Mapping Mountains – Significant Height Revisions – Y Trichant

Pen Rhiw Warren (ST 214 904)

There has been a Significant Height Revision to a hill that is now listed in the Y Trichant, with the summit height, bwlch height and their locations, the drop and status of the hill confirmed by LIDAR analysis conducted by Myrddyn Phillips.

LIDAR image of Pen Rhiw Warren (ST 214 904)

The criteria for the listing that this significant height revision applies to are:

Y Trichant – Welsh hills at or above 300m and below 400m in height with 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.

The name of the hill is Pen Rhiw Warren and its present state is the result of mine spoil, and it is adjoined to the Cymoedd Gwent group of hills, which are situated in the eastern part of South Wales (Region C, Sub-Region C2), and is positioned with the A468 road and the Afon Rhymni (Rhymney River) to its south, the B4251 and A467 roads and the Afon Sirhywi (Sirhowy River) towards the north, and has the town of Caerffili (Caerphilly) towards the west south-west.

This hill was not included in the main P30 list or the accompanying Hills to be surveyed sub list when the original Welsh 300m P30 hills were published on Geoff Crowder’s v-g.me website, as contemporary Ordnance Survey 1:50,000 Landranger and 1:25,000 Explorer maps of the day showed a bwlch height between 310m – 320m and a broken uppermost contour ring of 320m, therefore it did not meet the criteria used within this list.

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

The details for this hill were re-examined when the OS Maps website became available online.  This is the replacement for OS Get-a-map and has contours at 5m intervals and for the majority of land comprising old mine and quarry workings, and rail and road cuttings it shows present day continuous contours, as opposed to the broken contours on contemporary Ordnance Survey 1:50,000 Landranger and 1:25,000 Explorer maps.  The mapping on OS Maps shows this hill as having eight continuous 5m contour rings, implying that the hill has at least 35m of drop.

Extract from 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.

The summit height produced by LIDAR analysis and its position in relation to the contours shown on the contemporary Ordnance Survey 1:25,000 Explorer map 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.  As heights on different scaled Ordnance Survey maps are not consistent the height given on the 1:25,000 Explorer map is being prioritised in favour of the 1:50,000 Landranger map for detailing these revisions.

Therefore this hill’s new summit height is 350.5m and this was produced by LIDAR analysis, this is a dramatic increase in height and is 30.5m higher compared to the uppermost 320m contour ring on the contemporary Ordnance Survey 1:50,000 Landranger and 1:25,000 Explorer map, but is in accordance with the 5m contouring on the OS Maps website.

The inclusion of this hill to Trichant status and its dramatic increase in height is due to terra-forming by man’s intrusion and is the result of raised mine spoil.  As the hill in its present form is considered solid and stable its inclusion as a Trichant and its increase in known height is accepted as a present-day representation of the land.

ills of Wales, and are reproduced below@

The full details for the hill are:

Group:  Cymoedd Gwent

Name:  Pen Rhiw Warren

OS 1:50,000 map:  171

Summit Height (New height):  350.5m (LIDAR)

Summit Grid Reference:  ST 21463 90441 (LIDAR)

Bwlch Height:  313.9m (LIDAR)

Bwlch Grid Reference:  ST 21869 90427 (LIDAR)

Drop:  36.6m (LIDAR)

Myrddyn Phillips (August 2019)