Mapping Mountains – Trimble Surveys – Moel Hebog

16.02.14  Foel (SH 450 506)

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

Foel (SH 450 506)

Before detailing the third survey of Foel (SH 450 506) it may be best to outline a recent visit to Daresbury that John and I made to gather Trimble data at its Ordnance Survey Fundamental Benchmark (the height and position of Fundamental Benchmark’s are known to a high degree of accuracy).  The data sets collected indicated that there is a systematic error occurring of approximately 10cm when the Trimble is positioned on a pole with its external antenna.  However, further tests are required which have already started.

The third survey was a combined effort with Leica and Trimble technology employed.  The previous two surveys had resulted in 221.4m summit; 120.5m bwlch with 100.9m drop (1^st survey) and 221.7m summit, 120.1m bwlch with 101.6m drop (2^nd survey).  However, the first bwlch survey never attained the required o.1m accuracy and was the reason why the second survey took place and the second summit survey used the pole and external antenna and was positioned a few centimetres higher than the first summit survey position.  If the first bwlch and second summit results are discounted it indicates that the summit of Foel is 221.4m (plus about 7cm) and the bwlch is 120.1m, giving a drop of 101.4m.  It’ll be interesting to see how the processed results from the third survey compare to these measurements.

John, Graham and I met in the car park opposite the Texaco garage in Pen-y-groes (SH 470 529).  We drove through Llanllyfni and parked at SH 458 500, this is at the start of the track that leads to Llwyn Bedw Uchaf and crosses the critical bwlch.  As we got our boots on (wellies for me!) the sky was a radiant blue and the weather forecast was good for the rest of the day, which made a change from the incessant rain that had been sweeping over the country for weeks upon end.

We had a quick look at the area of the bwlch when we walked down the track and then headed for the summit.  Half an hour after leaving the cars we were approaching the final few metres of ascent.  John and Graham used the level and staff to find the highest part of the hill whist I noted all measurements and tried frantically to take a few photos in between plotting where each staff position was in relation to the position of the level and the summit cairn.  The highest point proved to be where the second Trimble summit survey had taken place; unfortunately we couldn’t take a relative height reading with the staff between the first and second survey positions as the Barnard boot had made a severe indent in the ground!

Surveying for summit position.

The Leica GS15 was soon set up on its 2m pole over the high point of Foel and we stood with our backs to the chilly breeze and chatted, eventually we took shelter on the leeward side of the breeze.  By this time cloud had materialised over Bwlch Mawr (SH 426 478), its bulbous nature was reminiscent of a volcanic explosion that had its centre at the summit of Bwlch Mawr, spewing its ash cloud inland to the higher peaks of Eryri.

The Leica GS15 set up on its 2m pole at the summit of Foel.

Once the Leica GS15 had gathered an hour of data we positioned the Trimble on the high point and gathered 11 minutes of data.  With all required summit data collected we reversed the inward route and headed down to the area of the bwlch, calling in at Llwyn Bedw Uchaf on the way to ask permission to take readings in the fields either side of the track.

The Trimble GeoXH 6000 gathering data at the summit of Foel.

We each placed a flag in the ground at the position where we thought the critical bwlch lay.  This is a practice we aim to do on most of the G&J Surveys outings as it will help in comparing how bwlch and also summit position is judged purely using eyesight when compared to the added advantage of using a surveyor’s level and staff.  I’m proud to say that my flag won – YYIIPPPEEEE!!!!

Surveying for bwlch position.

The critical bwlch was found to be positioned within 2cm in height and about 1m in position from each of the previous Trimble bwlch surveys.  John then assembled the Leica GS15 in a convenient spot in the field to the south of the track, whist I headed in to the field to the north of the track to chat with a farmer who had come up on quad bike to check on what we were doing.  He was the owner of the land that takes in the summit of the hill as well as the field to the north of the track.  We chatted for ten minutes or so and I explained why three people were wandering around with all manner of unusual looking bits of equipment.  He didn’t mind at all that we had visited the summit and kindly gave permission for us to take readings in the field and seemed quite interested in everything we were doing.  By the time I’d re-joined John and Graham the measurement offset between the critical bwlch and the Leica GS15 set up position had been measured with the level and staff.

Another hour of data was collected by the GS15 before we took three data sets with the Trimble.  Two on the Leica 2m pole (one calibrated to the ‘Top of notch’ and the next to ‘Bottom of antenna mount’) and the third with the Trimble positioned on the ground.  This will give us three sets of data to compare to the GS15 measurement and may answer the question why the Trimble seems to have a 10cm systematic error when operating with its external antenna on a pole.

John beside the Trimble set up position at the bwlch of Foel.

Five hours after setting off for the hill we arrived back at the cars.  John and Graham headed back to Mold and Chester respectively whilst I navigated my way around the narrow lanes to visit Mynydd Cennin (SH 458 449).

Survey Result:

Foel

Summit Height:  221.3m (converted to OSGM15, Trimble)  221.4m (converted to OSGM15, Leica GS15)

Summit Grid Reference:  SH 45021 50673

Bwlch Height:  120.2m (converted to OSGM15, Trimble)  120.2m (converted to OSGM15, Leica GS15)

Bwlch Grid Reference:  SH 45818 50103

Drop:  101.1m (Hump addition confirmed)

Dominance:  45.72%

The average of the three data sets for the Trimble bwlch data is 120.166m (converted to OSGM15) which is in good accordance with the 120.159m (converted to OSGM15) attained from the Leica GS15. 

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