Mapping Mountains – Hill Reclassifications – 30-99m Twmpau

Moel y Gadair (SH 521 391) – 30-99m Sub-Twmpau reclassified to 30-99m Twmpau 

There has been 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 a Trimble GeoXH 6000 survey conducted by Myrddyn Phillips and LIDAR analysis initially conducted by Aled Williams and subsequently by Myrddyn Phillips. 

Moel y Gadair (SH 521 391)

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. 

The 30-99m Twmpau by Myrddyn Phillips

The name the hill is listed by is Moel y Gadair 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 coast to its south and the A497 road to its north-west, and has the town of Cricieth towards the west south-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 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. 

When 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 31m of drop, based on the 67m summit spot height that appears on the Ordnance Survey 1:50,000 Landranger and 1:25,000 Explorer map and an estimated c 36m bwlch height based on interpolation of 10m contouring between 30m – 40m.  The details for this hill were subsequently reassessed and its drop value amended to an estimated c 30m based on an estimated bwlch height of c 37m. 

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

This hill was then surveyed on the 15^th June 2015 using the Trimble GeoXH 6000, resulting in a 66.7m summit height and a 35.9m bwlch height, with these values seemingly confirming its P30 status. 

The Trimble GeoXH 6000 gathering data at the summit of Moel y Gadair

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. 

One of a series of surveys conducted at the bwlch on the 15th June 2015 

One of the resources recently available online is the mapping on the OS Maps website and the details for this hill were subsequently re-assessed against this mapping.  This is the replacement for OS Get-a-map and until recent times had contours at 5m intervals which were proving consistently more accurate compared to the 5m contours that sometimes appear on the Ordnance Survey 1:25,000 Explorer map and used to appear on the online Vector Map Local.  This mapping had bwlch contouring between 35m – 40m, with interpolation placing the height of the bwlch as an estimated c 37m, with the 5m contouring helping to indicate the position of the bwlch. 

With the aid of a ten figure grid reference for the bwlch position taken from the 5m contouring on the OS Maps website the bwlch of this hill was re-surveyed using the Trimble GeoXH 6000 on the 23^rd February 2016, resulting in the hill being reclassified to 30-99m Sub-Twmpau status. 

One of two surveys conducted at the bwlch on the 23rd February 2016

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. 

Initial LIDAR analysis indicated that the position of the bwlch was at SH 52007 39290, which is relatively close to where the Trimble GeoXH 6000 was placed on the survey conducted on the 23^rd February 2016.  LIDAR gives this position as 36.8m in height and when coupled with the 66.7m summit height, these values gave this hill 29.9m of drop. 

LIDAR bwlch image with the joining of the red contour positioned on the road at SH 52007 39290 

However, the 36.8m bwlch height ascertained from LIDAR is placed on an artificially raised road which is considered a relatively recent man-made construct and therefore not taken as a part of the height of the bwlch.  Further LIDAR analysis was required. 

The initial re-assessment of LIDAR was undertaken by Aled Williams and he concluded that this hill is a natural P30, with only the height of the artificially raised road excluding it from being listed as a 30-99m Twmpau.  His conclusion was that the height of the bwlch could be listed as an estimated and conservative c 36m or if using remaining natural ground; 35.8m positioned at SH 52020 39324.  Either value classifies this hill as a P30. 

LIDAR image of Moel y Gadair with the white contour placed 30m below the summit indicating this hill is a natural P30 with only the artificially raised road stopping the two respective white contours from being continuous

Having studied LIDAR, I concur with Aled’s conclusion that this hill is a natural P30, the bwlch height quoted is 35.8m positioned at SH 52013 39319.  This position is indicated as a joining on the hill to hill traverse excluding the artificially raised road and is approximately four metres from remaining natural ground. 

LIDAR image of Moel y Gadair (SH 521 391) showing the various height contours used to conclude this hill is a natural P30 and its bwlch height and position

Therefore, the reclassification of this hill from 30-99m Sub-Twmpau status and its re-instatement as a 30-99m Twmpau is due to a Trimble GeoXH 6000 summit survey and LIDAR bwlch analysis, resulting in a 66.7m summit height and a 35.8m bwlch height, with these values giving this hill 30.9m of drop, which is sufficient for it to be classified as a 30-99m Twmpau. 

 

The full details for the hill are: 

Group:  Moel Hebog 

Name:  Moel y Gadair 

OS 1:50,000 map:  124

Summit Height:  66.7m (converted to OSGM15, Trimble GeoXH 6000) 

Summit Grid Reference:  SH 52165 39135 (Trimble GeoXH 6000)               

Bwlch Height:  35.8m (LIDAR) 

Bwlch Grid Reference:  SH 52013 39319 (LIDAR) 

Drop:  30.9m (Trimble GeoXH 6000 summit and LIDAR bwlch) 

 

Myrddyn Phillips (March 2022)