Mapping Mountains – Significant Height Revisions – Y Pellennig

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 which was initiated by a survey with the Trimble GeoXH 6000, with these details being retrospective as the survey that resulted in this height revision was conducted on the 13^th September 2015.

The criteria for this list appear below:

Y Pellennig –The Remotest Hills of Wales comprise all Welsh hills whose summit is 2.5km or more from the nearest paved public road and which have a minimum 15m of drop.  The list is co-authored by Myrddyn Phillips and Aled Williams.

The name of the hill is Ynys Arw and it forms one of the islands that make up the archipelago of Ynysoedd y Moelrhoniaid (The Skerries), these islands are positioned to the north-west of Ynys Môn (Anglesey). 

Ynys Arw is the most south-westerly island of this archipelago and has two high points, with the more southerly of these being the highest.  As Ynys Arw is a tidal island it is connected to its higher neighbour at high tide, the higher island is where The Skerries Lighthouse is situated and is the highest point of the archipelago and is also listed as a Pellennig hill.

The survey of this hill was suggested by Rob Woodall and it was conducted late in the afternoon.  Twelve of us visited Ynysoedd y Moelrhoniaid in a Rib using a company named RibRide who are based at the Holyhead Marina during the summer months, and the trip cost £35.00 per person.

The summit of Ynys Arw is easily accessed at low tide from the Lighthouse, although hands on rock are required to get down to the wet land between it and the higher island.  This connecting land is cut off from the higher island at high tide.  The ascent from the connecting land is initially over wet rock and then grass which has a multitude of burrowed Tern nests excavated into it.

The island of Ynys Arw was not classified in any list prior to the survey with the Trimble GeoXH 6000 as it is given an uppermost contour of only 10m on Ordnance Survey 1:25,000 Explorer maps.  It was surveyed with the Trimble GeoXH 6000 at being 15.2m (converted to OSGM15) high.

The 15.2m (converted to OSGM15) height produced by the Trimble GeoXH 6000 is not a dramatic height revision when compared to an estimated c 12m height for this island’s summit based on contour interpolation taken from its uppermost 10m map ring contour, but as the contours on the Ordnance Survey 1:25,000 Explorer map are at 5m intervals it does come within the parameters of the Significant Height Revisions used within this page heading, these parameters are:

The term Significant Height Revision applies to any listed hill whose Ordnance Survey summit spot height has a 2m or more discrepancy when compared to the surveyed height produced by the Trimble GeoXH 6000, also included are hills whose summit map data is missing an uppermost ring contour when compared to the data produced by the Trimble.  As heights on different scaled Ordnance Survey maps are not consistent the height given on the 1:25,000 map is being prioritised for detailing these revisions.

Therefore this hill’s new summit height is 15.2m (converted to OSGM15) which is 3.2m higher than the previously estimated height and 5.2m higher than its uppermost ring contour on Ordnance Survey 1:25,000 Explorer maps.

The full details for the hill are:

Cardinal Island:  Ynysoedd y Moelrhoniaid

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

Name:  Ynys Arw

OS 1:50,000 map:  114

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

Drop:  15.2m (Trimble GeoXH 6000 summit and LIDAR bwlch)

Gathering data from the summit of Ynys Arw which resulted in this island's significant height revision

For details on the survey that significantly revised the summit height of this hill

© 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)