Mapping Mountains – Hill Reclassifications – 100m Twmpau

Barclodiau (SH 466 848) – 100m Sub-Twmpau reclassified to 100m Twmpau

There has been confirmation of a reclassification to a hill that is listed in the 100m Twmpau, with the summit height, bwlch height and their locations, the drop and status of the hill confirmed by LIDAR analysis and a subsequent summit survey with the Trimble GeoXH 6000, with both conducted by Myrddyn Phillips and the latter taking place on the 28^th October 2018.

Barclodiau (SH 466 848)

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

100m Twmpau - Welsh hills at or above 100m and below 200m in height with 30m minimum drop, with an accompanying sub list entitled the 100m Sub-Twmpau with the criteria for this sub category being all Welsh hills at or above 100m and below 200m 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 name of the hill is Barclodiau, 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 has the A5025 road to its north-east and the B5111 road to its west, and is positioned between the small communities of Llannerch-y-medd towards its west and Moelfre towards its east north-east.  

When the original 100m height band of 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 that accompanied the main P30 list, as it did not meet the criteria then used in this list. 

When the sub list was standardised, and interpolated heights and drop values also included the details for this hill were re-evaluated and it was listed with an estimated c 30m of drop, based on the 168m summit spot height that appears on the Ordnance Survey 1:25,000 Explorer map and an estimated bwlch height of c 138m based on interpolation of 5m contouring.

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

The drop value for this hill was 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.  Although this mapping has the same 5m contour intervals as the 1:25,000 Explorer map its scale is greatly enlarged giving the opportunity for better interpolation.  This re-assessment resulted in the estimated c 30m drop value for this hill remaining the same.

Extract from the Ordnance Survey Vector Map Local hosted on the Geograph website

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.  

The confirmation of the reclassification of Barclodiau to 100m Twmpau status is due to LIDAR analysis, with a subsequent summit survey with the Trimble GeoXH 6000 resulting in a 168.6m Trimble summit height and a 137.1m LIDAR bwlch height, with these values giving this hill 31.5m of drop which is sufficient for it to be classified as a 100m Twmpau.

The Trimble GeoXH 6000 gathering data at the summit of Barclodiau

The full details for the hill are:

Group:  Ynys Môn

Name:  Barclodiau

OS 1:50,000 map:  114, 115

Summit Height:  168.6m (converted to OSGM15)

Summit Grid Reference:  SH 46674 84803

Bwlch Height:  137.1m (LIDAR)

Bwlch Grid Reference:  SH 46583 85001 (LIDAR)

Drop:  31.5m (Trimble summit and LIDAR bwlch)

For details on the summit survey of this hill

Myrddyn Phillips (May 2019)