Mapping Mountains – Significant Height Revisions – Y Pedwarau – The 400m Hills of Wales

Esgair Llwyn Gwyn (SN 885 790)

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

Approaching the summit of Esgair Llwyn Gwyn (SN 885 790)

The criteria for the list 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, the list is co-authored by Myrddyn Phillips and Aled Williams, with the Introduction to the Mapping Mountains publication of this list appearing on the 30th January 2017, and the list is now available in its entirety on Mapping Mountains in Google Doc format.

The name the hill is listed by is Esgair Llwyn Gwyn and it is adjoined to the Y Garn group of hills, which are situated in the north-western part of South Wales (Region B, Sub-Region B1), and it is positioned with the Afon Gwy (River Wye) and the A44 road to its north and the Afon Gwy (River Wye) and the A470 road to its south-east, and has the village of Llangurig towards the east north-east.

When the original Welsh 400m P30 list was published on Geoff Crowder’s v-g.me website, this hill was listed with a 438m summit height, based on the spot height positioned at SN 88588 79070 that appears on the Ordnance Survey 1:50,000 Landranger and 1:25,000 Explorer map.

Extract from the Ordnance Survey 1:50,000 Landranger map

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 image of Esgair Llwyn Gwyn (SN 885 790)

As this hill had now been surveyed with the Trimble GeoXH 6000, it is this result that is being prioritised for this hill and this summit height 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.

Gathering data at the summit of Esgair Llwyn Gwyn

Gathering data at the bwlch of Esgair Llwyn Gwyn

Therefore, the new listed summit height of this hill is 435.9m and this was derived from a Trimble GeoXH 6000 survey.  This is not a dramatic revision in height compared to some revised heights and is 2.1m lower than the previous listed height of 438m which was derived from the summit spot height on the Ordnance Survey 1:50,000 Landranger and 1:25,000 Explorer maps.

The full details for the hill are:

Group:  Y Garn

Name:  Esgair Llwyn Gwyn

OS 1:50,000 map:  135, 136, 147

Summit Height (New Height):  435.9m (converted to OSGM15)

Summit Grid Reference:  SN 88577 79070 

  

Bwlch Height:  402.3m (converted to OSGM15)

Bwlch Grid Reference:  SN 88120 78849

Drop:  33.6m

Myrddyn Phillips and Aled Williams (November 2019)