Mapping Mountains – Hill Reclassifications – Y Trichant – The 300m Hills of Wales

 

Dorth Siwgr (SN 834 427) – Sub-Trichant reclassified to Trichant

There has been a reclassification to the list of Y Trichant – The 300m Hills of Wales, 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 Dorth Siwgr (SN 834 427)

The criteria for the list that this reclassification applies to are:

Y Trichant – The 300m Hills of Wales – Welsh hills at or above 300m and below 400m in height that have 30m minimum drop, with an accompanying sub list entitled the Sub-Trichant, with the criteria for this sub category being all Welsh hills at or above 300m and below 400m in height with 20m or more and below 30m of drop.  The list is authored by Myrddyn Phillips with the Introduction to the list and the renaming of it appearing on Mapping Mountains on the 13^th May 2017, and the Introduction to the Mapping Mountains publication of the list appearing on the 1^st January 2022. 

Y Trichant - The 300m Hills of Wales by Myrddyn Phillips

The name the hill is now listed by is Dorth Siwgr and this was derived from online sources, and it is adjoined to the Mynydd Epynt group of hills, which are situated in the central part of South Wales (Region B, Sub-Region B1), and it is positioned with the A483 road to its north-west and a minor road to its south, and has the town of Llanymddyfri (Llandovery) towards the south-west.

When the original 300m 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 27m of drop, based on an estimated c 323m summit height and an estimated c 296m bwlch height, with both heights based on interpolation of 10m contouring that appear 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 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 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 326m summit spot height and when coupled with the c 296m estimated bwlch height; these values gave this hill an estimated c 30m of drop.  The 326m summit spot height also appears on the interactive mapping available on the Magic Maps website.

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 295m – 300m, with interpolation placing the height of the bwlch as an estimated c 297m, resulting in the drop value of this hill being amended to an estimated c 29m and its status changing from Trichant to Sub-Trichant. 

Extract from the interactive mapping that was hosted on the OS Maps 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 reclassification of this hill from Sub-Trichant status is due to LIDAR analysis, resulting in a 325.6m summit height and a 295.2m bwlch height, with these values giving this hill 30.4m of drop, which is sufficient for it to be classified as a Trichant. 

 

The full details for the hill are: 

Group:  Mynydd Epynt 

Name:  Dorth Siwgr 

OS 1:50,000 map:  147, 160

Summit Height:  325.6m (LIDAR)                                                           

Summit Grid Reference:  SN 83481 42793 (LIDAR) 

Bwlch Height:  295.2m (LIDAR) 

Bwlch Grid Reference:  SN 83703 42851 (LIDAR) 

Drop:  30.4m (LIDAR) 

 

Myrddyn Phillips (February 2024)