Mapping Mountains – Significant Height Revisions – The Welsh P15s

Allt y Geifr (SN 762 284) 

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 Allt y Geifr (SN 762 284)

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 listed by is Allt y Geifr, and it is adjoined to the Mynydd Du group of hills, which are situated in the southern part of South Wales (Region B, Sub-Region B5), and it is positioned encircled by minor roads with the A4069 road farther to its north-west and south-west, and has the village of Llangadog towards the west.

When the listing that became known as The Welsh P15s was being compiled, this hill was included in the main P15 list with an estimated c 16m of drop, based on the 174m 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 158m bwlch height, based on interpolation of 5m contouring between 155m – 160m that appeared on the OS Maps website. 

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 Allt y Geifr (SN 762 284)

The summit height produced by LIDAR analysis is 171.1m and when compared to detail on the Ordnance Survey Vector Map Local hosted on the Geograph website and which was entitled the Interactive Coverage 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 171.1m and this was derived from LIDAR analysis.  This is 2.9m lower than the 174m spot height that appeared on the Ordnance Survey Vector Map Local hosted on the Geograph website and which was entitled the Interactive Coverage. 

 ills of Wales, and are reproduced below@

The full details for the hill are: 

Group:  Mynydd Du 

Name:  Allt y Geifr 

OS 1:50,000 map:  146, 160

Summit Height (New Height):  171.1m (LIDAR)                                                           

Summit Grid Reference:  SN 76221 28491 (LIDAR) 

Bwlch Height:  156.9m (LIDAR) 

Bwlch Grid Reference:  SN 76558 28873 (LIDAR) 

Drop:  14.2m (LIDAR) 

 

Myrddyn Phillips (June 2024)