Thursday, 13 September 2018

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

Ynys Deullyn (SM 844 340)

There has been a Significant Height Revision to a hill that appears in the following lists; 30-99m Twmpau and Y Trechol – The Dominant Hills of Wales, with the hill’s height, drop, dominance and status confirmed by LIDAR analysis conducted by Myrddyn Phillips.

The criteria for the two listings that this significant height revision applies to are:

30-99m Twmpau – Welsh hills at or above 30m and below 100m in height that have a minimum 30m of drop, with the word Twmpau being an acronym standing for thirty welsh metre prominences and upward.

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

The name of the hill is Ynys Deullyn and as its name implies it is an island, or more strictly speaking a tidal island that is attached to mainland Wales at low tide.  The hill is adjoined to the Carn Llidi group of hills, which are situated in the south-western part of Mid and West Wales (Region B, Sub-Region B4), with the island being positioned on the northern part of the Pembrokeshire coast, and it has the small community of Abercastell towards its west south-west.

If wanting to visit the island permission to do so should be sought as it is not a part of designated open access land, for those wishing to do so caution is advised as the easiest approach will probably necessitate some form of scramble.

This hill did not appear in the main P30 list or the accompanying Hills to be surveyed sub list when the original Welsh P30 hills were published on Geoff Crowder’s website, as it was thought not to have 30m of drop and did not meet the criteria then used for the sub category; however this sub list has now been standardised including the addition of interpolated drop and height values.

This hill was included as a P30 shortly after the Ordnance Survey Interactive Coverage Map became available on the Geograph website, and prior to LIDAR analysis it was listed with 30m of drop based on the 30m summit spot height that appears on this map.

Extract from the Ordnance Survey Interactive Coverage Map hosted on the Geograph website

The summit height produced by 2m DSM LIDAR analysis is 32.6m, this is not a dramatic height revision when compared to some revised heights, but it does come within the parameters of the Significant Height Revisions used within this page heading, these parameters are:

2m DSM LIDAR image of Ynys Deullyn (SM 844 340)

The term Significant Height Revisions applies to any listed hill whose interpolated height and Ordnance Survey or Harvey map summit spot height has a 2m or more discrepancy when compared to the survey result produced by the Trimble GeoXH 6000 or by LIDAR analysis, also included are hills whose summit map data is missing an uppermost ring contour when compared to the data produced by the Trimble or by LIDAR analysis.  As heights on different scaled Ordnance Survey maps are not consistent the height given on the 1:25,000 Explorer map is being prioritised in favour of the 1:50,000 Landranger map for detailing these revisions.

Therefore, this hill’s new summit height is 32.6m and this was produced from LIDAR analysis, this is 2.6m higher than its previously listed height of 30m which appears on the Ordnance Survey Interactive Coverage Map hosted on the Geograph website.

The full details for the hill are:

Group:  Carn Llidi

Summit Height (New Height):  32.6m (LIDAR)

Name:  Ynys Deullyn

OS 1:50,000 map:  157

Summit Grid Reference:  SM 84479 34095 (LIDAR) 
Drop:  30.5m (LIDAR)

Dominance:  93.51% (LIDAR) 

Myrddyn Phillips (September 2018)

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