Mapping Mountains – Hill Reclassifications – 30-99m Twmpau and 100m Twmpau

Plas Newydd (SH 614 807) – 30-99m Sub-Twmpau reclassified to 100m Sub-Twmpau

There has been a reclassification to the listings of 30-99m Twmpau and 100m Twmpau, with the summit height, bwlch height and their locations, the drop and status of the hill derived from detail on contemporary maps produced from Ordnance Survey data and LIDAR analysis conducted by Myrddyn Phillips. 

LIDAR bwlch image of Plas Newydd (SH 614 807)

The criteria for the two listings that this reclassification applies to are: 

30-99m Twmpau – Welsh hills at or above 30m and below 100m in height that have 30m minimum drop, with an accompanying sub list entitled the 30-99m Sub-Twmpau with the criteria for this sub category being all Welsh hills at or above 30m and below 100m 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 30-99m Twmpau by Myrddyn Phillips

100m Twmpau – Welsh hills at or above 100m and below 200m in height that have 30m minimum drop, with an accompanying sub list entitled the 100m Sub-Twmpau with the criteria for this sub category being all Welsh hills at or above 100m and below 200m 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 100m Twmpau by Myrddyn Phillips

The name the hill is listed by is Plas Newydd and this was derived from the Tithe map, and it is adjoined to the Ynys Môn 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 coast to its north and a minor road to its south, and has the small community of Glan-yr-afon towards the west. 

When the original 30-99m height band of Welsh P30 hills were published on Geoff Crowder’s v-g.me website this hill was not included in the Hills to be surveyed sub list, as it was considered not to meet the criteria then used for this sub category. 

After the sub list was standardised, and interpolated heights and drop values also included the details for this hill were re-evaluated and it was listed with 27m of drop based on the 99m summit spot height and the 72m bwlch spot height that appear on the Ordnance Survey 1:25,000 Explorer map.  Therefore, the hill was an addition to the 30-99m Sub-Twmpau list. 

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

Since the original publication of the P30 lists of Geoff Crowder’s v-g.me website there have been a number of Ordnance Survey 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 are current and digitally updated such as the mapping on the WalkLakes website and the Magic Maps website. 

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 Ordnance Survey maps and for this hill a 100m spot height is given on the area of its summit and it is this height that is being favoured in preference to the 99m spot height that appears on the Ordnance Survey 1:25,000 Explorer map. 

Extract from the WalkLakes website

However, it was not until LIDAR became available that the bwlch 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 reclassification of this hill from 30-99m Sub-Twmpau status is due to detail on contemporary maps produced from Ordnance Survey data and LIDAR bwlch analysis, resulting in a 100m summit height and a 71.7m bwlch height, with these values giving this hill 28m of drop, which is sufficient for it to be classified as a 100m Sub-Twmpau. 

 

The full details for the hill are: 

Group:  Ynys Môn 

Name:  Plas Newydd 

OS 1:50,000 map:  114, 115

Summit Height:  100m (spot height) 

Summit Grid Reference:  SH 61430 80710 (spot height) 

Bwlch Height:  71.7m (LIDAR) 

Bwlch Grid Reference:  SH 60600 81404 (LIDAR) 

Drop:  28m (spot height summit and LIDAR bwlch) 

 

Myrddyn Phillips (May 2021)