Mapping Mountains – Hill Reclassifications – 500m Twmpau

Esgair Saeson (SN 795 603) – 500m Sub-Twmpau deletion

There has been a deletion 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 imge of Esgair Saeson (SN 795 603)

The criteria for the list that this deletion 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 name the hill is listed by is Esgair Saeson and it is adjoined to the Esgair Wen group of hills, which are situated in the central part of South Wales (Region B, Sub-Region B1), and it is positioned with a minor road to its north-west and south, and has the small community of Pontrhydfendigaid towards the north-west and the town of Tregaron towards the west.

After the list that later became known as the 500m Twmpau was first compiled, this hill was included in the accompanying sub list with an estimated c 24m of drop, based on the 500m summit spot height that appears on the Ordnance Survey 1:50,000 Landranger and 1:25,000 Explorer map and an estimated c 476m bwlch height, based on interpolation of 10m contouring between 470m – 480m. 

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 475m bwlch spot height positioned at SN 79101 60658, and when coupled with its 500m summit spot height these values gave this hill 25m of drop.

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 470m – 475m, with interpolation placing the height of the bwlch as an estimated c 473m, resulting in its drop value being amended to an estimated c 27m.

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 deletion of this hill from 500m Sub-Twmpau status is due to LIDAR analysis, resulting in a 498.4m summit height and a 473.8m bwlch height, with these values giving this hill 24.6m of drop, with its height insufficient for it to be classified as a 500m Sub-Twmpau. 

 

The full details for the hill are:

Group:  Esgair Wen

Name:  Esgair Saeson

OS 1:50,000 map:  146, 147

Summit Height:  498.4m (LIDAR)

Summit Grid Reference:  SN 79504 60391 (LIDAR)

Bwlch Height:  473.8m (LIDAR)

Bwlch Grid Reference:  SN 79174 60567 (LIDAR)   

Drop:  24.6m (LIDAR) 

 

Myrddyn Phillips (November 2023)