Wednesday, 12 August 2015

Mapping Mountains – Significant Height Revisions – 30m-99m Sub-Twmpau

Pt. 36.9, Ynys Hir (SH 566 398)

This is the third 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 third 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 new P30 is Ynys Hir (SH 56695 39393) which is adjacent to the hill this blog post relates to; Pt. 36.9m, Ynys Hir (SH 56649 39840).

The hill is listed in the 30m-99m height band of Sub-Twmpau hills and is situated to the north of Porthmadog in the area known as Traeth Mawr, the area of the hill is easily accessed from a lay-by on the A487 to the south.  But its ascent can be problematical due to the undergrowth.

The name of the hill is Pt. 36.9m, Ynys Hir with its adjacent and higher hill taking on the full name without the point (Pt.) notation.  The hill 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 24m.  Its new summit height is 36.9m, which is 12.9m higher than its previously estimated height and 16.9m higher than the 20m uppermost ring contour on the current Ordnance Survey map.

Although the new summit height of 36.9m 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):  36.9m (converted to OSGM15)

Name:  Pt. 36.9m, Ynys Hir

OS 1:50,000 map:  124

Summit Grid Reference:  SH 56649 39840
Drop:  25.8m

Setting up the Trimble at the summit of Pt. 36.9m, Ynys Hir.  Photo: Aled Williams

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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