Mapping Mountains – Hill Reclassifications – 30-99m Twmpau

Pt. 63.3m (SH 491 760) – 30-99m Twmpau reclassified to 30-99m Sub-Twmpau

There has been a reclassification to a hill that is listed in the 30-99m Twmpau, with the bwlch height and its location confirmed by LIDAR analysis, and a subsequent summit survey with the Trimble GeoXH 6000, both conducted by Myrddyn Phillips with the latter taking place on the 22^nd October 2018.

LIDAR image of Pt. 63.3m (SH 491 760)

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

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

The hill is now being listed by the point (Pt. 63.3m) notation as I do not know an appropriate name for it either from historic research or local enquiry.  The hill 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 encircled by minor roads with the B5420 to its south-west and the B5109 to its north-west, and has the town of Llangefni towards its west.

When the original Welsh P30 hills were published on Geoff Crowder’s v-g.me website this hill was included in the Hills to be surveyed sub list, as it did not at that stage meet the criteria used for the main P30 list. 

When the sub list was standardised, and interpolated heights and drop values also included for the P30 and sub list this hill was listed with an estimated c 30m of drop, based on the 64m summit spot height that appears on the Ordnance Survey 1:25,000 Explorer map and an estimated bwlch height of c 34m based on interpolation of 5m bwlch contouring between 30m – 35m that appears on the Ordnance Survey 1:25,000 Explorer map.

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

The drop value for this hill was next re-assessed when the Ordnance Survey Vector Map Local hosted on the Geograph website became available online; this mapping is entitled the Interactive Coverage Map, and the estimated drop value of c 30m remained the same.

However, it was not until LIDAR became available that the 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 bwlch image of Pt. 63.3m (SH 491 760)

The reclassification of Pt. 63.3m to 30-99m Sub-Twmpau status is due to LIDAR bwlch analysis and a subsequent summit survey with the Trimble GeoXH 6000 conducted by Myrddyn Phillips, resulting in a 63.3m summit height and a 35.1m bwlch height, with these values giving this hill 28.2m of drop which is insufficient for it to retain its 30-99m Twmpau status. 

 

The Trimble GeoXH 6000 gathering data at the summit of Pt. 63.3m (SH 491 760)

The full details for the hill are:

Group:  Ynys Môn

Name:  Pt. 63.3m

OS 1:50,000 map:  114, 115

Summit Height:  63.3m (converted to OSGM15)

Summit Grid Reference:  SH 49157 76080

Bwlch Height:  35.1m (LIDAR)

Bwlch Grid Reference:  SH 49043 76701 (LIDAR)

Drop:  28.2m (Trimble summit and LIDAR bwlch)

For details on the summit survey of this hill

Myrddyn Phillips (April 2019)