Wednesday, 9 August 2017

Mapping Mountains – Trimble Surveys – Y Mynydd Du



13.06.17  Fan Brycheiniog (SN 824 220, only bwlch surveyed SN 870 195)


Gathering data at the critical bwlch of Fan Brycheiniog

Standing beside an A road watching cars and lorries thunder past a small piece of surveying equipment could be thought of as an unusual pastime, to many it undoubtedly is, but there is a satisfaction to such a thing, albeit an unusual one.

Having visited Mynydd y Drum and the Nant Helen Opencast Mine and with a small walk above Llanfair-ym-Muallt (Builth Wells) pencilled in for later in the day I thought I’d better survey the critical bwlch of Fan Brycheiniog; one of south Wales’ highest mountains.  This bwlch is positioned on or beside the A 4067 as it heads southward from Sennybridge toward Ystradgynlais, it was a bwlch I drove over earlier in the day and when doing so I tried to judge where its critical point lay.

Thankfully the road has a number of large pull-ins close to the area of the bwlch and once out of my car I went to work.  This work consisted of bending over, kneeling and sometimes lying on the ground assessing where the uphill from the valley to valley traverse meets the downhill from the hill to hill traverse.  When doing this it is advantageous to gain height for an overall view of the scene, unfortunately the ground to the east of the road is steep and therefore I used a conveniently placed ladder stile to stand on top of and gain a little elevation to assess the lay of land.

It proved easy to judge where the upper part of the northern valley lay as the reed grass and wet surrounds of the Blaen-crai was positioned beside the ladder stile, which was only a few metres from the road.  I marched back to the lay-by where my car was parked and proceeded to kneel on the grass verge to look at the uphill part of the road, there seemed to be two points where the high point may lie, with a slight dip in the road between.

I ummed and aghed for ten minutes or so wondering which out of these two points to gather data from, on the eastern side of the road a drainage ditch beside the grass verge helped in judging where the downward southern land lay, however as drainage ditches aren’t necessarily consistent in their depth I decided only to treat this as an indicator of where the critical point may lie.

During this process vehicles sped past, one thing was certain the Trimble was not going to be set-up on the road, perhaps if this bwlch was on a country lane it would be all right to do so, but a busy A road was asking for trouble.  Deciding on my point I set the Trimble up on my rucksack and measured a 0.42m offset between the equipment’s internal antenna and the ground at the base of the rucksack and waited until the 0.1m accuracy level was attained before data should be logged.

The Trimble set-up position at the bwlch of Fan Brycheiniog

During data collection I made a note of how many vehicles whizzed past heading south, the total amounted to eight cars and four Lorries, all within five minutes; hopefully the data set won’t be compromised by this activity.

Once the allotted five minutes of data collection were up, I switched the equipment off, packed it away and headed north to the outskirts of Llanfair-ym-Muallt (Builth Wells) and the joys of Garth (SO 053 506), a hill I’d wanted to visit for many a year. 


Survey Result:


Fan Brycheiniog

Bwlch Height:  378.7m (converted to OSGM15, Trimble GeoXH 6000) 378.5m (LIDAR data, this is being prioritised)

Bwlch Grid Reference:  SN 87077 19560 (Trimble GeoXH 6000) SN 87056 19547 (LIDAR data, this is being prioritised)

Drop:  423.9m (converted to OSGM15, Trimble GeoXH 6000) 424.2m (Trimble summit and LIDAR bwlch, this is being prioritised)

Dominance:  52.84% (Dominant status confirmed) (based on Trimble summit and LIDAR bwlch)








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