Mapping Mountains – Significant Height Revisions – Y Trichant – The 300m Hills of Wales

 

Blaen Pathiog (SN 927 820) 

There has been a Significant Height Revision to a hill that is listed in the Y Trichant – The 300m Hills of Wales, with the summit height, bwlch height and their locations, the drop and status of the hill derived from LIDAR analysis and a subsequent Trimble GeoXH 6000 survey conducted by Myrddyn Phillips. 

LIDAR image of Blaen Pathiog (SN 927 820)

The criteria for the list this height revision affects are:

Y Trichant – The 300m Hills of Wales – Welsh hills at or above 300m and below 400m in height that have 30m minimum drop, with an accompanying sub list entitled the Sub-Trichant with the criteria for this sub category being all Welsh hills at or above 300m and below 400m in height with 20m or more and below 30m of drop.  The list is authored by Myrddyn Phillips, with the Introduction to the list and the renaming of it appearing on Mapping Mountains on the 13^th May 2017. 

Y Trichant - The 300m Hills of Wales by Myrddyn Phillips

The name the hill is listed by is Blaen Pathiog and it is adjoined to the Pumlumon group of hills, which are situated in the north-western part of Mid and West Wales (Region B, Sub-Region B2), and it is positioned with the A470 road to its south-east, and has the town of Llanidloes towards the north-east. 

When the original 300m 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 that accompanied the main P30 list, as it was considered not to meet the criteria then used for the main P30 category, and listed with a 377m summit height, based on the spot height that appears on the Ordnance Survey 1:25,000 Explorer map. 

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

After the P30 lists were standardised, and interpolated heights and drop values also included, this hill was listed with an estimated c 33m of drop based on the 379m summit spot height that appeared on the Ordnance Survey Vector Map Local that was hosted on the Geograph website and an estimated c 346m bwlch height, based on interpolation of 10m contouring between 340m – 350m.  The 379m summit spot height has latterly also appeared on the online mapping available on the Magic Maps website. 

Extract from the Magic Maps website

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 Blaen Pathiog

LIDAR analysis gives the summit height of this hill as 376.1m, and as the summit has now been surveyed with the Trimble GeoXH 6000 it is this result that is being prioritised for listing purposes 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 summit 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. 

The Trimble GeoXH 6000 gathering data at the summit of Blaen Pathiog

Therefore, this hill’s new listed summit height is 376.3m and this was derived from a Trimble GeoXH 6000 survey, this is positioned at SN 92775 82001 and is 2.7m lower than its previously listed height of 379m which 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 and which now appears on the online mapping available on the Magic Maps website. 

 

The full details for the hill are:

Group:  Pumlumon

Name:  Blaen Pathiog

OS 1:50,000 map:  136

Summit Height (New Height):  376.3m (converted to OSGM15, Trimble GeoXH 6000)

Summit Grid Reference:  SN 92775 82001 (Trimble GeoXH 6000)

Bwlch Height: 343.3m (LIDAR)

Bwlch Grid Reference:  SN 92760 81765 (LIDAR)

Drop:  33.1m (Trimble GeoXH 6000 summit and LIDAR bwlch)

 

Myrddyn Phillips (December 2020)