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

Mynydd Fynnon Wen (SN 724 797)

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 derived from LIDAR analysis conducted by Myrddyn Phillips.

LIDAR image of Mynydd Fynnon Wen (SN 724 797)

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 Fynnon Wen 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 the A44 road to its north and the A4120 road to its south and east, and has the village of Ponterwyd towards the east north-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 main P30 list, and listed with a 378m summit height, based on the spot height that appears on the Ordnance Survey 1:25,000 Explorer map.

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 Mynydd Fynnon Wen

The summit height produced by LIDAR analysis is 375.8m and this is positioned at SN 72420 79716, this is not a dramatic decrease in height compared to some revised heights, 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 375.8m and this was derived from LIDAR analysis, this is positioned at SN 72420 79716 and is 2.2m lower than its originally listed height of 378m 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 Fynnon Wen

OS 1:50,000 map:  135, 147

Summit Height (New Height):  375.8m (LIDAR)

Summit Grid Reference:  SN 72420 79716 (LIDAR)

Bwlch Height: 344.9m (LIDAR)

Bwlch Grid Reference:  SN 72479 79425 (LIDAR)

Drop:  30.9m (LIDAR)

Myrddyn Phillips (November 2020)