Mapping Mountains – Significant Height Revisions – The Welsh P15s

March Mynydd Ucha (SO 097 282) 

There has been a Significant Height Revision to a hill that was listed in The Welsh P15s, 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 March Mynydd Ucha (SO 097 282)

The criteria for the list that this hill used to be included in are:

The Welsh P15s – Welsh hills with 15m minimum drop, irrespective of their height, with an accompanying sub list entitled the Welsh Sub-P15s, with the criteria for this sub category being all Welsh hills with 14m or more and below 15m of drop.  The list is authored by Myrddyn Phillips, with the Introduction to the list appearing on Mapping Mountains on the 10^th May 2019. 

The Welsh P15s by Myrddyn Phillips

The name the hill is now listed by is March Mynydd Ucha and this was derived from the Tithe map, and it is adjoined to the Mynydd Epynt group of hills, which are situated in the central part of South Wales (Region B, Sub-Region B1), and it is positioned enclosed by minor roads, with the A470 road farther to its west and the A40 road farther to its south-west, and has the town of Aberhonddu (Brecon) towards the west.

When the listing that became known as The Welsh P15s was being compiled, this hill was included in the P14 sub list with an estimated c 14m of drop, based on the 270m summit spot height that appeared on the Ordnance Survey Vector Map Local hosted on the Geograph website and which was entitled the Interactive Coverage Map and an estimated c 256m bwlch height, based on interpolation of 10m contouring between 250m – 260m.  With the contemporary Ordnance Survey 1:50,000 Landranger and 1:25,000 Explorer map only having an uppermost 260m ring contour. 

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 March Mynydd Ucha (SO 097 282)

The summit height produced by LIDAR analysis is 271.1m and when compared to detail on the Ordnance Survey 1:50,000 Landranger and 1:25,000 Explorer map, 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 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, the new listed summit height of this hill is 271.1m and this was derived from LIDAR analysis.  This is 11.1m higher than the 260m uppermost ring contour that appears on the contemporary Ordnance Survey 1:50,000 Landranger and 1:25,000 Explorer map. 

 ills of Wales, and are reproduced below@

The full details for the hill are: 

Group:  Mynydd Epynt 

Name:  March Mynydd Ucha 

OS 1:50,000 map:  161

Summit Height (New Height):  271.1m (LIDAR)                                                           

Summit Grid Reference:  SO 09744 28233 (LIDAR) 

Bwlch Height:  257.4m (LIDAR) 

Bwlch Grid Reference:  SO 09741 28417 (LIDAR) 

Drop:  13.7m (LIDAR) 

 

Myrddyn Phillips (July 2024)