Mapping Mountains – Significant Height Revisions – Y Pedwarau - The 400m Hills of Wales

Mynydd Poeth (SH 953 513)

There has been a Significant Height Revision to a hill that is listed in the Y Pedwarau – The 400m Hills of Wales, with the summit height, bwlch height and their locations, the drop and status of the hill derived from a Trimble GeoXH 6000 survey and subsequent LIDAR analysis conducted by Myrddyn Phillips.

Mynydd Poeth (SH 953 513)

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

Y Pedwarau – The 400m Hills of Wales.  Welsh hills at or above 400m and below 500m in height that have 30m minimum drop, accompanying the main Y Pedwarau list are five categories of sub hills, with this hill being reclassified from the 400m Sub-Pedwar category.  The criteria for this sub category being all Welsh hills at or above 400m and below 500m in height that have 20m or more and below 30m of drop.  The list is co-authored by Myrddyn Phillips and Aled Williams and is published on Mapping Mountains in Google Doc format.

Y Pedwarau - The 400m Hills of Wales by Myrddyn Phillips and Aled Williams

The name the hill is listed by is Mynydd Poeth and it is adjoined to the Mynydd Hiraethog group of hills, which are situated in the northern part of North Wales (Region C1, Sub-Region C1A), and it is positioned with minor roads to its west and south, and the B4501 road to its east, and has the village of Cerrigydrudion towards the south.

When the original 400m height band of Welsh P30 hills was published on Geoff Crowder’s v-g.me website, this hill was included in the Hills to be surveyed sub list, as it was considered not to meet the criteria then used for the main P30 category.

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

After 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 417m summit spot height positioned at SH 95403 51069 that appears on the contemporary Ordnance Survey 1:50,000 Landranger and 1:25,000 Explorer map, and an estimated c 388m bwlch height, based on interpolation of 10m contouring between 390m – 400m, and these are the values given this hill when the 1^st edition of the Y Pedwarau was published by Europeaklist in May 2013.

The Trimble GeoXH 6000 gathering data at the summit of Mynydd Poeth

However, it was not until the Trimble GeoXH 6000 survey and when 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 Trimble GeoXH 6000 survey gives the highest ground on this hill as 419.3m positioned at SH 95375 51301, 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, Harvey or other interactive 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 with the data produced by the Trimble or by LIDAR analysis.

LIDAR image of Mynydd Poeth (SH 953 513)

Therefore, the new listed summit height of this hill is 419.3m and this was derived from a Trimble GeoXH 6000 survey, this is 2.3m higher than the previously listed summit height, which was based on the 417m spot height that appears on the contemporary Ordnance Survey 1:50,000 Landranger and 1:25,000 Explorer map.

The full details for the hill are:

Group:  Mynydd Hiraethog

Name:  Mynydd Poeth

OS 1:50,000 map:  116

Summit Height (New Height):  419.3m (converted to OSGM15, Trimble GeoXH 6000)

Summit Grid Reference:  SH 95375 51301 (Trimble GeoXH 6000)

Bwlch Height:  387.3m (LIDAR)

Bwlch Grid Reference:  SH 95072 51340 & SH 95070 51331 (LIDAR)

Drop:  33.0m (Trimble GeoXH 6000 summit and LIDAR bwlch)

Myrddyn Phillips and Aled Williams (August 2015)