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

 

Castell (SN 585 789) 

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 and status of the hill derived from LIDAR analysis and a subsequent Trimble GeoXH 6000 survey conducted by Myrddyn Phillips. 

The view from the summit of Castell (SN 585 789)

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 with 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, with the Introduction to 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 and it is adjoined to the Elenydd group of hills which are situated in the central part of the Mid and West Wales Region (Region B, Sub-Region B2), and it is positioned with the coast to its west, the Afon Ystwyth to its east and the A487 road farther to its east, and has the town of Aberystwyth towards the north. 

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 that accompanied the main P30 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 29m of drop, based on the 73m summit spot height that appears on the Ordnance Survey 1:25,000 Explorer map and an estimated c 44m bwlch height based on interpolation of 10m contouring between 40m – 50m. 

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

The details for this hill we 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 a 41m spot height appeared on the area of its bwlch and when coupled with the 73m summit spot height these values give this hill 32m 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. 

LIDAR image of Castell

The summit height produced by LIDAR analysis is 77.4m, and as the summit has now been surveyed with the Trimble GeoXH 6000 giving 77.2m it is this result that is being prioritised for listing purposes, 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, this hill’s new listed summit height is 77.2m and this was derived from a Trimble GeoXH 6000 survey, this is 4.2m higher than the previous listed summit height of 73m which was based on the spot height that appears on contemporary Ordnance Survey 1:25,000 Explorer maps.

 

ills of Wales, and are reproduced below@

The full details for the hill are:

Group:  Elenydd 

Name:  Castell 

OS 1:50,000 map:  135

Summit Height (New Height):  77.2m (converted to OSGM15, Trimble GeoXH 6000) 

Summit Grid Reference:  SN 58516 78997 (Trimble GeoXH 6000) 

Bwlch Height:  42.3m (LIDAR) 

Bwlch Grid Reference:  SN 58197 78954 (LIDAR) 

Drop:  34.9m (Trimble GeoXH 6000 summit and LIDAR bwlch) 

Dominance:  45.21% (Trimble GeoXH 6000 summit and LIDAR bwlch) 

 

Myrddyn Phillips (April 2021)