Mapping Mountains – Significant Height Revisions – 100m Twmpau

Y Gaer (SJ 204 155)  

The summit of this hill has been re-surveyed with the Trimble GeoXH 6000 and its bwlch analysed by LIDAR, resulting in a 129.3m summit at SJ 20417 15566 and a 91.2m bwlch at SJ 21177 15977, giving 38.2m drop and 29.51% dominance.  The above values are taken to remaining natural ground at the base of the covered reservoir at the summit and the raised road at the bwlch, with the below post retained in its original format for historical purposes.

There has been a Significant Height Revision initiated by a survey with the Trimble GeoXH 6000 to a hill that is listed in the 100m Twmpau (thirty welsh metre prominences and upward), with these details being retrospective as the survey that resulted in this height revision was conducted on 8^th August 2015.  With the criteria for 100m Twmpau status being all Welsh hills at or above 100m and below 200m in height that have a minimum 30m of drop.

The name of the hill is Y Gaer and it is situated in the Carnedd Wen range, and is positioned above the small community of Trefnanney which is beside the Afon Efyrnwy (River Vyrnwy) and the Dyffryn Meifod.

Y Gaer was surveyed in the company of Alex Cameron during a multi hill bagging day when we had glorious weather and visited nine P30 Twmpau hills with seven of them being Trimbled.

The hill has two uppermost 125m ring contours on Ordnance Survey 1:25,000 Explorer maps, these are bisected by a lane that follows this hill’s summit ridge in a south-west to north-east direction.  It is from the top of this narrow lane that easy access to the high point of the hill can be found.

The larger of the two uppermost ring contours is positioned to the west of the smaller one, and it is the larger ring contour where the remains of an ancient earthen embankment is indicated on Ordnance Survey maps.  It is this ancient hill fort that gives the hill its name.

Prior to the survey with the Trimble GeoXH 6000 the height of the hill was estimated as being c 129m based on the dimension of the uppermost westerly 125m contour ring against the steepness of ground as indicated by contours on this hill’s northern, western and southern slopes.

However, the summit of this hill now possesses a covered reservoir which is indicated as a small round circle on Ordnance Survey maps, and as a number of P30s comprising many walks took place on the day of this hill's survey the lay of land was quickly judged and the Trimble GeoXH 6000 placed on land that probably forms a a part of the covered reservoir.  If the hill is visited again the land will be re-assessed, but until then the height and position of the summit is being taken as that produced by the Trimble, a second visit would also give chance to survey the highest point of the easterly contour.

The 130.5m (converted to OSGM15) height produced by the Trimble GeoXH 6000 is not a dramatic height revision when compared to this hill’s previously listed height of c 229m, but as the contours on the Ordnance Survey 1:25,000 Explorer map are at 5m intervals it does come within the parameters of the Significant Height Revisions used within this page heading, these parameters are:

The term Significant Height Revision applies to any listed hill whose Ordnance Survey summit spot height has a 2m or more discrepancy when compared to the height surveyed with the Trimble GeoXH 6000, also included are hills whose summit map data is missing an uppermost ring contour when compared to the data produced by the Trimble.  As heights on different scaled Ordnance Survey maps are not consistent the height given on the 1:25,000 map is being prioritised for detailing these revisions.

Therefore this hill’s new summit height is 130.5m (converted to OSGM15) which is 1.5m higher than its previously estimated height and 5.5m higher than its uppermost ring contour on current Ordnance Survey 1:25,000 Explorer maps.

The full details for the hill are:

Cardinal Hill:  Carnedd Wen

Summit Height (New Height):  130.45m (converted to OSGM15)

Name:  Y Gaer

OS 1:50,000 map:  126

Summit Grid Reference:  SJ 20413 15563

  

Drop:  c 39m

Gathering data from the summit of Y Gaer 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 (September 2015)