Mapping Mountains – Significant Height Revisions – The Welsh P15s

 

Porfa Bwlch (SN 987 491) 

There has been a Significant Height Revision to a hill that is 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 Porfa Bwlch (SN 987 491)

The criteria for the list that this height revision applies to 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 Porfa Bwlch 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 with a minor road to its north and east, and the A483 road farther to its north, and has the town of Llanfair-ym-Muallt (Builth Wells) towards the east north-east.

When the original 200m height band of Welsh P30 hills were published on Geoff Crowder’s v-g.me website, this hill was not included in the accompanying Hills to be surveyed sub list, as it was considered not to meet the criteria then used for this sub category.

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 2om of drop, based on the 276m 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. 

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 Porfa Bwlch (SN 987 491)

The summit height produced by LIDAR analysis is 273.8m and when compared to the originally listed summit height 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.

Therefore, the new listed summit height of this hill is 273.8m and this was derived from LIDAR analysis.  This is 2.2m lower than the originally listed summit height of 276m that appeared as a spot height on the Ordnance Survey Vector Map Local hosted on the Geograph website and which was entitled the Interactive Coverage Map.

 

ills of Wales, and are reproduced below@

The full details for the hill are: 

Group:  Mynydd Epynt 

Name:  Porfa Bwlch 

OS 1:50,000 map:  147

Summit Height (New Height):  273.8m (LIDAR) 

Summit Grid Reference:  SN 98775 49136 & SN 98777 49139 (LIDAR) 

Bwlch Height:  253.8m (LIDAR) 

Bwlch Grid Reference:  SN 98928 49112 (LIDAR) 

Drop:  19.95m (LIDAR) 

 

Myrddyn Phillips (February 2025)