Mapping Mountains – Hill Reclassifications – 30-99m Twmpau

 

Ynys Llanddwyn (SH 388 629) – Double Sub-Twmpau addition 

There has been an addition to the list of Double Sub-Twmpau adjoined to the 30-99m Twmpau, with the summit height, bwlch height and their locations, the drop and status of the hill derived from LIDAR analysis with a previous Trimble GeoXH 6000 survey conducted by Myrddyn Phillips. 

Ynys Llanddwyn (SH 388 629)

The criteria for the list that this addition applies to are:

30-99m Twmpau - Welsh hills at or above 30m and below 100m in height with 30m minimum drop, this height band of hills has two accompanying sub lists, the first of which is entitled the 30-99m Sub-Twmpau with the criteria for this sub category being all Welsh hills at or above 30m and below 100m in height with 20m or more and below 30m of drop, and the second sub category is entitled the Double Sub-Twmpau with the criteria for this sub category being all Welsh hills at or above 20m and below 30m in height with 20m or more and below 30m of drop, with the word Twmpau being an acronym standing for thirty welsh metre prominences and upward. 

The 30-99m Twmpau by Myrddyn Phillips

The name the hill is listed by is Ynys Llanddwyn, and it is adjoined to the Ynys Môn group of hills, which are situated in the north-western part of North Wales (Region A, Sub-Region A1), and as it is a tidal island it is adjoined to Ynys Môn in the south-west of the island, and is positioned with the A480 road and the village of Niwbwrch (Newborough) towards the north-east. 

When the original 30-99m height band of Welsh P30 hills were published on Geoff Crowder’s v-g.me website this hill was not included in the Hills to be surveyed sub list, as it was considered not to meet the criteria then used for this sub category.

Extract from the Ordnance Survey 1:25,000 Explorer map

When the sub list was standardised, and interpolated heights and drop values also included the details for this hill were re-assessed and it was listed with an estimated c 20m of drop, based on an estimated c 20m summit height which was based on interpolation of the uppermost contour on contemporary Ordnance Survey 1:25,000 Explorer maps.  The summit and bwlch of this hill were subsequently surveyed with the Trimble GeoXH 6000; however it is LIDAR that is being prioritised for each. 

The Trimble GeoXH 6000 gathering data at the summit of Ynys Llanddwyn

However, it was not until LIDAR became available that the survey details for this hill could be accurately re-assessed.  The LIDAR (Light Detection & Ranging) technique produced highly accurate height data that is now freely available for much of England and Wales. 

LIDAR image of Ynys Llanddwyn (SH 388 629)

Therefore, the addition of this hill to Double Sub-Twmpau status is due to LIDAR analysis, resulting in a 21.8m summit height and a 1.5m bwlch height, with these values giving this hill 20.3m of drop, which is sufficient for it to be classified as a Double Sub-Twmpau. 

 

The full details for the hill are: 

Group:  Ynys Môn

Name:  Ynys Llanddwyn

OS 1:50,000 map:  114

Summit Height:  21.8m (LIDAR)

Summit Grid Reference:  SH 38841 62953 (LIDAR)

Bwlch Height:  1.5m (LIDAR)

Bwlch Grid Reference:  SH 39107 63441 & SH 39099 63431 (LIDAR) 

Drop:  20.3m (LIDAR)

 

Myrddyn Phillips (March 2021)