Wednesday, 12 August 2015

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

Pt. 36.9, Ynys Hir (SH 566 398)

There has been a Significant Height Revision in the 30-99m Sub-Twmpau (thirty welsh metre prominences and upward) list initiated by a survey with the Trimble GeoXH 6000, with the following details being retrospective as the  survey that resulted in this height revision was conducted on 1st February 2014.

The criteria for the listing that this height revision affects are:

30-99m Twmpau - these are the Welsh hills at or above 30m and below 100m in height that have a minimum drop of 30m. 

30-99m Sub-Twmpau - these are the Welsh hills at or above 30m and below 100m in height with 20m or more and below 30m of drop

The listed 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, and it is situated in the lowlands outside of Porthmadog in north-west Wales, and the survey was conducted 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!  With the new P30 being 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 situated to the north of Porthmadog in the area known as Traeth Mawr, and it can be accessed from a lay-by on the A487 to the south of the hill, however its ascent can be problematic due to the undergrowth.

The summit height of this hill has been dramatically increased from its current Ordnance Survey 1:50,000 Landranger and 1:25,000 Explorer 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 both of these maps, with an estimated summit height of c 24m.  Its new summit height is 36.9m (converted to OSGM15), which is 12.9m higher than its previously estimated height and 16.9m higher than the 20m uppermost ring contour on current Ordnance Survey maps.

Although the new summit height of 36.9m (converted to OSGM15) is dramatically different to current Ordnance Survey map detail, the Ordnance Survey Historical 1:25,000 map has an uppermost ring contour of 100ft for this hill which is correct when compared to the surveyed height produced by 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 (converted to OSGM15)

Setting up the Trimble geoXH 6000 at the summit of Pt. 36.9m, Ynys Hir.  Photo: Aled Williams

Myrddyn Phillips (August 2015)

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