Mapping Mountains – Significant Height Revisions – Y Trichant

Mynydd y Grug (ST 177 906)

There has been a Significant Height Revision to a hill that is 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 Mynydd y Grug (ST 177 906)

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 Mynydd y Grug 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 it is positioned with the A469 road to its west, the B4251 and the Afon Sirhywi (Sirhowy River) to its north-east and the A468 road to its south, and has the town of Caerffili (Caerphilly) towards the south-west.

When the original Welsh 300m P30 list was published on Geoff Crowder’s v-g.me website this hill was listed with a 348m summit height, based on the spot height positioned at ST 17688 90958 that appears on the Ordnance Survey 1:25,000 Explorer map, with an accompanying note stating; Workings probably obliterated 350c top at ST176908 on 1986 1:50000 map.  As heights on 1:25,000 scale mapping were prioritised in this list over those on 1:50,000 scale mapping this hill’s summit was listed to where the 348m spot height appears on the 1:25,000 Explorer map.

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

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

The details for this hill were re-assessed when the Ordnance Survey Vector Map became available online and this shows a 375m summit spot height with contemporary Ordnance Survey 1:25,000 Explorer maps showing an uppermost 370m ring contour and a raised summit area which is the bi-product of mine waste spoil.

Extract from the contemporary 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 Mynydd y Grug

The summit height produced by LIDAR analysis and its position in relation to the 348m spot height 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.

The summit height produced by LIDAR analysis is 374.2m and is positioned at ST 17753 90669, this is a dramatic increase in height over the 348m spot height that appeared on contemporary Ordnance Survey 1:25,000 Explorer maps of the day when the Welsh 300m P30s were first listed, and is 24.2m higher than the uppermost 350m ring contour on contemporary Ordnance Survey 1:50,000 Landranger maps.  However, the 374.2m height produced by LIDAR is in accordance with the uppermost 370m ring contour on contemporary Ordnance Survey 1:25,000 Explorer maps and detail on the OS Maps website.

ills of Wales, and are reproduced below@

The full details for the hill are:

Group:  Cymoedd Gwent

Name:  Mynydd y Grug

OS 1:50,000 map:  171

Summit Height (New height):  374.2m (LIDAR)

Summit Grid Reference:  ST 17753 90669 (LIDAR)

Bwlch Height:  169.85m (LIDAR)

Bwlch Grid Reference:  ST 16783 95144 (LIDAR)

Drop:  204.3m (LIDAR)

Myrddyn Phillips (July 2019)