid Reference:  SH 66333 32260 (LIDAR) 

Drop:  20.1m (LIDAR)

 

Myrddyn Phillips (July 2023)