Mapping Mountains – Significant Height Revisions – Y Pellennig - The Remotest Hills of Wales

Ynys Arw (SH 266 945) 

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

Ynys Arw (SH 266 945) 

The criteria for the list that this height revision applies to are: 

Y Pellennig –The Remotest Hills of Wales - Welsh hills whose summit is at least 2.5km from the nearest paved public road and the hill has a minimum 15m of drop.  The list is co-authored by Myrddyn Phillips and Aled Williams and is available as a downloadable e-booklet or print-booklet version on Mapping Mountains Publications with the up-to-date master list available on Mapping Mountains to download in Google Doc format.

Y Pellennig - The Remotest Hills of Wales by Myrddyn Phillips and Aled Williams

The name the hill is listed by is Ynys Arw, and it is adjoined to the Mynydd Twr group of hills, which are situated in the north-western part of North Wales (Region C1, Sub-Region C1A), and it is positioned to the north-west of Ynys Môn on an archipelago that is known as Ynysoedd y Moelrhoniaid and The Skerries.

When this list was originally compiled this hill was not included as detail on the Ordnance Survey 1:25,000 giving an uppermost 10m contour ring was deemed insufficient for it to be considered as a Pellennig, with subsequent interpolation giving it an estimated c 12m summit height.

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

However, it was not until the Trimble GeoXH 6000 survey that an accurtae height could be given for the summit and when coupled with subsequent LIDAR analysis supplied by © Crown:  CHERISH PROJECT 2019 and conducted by Aled Williams that an accurate drop value could be given to this hill.  The LIDAR (Light Detection & Ranging) technique produced highly accurate height data that is now freely available for much of England and Wales.

The Trimble GeoXH survey gives the summit height of this hill as 15.2m 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 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.

The Trimble GeoXH 6000 gathering data at the summit of Ynys Arw

Therefore, the new listed summit height of this hill is 15.2m and this was derived from a Trimble GeoXH 6000 survey, this is 3.2m higher than the interpolated height previously given this hill.

The full details for the hill are: 

Group:  Mynydd Twr

Name:  Ynys Arw

OS 1:50,000 map:  114

Summit Height (New Height):  15.2m (converted to OSGM15, Trimble GeoXH 6000)

Summit Grid Reference:  SH 26647 94598 (Trimble GeoXH 6000)

Bwlch Height:  N/A

Bwlch Grid Reference:  N/A   

Drop:  15.2m (Trimble GeoXH 6000) 

Remoteness:  4.850km

© Crown: CHERISH PROJECT 2019. Produced with EU funds through the Ireland Wales Co-operation Programme 2014-2020. All material made freely available through the Open Government Licence.

Myrddyn Phillips and Aled Williams (October 2015)