Sunday, 9 August 2015

Mapping Mountains – Significant Height Revisions – 30-99m Twmpau and Y Trechol - The Dominant Hills of Wales

Ynys Hir (SH 566 396)

This is the second 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 1st February 2014.

The second major height revision initiated from a survey with the Trimble GeoXH 6000 was conducted in the lowlands outside of Porthmadog in north-west Wales in the company of Aled Williams who had suggested the walk, as he considered at least one of the hills he proposed for us to visit would become a new P30 – he was right!

The hill is listed in the 30-99m height band of Twmpau hills and is also now listed as a Dominant hill, and is situated to the north of Porthmadog in the area known as Traeth Mawr, the hill is easily accessed from a lay-by on the A487 to the south-west.  The term ‘easily accessed’ is only applicable to where the road is situated in relation to the hill as it can be a wee battle to reach its top.

The name of the hill is Ynys Hir which should not be confused with its higher Cardinal Hill of Hir Ynys.  Its summit height has been dramatically increased from its current Ordnance Survey map height, as prior to the survey with the Trimble GeoXH 6000 the hill was not listed as it only had an uppermost ring contour of 20m on current Ordnance Survey maps, with an estimated summit height of c 23m.  Its new summit height is 37.3m (converted to OSGM15), which is 14.3m higher than its previously estimated height and 17.3m higher than the 20m uppermost ring contour on the current Ordnance Survey map.

Although the new summit height of 37.3m (converted to OSGM15) is dramatically different to current Ordnance Survey map detail, the Historical 1:25,000 map has an uppermost ring contour of 100ft for this hill which is correct when compared to the surveyed height from the Trimble GeoXH 6000.  

The full details for the hill are:

Cardinal Hill:  Hir Ynys

Summit Height (New Height):  37.3m (converted to OSGM15)

Name:  Ynys Hir

OS 1:50,000 map:  124

Summit Grid Reference:  SH 56695 39693 
Drop:  c 34m

Beyond the trees, vertical cliffs and undergrowth is the summit of Ynys Hir, a new P30 which has a significant height revision

For details on the survey that significantly revised the summit height of this hill please click {here}

Myrddyn Phillips (February 2014)

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