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

Castell (SN 949 961) – Sub-Trichant deletion

There has been confirmation of a deletion 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 Castell (SN 949 961)

The criteria for the list that this deletion 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 listed by is Castell and this was derived from the Ordnance Survey series of Six-Inch maps, and it is adjoined to the Pumlumon group of hills, which are situated in the northern part of South Wales (Region B, Sub-Region B1), and it is positioned with minor roads to its north and south, and the A470 road to its north-east, and has the village of Carno towards the east north-east.

When the original 300m 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 this sub 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 20m of drop, based on the 325m summit spot height that appears on the Ordnance Survey 1:50,000 Landranger and 1:25,000 Explorer map and the 305m bwlch spot height that appeared on the Ordnance Survey Vector Map Local hosted on the Geograph website and which was entitled the Interactive Coverage Map.  With the caveat that the bwlch spot height was noted to be too low. 

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

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 305m – 310m, with interpolation placing the height of the bwlch as an estimated c 307m, resulting in an estimated c 18m of drop.

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 deletion of this hill from Sub-Trichant status is due to LIDAR analysis, resulting in a 323.7m summit height and a 306.4m bwlch height, with these values giving this hill 17.3m of drop, which is insufficient for it to be classified as a Sub-Trichant. 

 

The full details for the hill are: 

Group:  Pumlumon 

Name:  Castell 

OS 1:50,000 map:  136

Summit Height:  323.7m (LIDAR)                                                           

Summit Grid Reference:  SN 94917 96119 (LIDAR) 

Bwlch Height:  306.4m (LIDAR) 

Bwlch Grid Reference:  SN 94423 95831 (LIDAR) 

Drop:  17.3m (LIDAR) 

 

Myrddyn Phillips (November 2022)