Mapping Mountains – Trimble Surveys – Moel y Gamelin

18.03.14 and 08.04.14  Bryn y Beili (SJ 235 643)

Bryn y Beili (SJ 235 643)

And what a dramatic day this turned out to be!  It had started with a visit to Moel Ffagnallt (SJ 189 700) a rather unimposing small hill above the hamlet of Rhes-y-cae.  I was with Mark Trengove who had suggested a day spent looking for ‘new’ P30’s, Moel Ffagnallt was the first possibility (it just failed with 27.2m of drop).  The second possibility was Bryn y Beili (Bailey Hill in English).

At the top of Bryn y Beili is an impressive motte and bailey built on an old glacial mound, just below the summit is a bowling green which has a 135m spot height on it on the Ordnance Surveyed enlarged Geograph map.  It also has a small recreational area with a slide, and a stone gorsedd circle where John, Graham and I filmed one of the introductory videos to the survey of Tryfan.  We used the upright stones to demonstrate how we planned to survey Adam or Eve.

The rise from the bowling green to the top of the motte is steep; we estimated it as a minimum of 5m, placing the height of the summit at approximately 140m (+/- 2m).  The critical bwlch for the hill lies in the cobweb of roads near to the centre of Mold with the northerly valley to valley contour making its way up (south) Ffordd Pennant until it reaches the junction with Gwernaffield Road.  Just east of this road junction is Park Avenue, which is the opposing road on the valley to valley traverse that makes its way up (north) until it reaches the junction with Gwernaffield Road.  This latter road junction has a 111m spot height on the Ordnance Survey enlarged Geograph map.

We parked on Ffordd Pennant and assessed the area of the bwlch as we walked into town for a relaxing hour in a pub with dinner time grub.  Whilst walking from the car down (or should that be up) Gwernaffield Road we realised that we would probably need two data sets from the road as it seems quite flat for 200 metres or so.  From one direction it seemed to be going up and when looking back at the previously sighted point from the point of upness it also seemed to be going up.  We debated this predicament and assessed the upness from a number of points.  During lunch we struck upon a cunning plan and decided that a shop in Mold must sell a surveying implement that may solve the ‘upness’ problem.  The surveying implement was found in a joke shop, it cost £2.50 (money well spent!), with the theory being that when the newly bought, expensive surveying implement was placed on the ground it would roll downhill.  Unfortunately it was a little windy and the technicalities of the surveying procedure quickly got out of hand as the surveying implement kept being blown uphill.  This resulted in a number of giggles and the thought of what John and Graham would think about the whole thing!  A photograph of the expensive surveying implement appears below.

Mark proudly holds the expensive surveying implement

Once we were fed we walked back to the area of the bwlch and visited the car to pick up all necessary gear, we deposited the football in the boot as its surveying life was probably not a lengthy one.  Our next stop was the junction of Gwernaffield Road and Park Avenue, this is where the 111m spot height appears on the OS enlarged Geograph map.  The Trimble was placed on the pavement near to the road and whilst it gathered its ten minutes of data Mark headed south to assess the bwlch from that direction, and I kept my eye fixed on all traffic as I didn’t want the Trimble to end up as road kill.  We then headed further down / up Gwernaffield road to the second point where we wanted to gather prospective bwlch data from.

At the junction of Gwernaffield Road and Park Avenue - gathering data next to roads can be highly dangerous for Trimbles

Again the Trimble was positioned on the pavement near to the road, we stood about four metres away, sufficiently close to grab it if any rogue car decided to try and squash it, but also sufficiently far away from it that we wouldn’t disrupt satellite coverage.  It took quite some time to get toward the required 0.1m accuracy before starting to ‘Log’ data; this was due to it having cars passing it every few seconds.  During this time Mark and I chatted and waited patiently.  I remember looking to my right and being shocked to see a car on the pavement, having cut the corner of the road junction, at that stage I remember being quite horrified as its front wheel had already passed the Trimble by inches, but its rear wheel was on a collision course with a very expensive bit of surveying equipment.

DON'T SQUASH THE TRIMBLE!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!

I shouted in desperation for the driver to stop and then watched as he drove straight over the Trimble.  The car’s rear wheel went straight over the centre of the Trimble and kicked it up slightly from the pavement.  I expected to see it completely crushed and splintered across the pavement, remarkably it sat there in one piece.  Hard little beasts these Trimbles!  The driver realised that something was amiss because of me dashing towards his car shouting just before he tried to kill it.  He stopped and got out and was very apologetic, we spoke for two or three minutes, he apologised profusely, and as it was still in one piece and I was probably more concerned with its immediate welfare than anything else, I didn’t think about asking or swapping insurance details.  After a few minutes he got back in to his car and drove off.  I inspected the Trimble and the only external damage seemed to be on the outer casing on the back, where the force of the impact had scuffed the black surround.  Slight external damage is one thing but it is the prospect of internal damage that is the concern.

Battered

Bruised

But still intact

On our way to the top of Bryn y Beili we chatted about the incident.  Once on top we set the Trimble up, firstly on the floor operating with its internal antenna, and secondly on its 2m pole, operating with its external antenna.  On both occasions it didn’t reach its required 0.1m accuracy, therefore we did not ‘Log’ any data.  The nearest it reached was 0.12m with its external antenna.  The reason for this may be because of the mature tree coverage around the summit area.  But of course it may be because the Trimble was in a state of unrepairable shock.

Mark suggested we take a data set on a field close to his house, which we did.  The accuracy of 0.1m was reached; it gathered data and this was then post-processed with no seeming problem.  I then gathered a data set from a datum point outside my bungalow later that night and post-processed the data the following morning.  The standard deviation was o.6 which is very poor and the height was 0.5m below the usual measurement.  This latter data set coupled with the screen showing signs of flickering when trying to gather data at the top of Bryn y Beili is a sign for concern.

The Trimble will have to be looked at by the experts at KOREC UK.  This has been organised and I await their assessment.  I suspect there will be no more Trimbling for a few weeks.  Hopefully when looked at/ repaired/ replaced we can come back to Bryn y Beili and survey its summit.  But until then the GeoXH 6000 is in the Trimbe hospital being assessed.

I handed the Trimble over to Martyn Palmer at a service station off the M54 near to Telford.  He set it running next to a new 7series Trimble, operating each machine in sync with the other, his first assessment was encouraging as he said he would be amazed if there was any internal damage.  After a coffee and a good chat Martyn took the Trimble off to the workshop at KOREC UK in Huntington.

Can I have the nice pretty new one on the right please sir?

Martyn Palmer assessing the XH 6000 against the new 7series

Two weeks later it was delivered back to me with a clean bill of health, quite remarkable considering a car tried to squish it to smitherines.  Bryn y Beili awaited and it had to be surveyed with Mark.  We decided upon this evening, a lovely early spring day with blue sky, a slight breeze and daylight hours extending to just beyond 8.00pm.

I parked in Mold and by 5.00pm I was on top of the hill, having paid my respects to the scene of the crime.  Mark was due to meet me at 6.00pm.  I first tried the Trimble on the high point using its internal antenna whilst on the ground.  It achieved 0.12m accuracy, but never the 0.1m required to start logging data; pesky tree coverage was to blame.

The scene of the crime with Bryn y Beili in the background

I’d brought my old measuring staff as the bowling green on the south of the motte has a 135m spot height on it on the Ordnance Survey enlarged Geograph map, it also has good clear sky above it, so if the tree coverage hampered data gathering at the summit, we could at least gather data on the bowling green and then survey from this point to the summit using my old staff.

Although the Trimble didn’t achieve its required 0.1m accuracy it was great to be out with it again.  I then decided to try it at the gorsedd circle just to the north of the motte, if I could get data from here we could use the old staff to measure up to the high point.  Twenty minutes later and it still hadn’t reached the required accuracy; it slowly crept down toward 0.1m but kept bouncing back up when reaching 0.15m.  When this happens experience has taught me to be patient, as each time it creeps back toward the required accuracy it seems to get closer before bouncing back up again.  But after the third or fourth time of this happening I decided to visit the Bowling Green and its view of clear sky.

The view of the motte from the gorsedd stone circle

I paced out the bowling green and put the Trimble where I imagined the spot height appeared on the map.  It was 5.50pm by the time the Trimble attained 0.1m and I celebrated by quickly pressing ‘Log’.  A few minutes later and Mark appeared in his trendy surveying threads.  We gathered data at the bowling green for 20 minutes and then line surveyed from this point to the summit; the height difference came to 26½ft / 8.1m.  We’d already estimated that if this figure came to 7m or more Bryn y Beili would probably be a new P30.  But we really needed to get a good data set from the summit.

Gathering data on the bowling green with the motte behind the trees in the background

One last try and this time we set the Trimble up on its 2m pole, pushing this in to the ground for extra stability whilst also using its bipod.  The distance from bottom of antenna to the ground was measured and input as a measurement offset and we then waited and hoped that the magical 0.1m would appear on the screen.  Every few minutes I checked it and every few minutes the figure crept down; 0.19, 0.15, 0.13 and then 0.11m, the closest yet.  A few minutes later and there it was – 0.1m – at last!

Mark watches for rogue cars as the Trimble GeoXH 6000 eventually attains its 0.1m required accuracy

It only took a road accident, over three weeks and four goes on the summit to get the required accuracy.  Once ‘Log’ was pressed I joined Mark and we waited until over 30 minutes of data had been collected.  All that was left was to pack everything away and visit a pub for an evening meal, an excellent way to end a very fulfilling evening’s survey.

Survey Result:

Bryn y Beili

Summit Height:  143.0m (converted to OSGM15) (significant height revision) 

Summit Grid Reference:  SJ 23544 64367

Bwlch Height:  111.5m (converted to OSGM15)

Bwlch Grid Reference:  SJ 23251 64429

Drop:  31.5m (100m Twmpau addition)

Dominance:  22.02%

The survey on the bowling green came to 135.276m (converted to OSGM15), the height from this point to the summit using the old staff was measured as 8.077m.  Therefore the absolute height of the hill from this back-up survey is 143.4m.  Whilst doing the survey Mark asked me the margin of uncertainty associated with using the old measuring staff over the distance and terrain from the bowling green to the summit, I estimated +/- 1ft / 0.3m.

For further details please consult the Trimble survey spreadsheet click {here}