Mapping Mountains – Significant Height Revisions – Y Trichant – The 300m Hills of Wales

Mynydd Derw Llwydion (SN 904 900)

There has been a Significant Height Revision to a hill that is listed in the Y Trichant – The 300m Hills of Wales, with the summit height, bwlch height and their locations, the drop and status of the hill confirmed by a Trimble GeoXH 6000 survey conducted by Myrddyn Phillips.

Mynydd Derw Llwydion (SN 904 900)

The criteria for the list this height revision affects 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 13th May 2017.

Y Trichant - The 300m Hills of Wales by Myrddyn Phillips

The name the hill is listed by is Mynydd Derw Llwydion and it is adjoined to the Pumlumon group of hills, which are situated in the north-western part of Mid and West Wales (Region B, Sub-Region B2), and it is positioned with minor roads to its north and west, and the B4518 road to its south, and has the town of Llanidloes towards the south-east.

When the original 300m height band of Welsh P30 hills were published on Geoff Crowder’s v-g.me website this hill was included in the accompanying Hills to be surveyed sub list, and listed with a 387m summit height, based on the spot height that appears on the Ordnance Survey 1:25,000 Explorer map and which is positioned at SN 90722 90066.

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

The summit and bwlch of this hill were surveyed using the Trimble GeoXH 6000 on the 18^th April 2014, resulting in a 388.1m summit height taken to the position of the 387m spot height and a 358.1m bwlch height, with the hill being reclassified to Trichant status due to this survey.

Gathering data during the first Trimble GeoXH 6000 summit survey

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 Derw Llwydion

LIDAR analysis gave the position of the summit to the west of where the 387m spot height appears on the map, which is also where the first Trimble GeoXH 6000 survey was conducted, therefore the summit position given by LIDAR was visited on the 11^th July 2020 and a data set taken from its high point.

Gathering data during the second Trimble GeoXH 6000 summit survey

The summit height produced by the second Trimble GeoXH 6000 survey is 389.3m and this is positioned at SN 90460 90041, this is not a dramatic increase in height compared to some height revisions, but it does come 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 summit 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 389.3m and this was derived from a Trimble GeoXH 6000 survey, this is positioned at SN 90460 90041 and is 2.3m higher than its originally listed height of 387m which appears as a spot height on the Ordnance Survey 1:25,000 Explorer map.

The full details for the hill are:

Group:  Pumlumon

Name:  Mynydd Derw Llwydion

OS 1:50,000 map:  136

Summit Height (New Height):  389.3m (converted to OSGM15, Trimble GeoXH 6000)

Summit Grid Reference:  SN 90460 90041 (Trimble GeoXH 6000)

Bwlch Height: 358.1m (converted to OSGM15, Trimble GeoXH 6000)

Bwlch Grid Reference:  SN 90391 90264 (Trimble GeoXH 6000)

Drop:  31.3m (Trimble GeoXH 6000)

For details on the first Trimble GeoXH 6000 survey of this hill

For details on the second Trimble GeoXH 6000 survey of this hill

Myrddyn Phillips (October 2020)