Monday, 3 February 2014

Mapping Mountains – Trimble Surveys – Moel Hebog


02.02.14  Foel (SH 450 506)

This is the second of three Trimble surveys of Foel, to read the first survey post please click {here} and the third please click {here}


Foel (SH 450 506)
The first survey of Foel was conducted late in the afternoon on the 30th January and produced a drop result of 100.9m.  As the hill is currently listed as a Sub-Hump with 99m of drop I wanted to repeat the survey to compare data sets, but also attain adequate bwlch data as the previous data set had a high standard deviation and its estimated accuracies was also poor.

I was at the base of the hill by 10.00am and walked down the track to the position of the bwlch.  Thankfully the weather was set fine for most of the day and the sun was shining through the trees that lined the sides of the track.

On my previous visit the Trimble had been placed beside the track for it to gather data with its internal antenna, it never achieved its 0.1m accuracy which indicates that the equipment is ready to ‘Log’ data.  This may be due to the closeness of trees at the position of the bwlch, as activating the equipment before the required accuracy had been attained probably resulted in the poor data set that was gathered. 

For this second survey I’d come fully prepared with pole, tripod and external antenna.  I wanted to place the equipment in the same position as I had done during the first survey and within a few minutes the Trimble was connected to its Zephyr antenna, which was mounted on the Draper tripod.


The Trimble on its Draper tripod beside the track at the bwlch
The advantage of the Draper tripod is that it only weighs 1Kg which is beneficial when carrying a lot of surveying gear around the hills.  It was Alan Dawson who first showed me the tripod whist in Scotland last year.  Another advantage is its cost as it’s less than £20.00 including postage and packing from good retail outlets on the Internet.  However, its weight is also its major disadvantage as any strong wind would probably blow it over.  A freshening wind was forecast for the morning so I hoped the low elevation and sheltered spot of the bwlch would enable the Trimble to be set up on its tripod. 

During the first survey of the bwlch the required accuracy of the Trimble slowly edged its way down to the required 0.1m but only ever attained 0.12m and then bounced back up to over 1.0m.  This happened on three or four occasions and was the reason why I activated the equipment to gather data with the 0.1m accuracy not being attained.  Once the Trimble was set up on the Draper tripod I wondered if I was in for a long wait for the accuracy to be attained, if it ever was attained.  After about a minute I looked at the screen and thankfully the 0.1m accuracy was showing, I pressed ‘Log’ and stood away from the equipment a few metres up the lane.


The Trimble positioned at the bwlch with Foel in the background


The track crosses the bwlch on the hill to hill traverse.  With the valley to valley traverse bisecting the track at the approximate position of the Trimble.  The field on the left of photo was visually dismissed as lower than the track, whilst the field on the right of photo had data gathered in two positions during the surveys
During this process a number of cars and vans had pulled up and parked next to my car, which was only a couple of minutes’ walk away.  A number of people walked across the adjacent field, rifles in hand and after another few minutes a couple came down the lane to investigate what I was doing.  We chatted for ten minutes, I explained why I was there and what a Hump was, they seemed a little interested and slightly bemused but did not mock!  They were out trying to find a fox that had been taking chickens from two nearby farms.  I asked if it was all right to visit the summit and also the adjacent field, they didn't mind at all.


The field that was visually dismissed as lower than the track is on the right of this photo
After 20 minutes of data had been gathered I packed the equipment away and headed up to the summit.  The mark where the Trimble had been set up on the ground beside the summit cairn on the 30th January was still there, but I chose a slightly different spot about 20cm away that seemed a little higher.  As a brisk breeze was blowing across the hill I employed a method used by Alan Dawson and pushed the pole in to the ground, attached the external antenna, connected it to the Trimble and measured the distance between the tip of the screw thread just below the bottom of the antenna and the ground, and input this in to the equipment as a measurement offset.  The 0.1m accuracy was soon attained and another 20 minutes of data was gathered.


The set up position next to the summit cairn
The Trimble set up at the summit with Bwlch Mawr (SH 426 478) in the background
During this wait I wandered around the earthen ramparts of the ancient hill fort and took a number of photos of the equipment.  The hill has good views from its summit with the coastline heading north toward Caernarfon and Ynys Môn and the Yr Eifl range of hills on view toward the south-west.


The screen on the Trimble, giving 21 satellites locked onto, battery approximately half full, 10cm accuracy attained and 321 points (seconds) of data gathered.
After the equipment had been packed away I headed back to the bwlch as I wanted to get a data set from the adjacent field.  I’d done the same thing during the first survey, as I’d now got permission to visit the field I had confidence in wandering about assessing the line of the bwlch on the valley to valley traverse.  The conclusion was the same as last time; the critical bwlch is placed on or beside the track.  I picked a spot close to a telegraph pole and placed the Trimble on the grass in the field and collected 11 minutes of data with its internal antenna, and happy in the knowledge that I’d done as much as I could with the equipment at hand I headed back to the car.


Survey Result:


Foel

Summit Height:  221.7m (converted to OSGM15)

Summit Grid Reference:  SH 45021 50673

Bwlch Height:  120.1m (converted to OSGM15)

Bwlch Grid Reference:  SH 45819 50104

Drop:  101.6m

Dominance:  45.82% (final 3rd survey gives 45.72% Dominance)


The first survey gave a drop of 100.9m with an inadequate data set from the bwlch.  The second survey gives 101.6m with a good data set from the bwlch and a slightly higher summit value, which is partly based on a slightly higher position for the Trimble.  On each survey data was gathered from a position in the adjacent field to the bwlch, both results show map contouring at the bwlch to be on the high side as each result is just below 120m.  Therefore this hill is a proposed new Hump.



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

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