Mapping Mountains – Significant Height Revisions – Y Pedwarau

Bryn Serth (SO 148 109)  

There has been a Significant Height Revision to a hill that is listed in the Y Pedwarau that was initiated by a survey with the Trimble GeoXH 6000 and subsequently confirmed via LIDAR analysis conducted by Aled Williams, with these details being retrospective as the Trimble survey that resulted in this height revision was conducted on 12^th August 2015.  The hill appeared in the listing of Y Pedwarau as a Pedwar with c 34m of drop and was reclassified to a 400m Sub-Pedwar due to the survey with the Trimble GeoXH 6000. 

The criteria for Pedwar status is all Welsh hills at or above 400m and below 500m in height that have 30m minimum drop.  Whilst the criteria for 400m Sub-Pedwar status is all Welsh hills at or above 400m and below 500m in height that have 20m or more and below 30m of drop.

The name of the hill is Bryn Serth and it is situated in the hill range known as Cymoedd Gwent which is positioned in south Wales, and the survey was conducted during a walk in the company of Mark Trengove who had suggested visiting the hill.

Bryn Serth is situated on the northern outskirts of Tredegar at the head of the Cwm Sirhywi (Sirhowey Valley) and it can be easily accessed from a car park beside the busy A4047 which is positioned south-east of the summit.  This car park also gives access to the Aneurin Bevan Memorial Stones; these consist of four monoliths and are set against the landscape of Ebbw Vale, Tredegar and Rhymney which form the constituencies which Aneurin Bevan represented during his years in Westminster.

Prior to the survey with the Trimble GeoXH 6000 the hill was listed as a Pedwar with its summit height estimated as c 410m based on a small uppermost 410m ring contour that appears on the Ordnance Survey 1:50,000 Landranger and 1:25,000 Explorer maps.  However, this point is not the high point of the hill as land approximately 100 metres further north was surveyed with the Trimble as being 2.0m higher than the position where the uppermost contour ring appears on the ground.

The two points surveyed with the Trimble GeoXH 6000 gave results that are significantly lower than current Ordnance Survey map data, these results are:

Northern summit:  403.927m (converted to OSGM15) at SO 14794 10908

Southern summit:  401.890m (converted to OSGM15) at SO 14782 10809

It is the southern summit where the small 410m ring contour appears on contemporary Ordnance Survey maps.

Since processing the data sets from this survey Aled has scrutinised on-line mapping and found that the Ordnance Survey Six-Inch map has a 1,327ft (404.47m) height on the area of the summit and a 1,242.5ft (378.71m) height on the area of the bwlch.  These Six-Inch map values give Bryn Serth a drop of 25.76m which is comparable to the result produced by the Trimble GeoXH 6000 and by LIDAR analysis.

Therefore, this hill’s new summit height is 404.0m and this was derived from LIDAR analysis, and thus is 6m lower than its previously estimated height of c 410m and its uppermost ring contour on Ordnance Survey 1:50,000 Landranger and 1:25,000 Explorer maps.

The full details for the hill are:

Group:  Cymoedd Gwent

Summit Height (New Height):  404.0m (LIDAR)

Name:  Bryn Serth

OS 1:50,000 map:  161

Summit Grid Reference:  SO 14807 10929 (LIDAR)

Drop:  25.8m (LIDAR)

Gathering data at the summit of Bryn Serth which resulted in this hill's significant height revision

For details on the survey that significantly revised the summit height of this hill

Myrddyn Phillips and Aled Williams (September 2015)