Mapping Mountains – Hill Reclassifications – 30-99m Twmpau

 

Pt. 32m (SH 281 756) – 30-99m Sub-Twmpau addition

There has been an addition to the list of 30-99m 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 in combination with LIDAR analysis conducted by Myrddyn Phillips. 

The criteria for the list that this addition 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

The hill is being listed by the point (Pt. 32m) notation as an appropriate name for it either through local enquiry and / or historic research has not been found by the author, 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 a series of minor roads to its north-west, and has the village of Trearddur towards the north-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 either 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 an estimated c 19m of drop based on the 32m summit spot height that appeared on the Ordnance Survey Vector Map Local hosted on the Geograph website and which was entitled the Interactive Coverage Map, and an estimated c 13m bwlch height based on interpolation of 5m contouring between 10m – 15m that appears on the Ordnance Survey 1:25,000 Explorer map. 

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

Since the original publication of the P30 lists on Geoff Crowder’s v-g.me website and the re-assessment of this hill’s details 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 Magic Maps website. 

Extract from the Magic Maps website

One of the mapping resources now available online is the Magic Maps website which hosts an interactive map originated from Ordnance Survey data.  This mapping has many spot heights not on other publicly available Ordnance Survey maps and for this hill the 32m summit spot height is also given on the area of its summit, with the caveat that the Ordnance Survey 1:50,000 Landranger map gives a 33m summit spot height in approximately the same position. 

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. 

LIDAR bwlch image of Pt. 32m (SH 281 756)

Therefore, the addition of this hill to 30-99m Sub-Twmpau status is due to detail on contemporary maps produced from Ordnance Survey data in combination with LIDAR bwlch analysis, resulting in a 32m summit height and an 11.9m bwlch height, with these values giving this hill 20m of drop, which is sufficient for it to be classified as a 30-99m Sub-Twmpau. 

 

The full details for the hill are: 

Group:  Ynys Môn 

Name:  Pt. 32m 

OS 1:50,000 map:  114

Summit Height:  32m (spot height) 

Summit Grid Reference:  SH 28120 75648 (spot height) 

Bwlch Height:  11.9m (LIDAR) 

Bwlch Grid Reference:  SH 27589 76108 (LIDAR) 

Drop:  20m (spot height summit and LIDAR bwlch) 

 

Myrddyn Phillips (April 2021)