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)

There has been a Significant Height Revision to the 30-99m Twmpau and Y Trechol - The Dominant Hills of Wales listings 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 two listings 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. 

Y Trechol - The Dominant Hills of Wales - these are the Welsh P30 hills whose prominence  equal or exceed half that of their absolute height.

The name of the hill is Ynys Hir 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!

The hill is situated to the north of Porthmadog in the area known as Traeth Mawr, and it can be 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 bit of a battle to reach its top.

The summit height of Ynys Hir 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 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 maps.

Although the new summit height of 37.3m (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):  37.3m (converted to OSGM15)

Name:  Ynys Hir

OS 1:50,000 map:  124

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

Dominance:  91.21%

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

Myrddyn Phillips (August 2015)

No comments: