Mapping Mountains – Hill Reclassifications – 30-99m Twmpau

Geirth Mawr (SH 535 393) – 30-99m Twmpau reclassified to 30-99m Sub-Twmpau 

There has been confirmation of a reclassification 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 LIDAR analysis conducted by Myrddyn Phillips. 

LIDAR image of Geirth Mawr (SH 535 393)

The criteria for the list 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. 

30-99m Twmpau by Myrddyn Phillips

The name the hill is listed by is Geirth Mawr, and it is adjoined to the Moel Hebog 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 A497 road to its north and minor roads to its west and east, and has the town of Porthmadog towards the east south-east.

When the original 30-99m height band of Welsh P30 hills were published on Geoff Crowder’s v-g.me website, this hill was included in the Hills to be surveyed sub list, as it was considered not to meet the criteria then used for the main P30 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 30m of drop, with an estimated c 58m summit height, based on interpolation of its uppermost 50m ring contour, and a 28m bwlch spot height that appears on the Ordnance Survey 1:25000 Explorer map. 

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

Since the original publication of the Welsh P30 lists on Geoff Crowder’s v-g.me website 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 56m summit spot height and when coupled with the 28m bwlch spot height, these values gave this hill 28m of drop and therefore it was reclassified back to 30-99m Sub-Twmpau status.

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 57m summit spot height which when coupled with the 28m bwlch spot height would give this hill 29m of drop. 

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 confirmation of the reclassification of this hill to 30-99m Sub-Twmpau status is due to LIDAR analysis, resulting in a 57.4m summit height and a 27.9m bwlch height, with these values giving this hill 29.5m of drop, which is insufficient for it to be classified as a 30-99m Twmpau. 

 

The full details for the hill are: 

Group:  Moel Hebog 

Name:  Geirth Mawr 

OS 1:50,000 map:  124

Summit Height:  57.4m (LIDAR) 

Summit Grid Reference:  SH 53511 39363 (LIDAR)               

Bwlch Height:  27.9m (LIDAR) 

Bwlch Grid Reference:  SH 53302 39651 & SH 53311 39649 (LIDAR) 

Drop:  29.5m (LIDAR) 

 

Myrddyn Phillips (May 2024)