Sunday, 23 July 2017

Mapping Mountains – Trimble Surveys – Moel Hebog

04.06.17  Moel Tryfan (SH 517 559) and Moel Tryfan (SH 515 561)

Moel Tryfan (SH 515 561)

Moel Tryfan stands as the north-western outlier of the higher Mynydd Mawr, and is designated a site of special scientific interest  and is one of the most important places in the country to examine geology, as in many places the rocks have been exposed due to quarrying activities that have resulted in the upper part of the hill being split in two, with the south-easterly part lower, it is also this lower part of the hill that Aled analysed via LIDAR data and through this a new 400m Sub-Pedwar has been listed, as in affect the quarry has created a north and south bwlch between the two tops with the land between having been ripped apart, leaving a gaping hole.

My plan was to survey each summit and bylchau and compare the Trimble data with that produced by LIDAR and also that produced for the higher summit by the survey with the Leica GS15 which was conducted in July 2013.

Access to the hill from the south is easy as a paved road that turns into a gravelled track leads from the last few houses east of Y Fron.  As I walked up the track a runner sped down, we stopped and chatted and she told me that the gun club were firing just below the summit, I appreciated hearing this as otherwise the sound of gunfire may have proved a little disconcerting.

Away to the south grey cloud was already massing around the higher peaks and with a forecast of heavy showers in the central part of the country I hoped that I’d picked a good and dry place to walk today.

Approaching Moel Tryfan

The track I was on headed toward the southern bwlch which is the higher and critical bwlch for the 400m Sub-Pedwar, the view north was down to exposed quarry lakes and the hills beyond.  I used the Trimble as a hand-held GPS and zeroed in to the grid reference produced by Aled’s LIDAR analysis, I’ve learnt to have confidence in these positions as LIDAR is proving a revolutionary tool for positions and heights associated with hills.

Once the Trimble was set up and gathering data I walked down the track to gain a better view of the 400m Sub-Pedwar, the whole of its western face was now exposed, having been ripped apart leaving an extended cliff and waste spoil of rubble.

Gathering data at the critical bwlch of Moel Tryfan (SH 517 559)

Moel Tryfan (SH 517 559) the recent addition to the 400m Sub-Pedwarau

Packing the Trimble away a narrow path led across a flat rubbled land bridge and continued toward the summit, which proved to be the second high point amongst heather overlooking a large drop to the quarry lakes below.

When surveying with John and Graham one of the most enjoyable times for me was the wait during data collection, this was a time to talk, but also a time to watch the landscape in all its moods as light appeared and shadow cast down, and although the timeframe  surveying with the Trimble is very different as five minutes of data gives accurate results, this waiting process is still one where I stand and look and appreciate the surroundings, and whilst the Trimble gathered its allotted summit data I looked out to the rock outcrop atop the higher of the two Moel Tryfan summits and down into the great gash of the quarry, which is now a peaceful place where once man’s activities coursed the whole eastern part of the upper section of this hill to be ripped apart.

Gathering data at the summit of Moel Tryfan (SH 517 559)

The Trimble set-up position at the summit of Moel Tryfan (SH 517 559)

Leaving the lower summit I soon joined another track leading to the northern bwlch, and again the ten figure grid reference produced by LIDAR took me to the point for the Trimble set-up position.  As the Trimble gathered data the sky turned deep grey and a few wind-blown raindrops sped across the hill, not wanting to linger I quickly closed the equipment off after it had gathered five minutes of data and headed up to the main summit on a rutted and earthen track before a grassed path led to the rock outcrop at the top.

Gathering data at the lower northern bwlch

The rock outcrop at the summit of Moel Tryfan (SH 515 561)

An information board is pinned to the lower face of the rock outcrop and states that on Sunday 26th June 1842 Charles Darwin stood on the summit of Moel Tryfan and studied its rock formations.  Today the summit was quiet, except for the Trimble beeping away gathering datum points perched on the high point of the hill and wedged in place with two small retaining rocks that I’d brought for this specific purpose.

The information board pinned to the rock outcrop

As the Trimble gathered data I busied myself writing down all required information and taking a number of photos, once the equipment was closed off and packed away I followed a path down to the southern bwlch and retraced my inward route back to the car.

The summit of the higher of the two Moel Tryfan peaks with the Trimble gathering data perched on top of the rock outcrop

The Trimble set-up position at the summit of Moel Tryfan (SH 515 561)

Looking across Dyffryn Nantlle to Y Garn

Survey Result:

Moel Tryfan

Summit Height:  405.4m (converted to OSGM15) 
Summit Grid Reference:  SH 51780 55926

Bwlch Height:  384.0m (converted to OSGM15)

Bwlch Grid Reference:  SH 51522 55908

Dominance:  5.28%

Moel Tryfan

Summit Height:  428.9m (converted to OSGM15, Trimble GeoXH 6000) 429.0m (converted to OSGM15, Leica GS15)  
Summit Grid Reference:  SH 51528 56190

Drop:  102.6m (converted to OSGM15, Leica GS15 summit and bwlch)

Dominance:  23.92%

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