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

Mynydd Epynt (SN 961 464)

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

Mynydd Epynt (SN 961 464)

The criteria for the two listings 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 list are five categories of sub hills, with this hill being included in 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

Y Trechol – The Dominant Hills of Wales – Welsh P30 hills whose prominence equal or exceed half that of their absolute height.  With the criteria for Lesser Dominant status being those additional Welsh P30 hills whose prominence is between one third and half that of their absolute height.  The list is authored by Myrddyn Phillips with the Introduction to the start of the Mapping Mountains publication of this list appearing on the 3^rd December 2015, and the list is now available in its entirety on Mapping Mountains in Google Doc format. 

Y Trechol - The Dominant Hills of Wales by Myrddyn Phillips

The name the hill is listed by is Mynydd Epynt, and it is adjoined to its own group of hills; Mynydd Epynt, which are situated in the central part of South Wales (Region C2, Sub-Region C2A), and it is positioned with the B4519 road to its north-east, and has the town of Llanfair-ym-Muallt (Builth Wells) towards the north-east.

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 main P30 list with a summit height of 478m, based on the spot height that appears on the Ordnance Survey 1:50,000 Landranger and 1:25,000 Explorer map.

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

However, it was not until LIDAR became available and the subsequent Trimble GeoXH 6000 survey 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 Epynt (SN 961 464)

The summit height produced by the Trimble GeoXH 6000 is 475.7m positioned at SN 96124 46428, 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.

The Trimble GeoXH 6000 gathering data at the summit of Mynydd Epynt (SN 961 464)

Therefore, the new listed summit height of this hill is 475.7m and this was derived from a Trimble GeoXH 6000 survey, this is 2.3m lower than the previously listed summit height, which was based on the 478m spot height that appears on the Ordnance Survey 1:50,000 Landranger and 1:25,000 Explorer map.

The full details for the hill are:

Group:  Mynydd Epynt

Name:  Mynydd Epynt

OS 1:50,000 map:  147

Summit Height (New Height):  475.7m (converted to OSGM15, Trimble GeoXH 6000)

Summit Grid Reference:  SN 96124 46428 (Trimble GeoXH 6000)  

Bwlch Height:  277.9m (LIDAR)

Bwlch Grid Reference:  SN 84054 43300 (LIDAR) 

Drop:  197.8m (Trimble GeoXH 6000 summit and LIDAR bwlch)

Dominance:  41.58% (Trimble GeoXH 6000 summit and LIDAR bwlch)

Myrddyn Phillips and Aled Williams (January 2017)