Mapping Mountains - Hill Reclassifications – 300m Twmpau

Mynydd Derw Llwydion (SO 904 900) - 300m Sub-Twmpau reclassified to 300m Twmpau

There is an addition to the 300m Twmpau list due to a survey with the Trimble GeoXH 6000, subsequent LIDAR analysis and a further survey with the Trimble, all of which were conducted by Myrddyn Phillips.  

The hill was previously listed as a 300m Sub-Twmpau with 29m of drop, based on the 387m summit spot height and the 358m bwlch spot height that appear on the Ordnance Survey 1:25,000 Explorer map, and it is situated in the Pumlumon range and positioned between Llyn Clywedog and Penffordd-las (Staylittle).  

The Trimble survey gave the hill sneaking in to P30 status with 14 millimetres to spare!  This margin is within the uncertainty applied to the equipment and its positioning at the summit and the bwlch.  However, reclassifications are taking place in the listings I have compiled and those jointly co-authored with Aled Williams, from best available evidence on any given day and as the data produced by the Trimble is more accurate that spot heights produced by photogrametry, the Trimble result was taken for the recalssification of this hill.  

However, since this survey LIDAR is now available and the subsequent height and position derived from LIDAR prompted a further survey with the Trimble GeoXH 6000.

The full details for the hill are:

Group:  Pumlumon

Name:  Mynydd Derw Llwydion

OS 1:50,000 map:  136

Summit Height:  389.3m (converted to OSGM15, Trimble GeoXH 6000)

Summit Grid Reference:  SN 90460 90041

Bwlch Height:  358.1m (converted to OSGM15, Trimble GeoXH 6000)

Bwlch Grid Reference:  SN 90391 90264

Drop:  31.3m

LIDAR image of Mynydd Derw Llwydion (SN 904 900)

For details on the survey that reclassified this hill to 300m Twmpau status please click {here}

For details on the second Trimble GeoXH 6000 survey of this hill

Myrddyn Phillips (April 2014)