Mapping Mountains – Hill Reclassifications – 30-99m Twmpau

 

Ynysoedd y Moelrhoniaid (SH 267 947) – Double Sub-Twmpau addition 

There has been confirmation of an addition to the list of Double Sub-Twmpau adjoined to the 30-99m Twmpau, with the summit height and its location, the drop and status of the hill confirmed by a Trimble GeoXH 6000 survey conducted by Myrddyn Phillips.

Ynysoedd y Moelrhoniaid (SH 267 947)

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 Ynysoedd y Moelrhoniaid, 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 it is positioned to the north-west of Ynys Môn on an archipelago that is also known as The Skerries. 

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.  As the summit of this hill has now been surveyed with the Trimble GeoXH 6000, it is this result that is being prioritised for listing purposes. 

The Trimble GeoXH 6000 gathering data at the summit of Ynysoedd y Moelrhoniaid

Therefore, the confirmation of the addition of this hill to Double Sub-Twmpau status is due to a Trimble GeoXH 6000 survey, resulting in a 21.7m summit height, which is also its drop value as it is the highest point of an island, and this is sufficient for it to be classified as a Double Sub-Twmpau. 

 

The full details for the hill are: 

Group:  Ynys Môn

Name:  Ynysoedd y Moelrhoniaid

OS 1:50,000 map:  114

Summit Height:  21.7m (converted to OSGM15)

Summit Grid Reference:  SH 26777 94759

Bwlch Height:  not applicable, sea level

Bwlch Grid Reference:  not applicable, sea level 

Drop:  21.7m (LIDAR)

 

Myrddyn Phillips (March 2021)