Mapping Mountains – Significant Height Revisions – 30-99m Twmpau and Y Trechol – The Dominant Hills of Wales

 

Castell y Dryslwyn (SN 554 203) 

There has been a Significant Height Revision to a hill that is listed in the 30-99m Twmpau and Y Trechol – The Dominant Hills of Wales, with the summit height, bwlch height and their locations, the drop, dominance and status of the hill confirmed by LIDAR analysis conducted by Myrddyn Phillips. 

LIDAR image of Castell y Dryslwyn (SN 554 203)

The criteria for the two listings that this height revision 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

Y Trechol – The Dominant Hills of Wales – Welsh P30 hills whose prominence equal or exceed half that of their absolute height.  With the criteria for Lesser Dominant status being those additional Welsh P30 hills whose prominence is between one third and half that of their absolute height.  The list is authored by Myrddyn Phillips with the Introduction to the start of the Mapping Mountains publication of this list appearing on the 3^rd December 2015, and the list is now available in its entirety on Mapping Mountains in Google Doc format. 

Y Trechol - The Dominant Hills of Wales by Myrddyn Phillips

The name the hill is listed by is Castell y Dryslwyn and it is adjoined to the Mynydd Mallaen 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, the B4297 road to its west and the Afon Tywi to its south, and has the town of Llandeilo towards the east north-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 main P30 list with a summit height of 71m based on the spot height that appears on the Ordnance Survey 1:50,000 Landranger map. 

Extract from the Ordnance Survey 1:50,000 Landranger map

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 50m of drop, based on an estimated c 76m summit height based on interpolation of the uppermost 75m ring contour that appears on the Ordnance Survey 1:25,000 Explorer map, and an estimated c 26m bwlch height based on interpolation of 5m contouring between 25m – 30m. 

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

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. 

LIDAR summit image of Castell y Dryslwyn (SN 554 203)

The summit height produced by LIDAR analysis is 72.4m and is positioned at SN 55420 20301, and this comes within the parameters of the Significant Height Revisions used within this page heading, these parameters are:

The term Significant Height Revisions applies to any listed hill whose interpolated height and Ordnance Survey or Harvey map summit spot height has a 2m or more discrepancy when compared to the survey result produced by the Trimble GeoXH 6000 or analysis of data produced via LIDAR, also included are hills whose summit map data is missing an uppermost ring contour when compared to the data produced by the Trimble or by LIDAR analysis.

Therefore, the new listed summit height of this hill is 72.4m and this was derived from LIDAR analysis, this is 3.6m lower than the previously listed summit height of c 76m, which was based on interpolation of 5m contouring that appear on the Ordnance Survey 1:25,000 Explorer map, and 1.4m higher than its originally listed summit height which was based on the 71m spot height that appears on the Ordnance Survey 1:50,000 Landranger map.

 

ills of Wales, and are reproduced below@

The full details for the hill are:

Group:  Mynydd Mallaen 

Name:  Castell y Dryslwyn 

OS 1:50,000 map:  159

Summit Height (New Height):  72.4m (LIDAR) 

Summit Grid Reference:  SN 55420 20301 (LIDAR)               

Bwlch Height:  25.3m (LIDAR) 

Bwlch Grid Reference:  SN 55696 20577 (LIDAR) 

Drop:  47.1m (LIDAR) 

Dominance:  65.02% (LIDAR) 

 

Myrddyn Phillips (May 2022)