Friday, 16 October 2015

Mapping Mountains – Significant Height Revisions – Y Pellennig


Ynys Arw (SH 266 945) 

This is the twentieth post under the heading of Significant Height Revisions, and the following details are retrospective as the Trimble survey that resulted in this height revision was conducted on the 13th September 2015.

The twentieth major height revision initiated from a survey with the Trimble GeoXH 6000 was conducted on one of the islands that make up Ynysoedd y Moelrhoniaid, these islands are positioned to the north-west of Ynys Mon, and the islands name is Ynys Arw.

Ynys Arw (SH 266 945) is the most south-westerly island of this archipelago and has two high points, with the more southerly of these being the highest.  This island is tidal and 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.

This hill’s survey was suggested by Rob Woodall and it was conducted in the late afternoon.  In all twelve of us visited Ynysoedd y Moelrhoniaid, we went over to the island by 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 the higher island; Ynysoedd y Moelrhoniaid, and Ynys Arw.  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 as it is given an uppermost contour of only 10m on current Ordnance Survey maps.  It was surveyed with the Trimble GeoXH 6000 at being 15.2m (converted to OSGM15) high, and as the criteria for qualification to the Pellennig list is a hill whose summit is a minimum of 2.5 km from the nearest paved public road and which has a minimum drop of 15m, this hill easily qualifies on the distance criterion and now meets the necessary minimum drop criterion to become the latest entry to the Pellennig list.

The 15.2m (converted to OSGM15) height produced by the Trimble 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 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 Trimble height, 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 its previously estimated height and 5.2m higher than its uppermost ring contour on current Ordnance Survey maps.


The full details for the hill are:

Cardinal Island:  Ynysoedd y Moelrhoniaid

Summit Height (new height):  15.2m (converted to OSGM15)

Name:  Ynys Arw

OS 1:50,000 map:  114

Summit Grid Reference:  SH 26647 94598

Drop:  15.2m


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 please click {here}

Myrddyn Phillips and Aled Williams (September 2015)











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