Monday, 25 April 2016

Mapping Mountains – Trimble Surveys – Pegwn Mawr


20.04.16  Pt. 436.3m (SO 062 850) 

Pt. 436.3m (SO 062 850)

During last summer I’d visited this hill and Trimbled its bwlch and summit with the result giving the hill 29.983m of drop, after discussing this result with Aled we decided to leave the hill listed as a Pedwar until a further survey could be conducted.  As the Trimble has limitations on its accuracy when the margin between 30m drop qualification is only 17mm, I knew that the ideal way to re-survey this hill would be by line survey; however I could at least take a series of data sets with the Trimble and assess the results with Aled afterward. 

The forecast for the day was ideal with only a breath of breeze and sunshine predicted during daylight hours.  Thankfully the hill is easily accessed from a minor lane to the south of its summit.  I set off walking at 8.00am down the green vehicle track that leads to the bwlch.  Looking toward the hill the sky gave a dulled pale sheen as blue merged with white on the horizon.  All round was quiet and except for an occasional farmer out feeding sheep, it remained so for the four hours it took me to take a further 19 data sets, with 9 of these being at the bwlch and 10 at the summit.

When approaching this hill from the south the critical bwlch is crossed on the way to the summit, I’d come duly prepared with ten figure grid references for the positions where I’d previously taken data from, as well as the ten figure grid references for where the 406m bwlch and 437m summit spot heights appear on the ground.

As the green vehicle track crosses the area of the bwlch the land descends on its west, whilst on its east is a stagnant reed infested pool, the water from which does not cross the track, therefore unless water flowed in to the pool from its east the critical bwlch lay either on, or beside the vehicle track.

The first data set I took at the bwlch matched the co-ordinates of my previous survey from last summer, I then spent a long time assessing the ground and placed the Trimble approximately 18 inches from this first placement and gathered another five minutes of data.  Three further data sets were repeated from this second point, each time alternating between the Trimble being placed on the ground and being placed on top of my rucksack with a 0.41m measurement offset between the internal antenna within the Trimble and the ground at the base of my rucksack.

Looking south from the bwlch with the green vehicle track heading back to my car 

Looking north from the bwlch toward the summit of the hill during one of nine data sets taken at the bwlch

After each data set was gathered from different points I marked the position of the Trimble so if need be another data set could be gathered from the same position and the resulting data then averaged.

Following these surveys I took a data set from where the 406m spot height appears on the ground and a further two data sets from the vehicle track adjacent to the spot height position, one from each side of the track.

The only remaining data set to gather at the bwlch was from the opposing easterly side of the pool, this would prove that it was lower than the track although visually it was significantly lower, I only wanted to do this if the water flowed in to the pool and not out of it, as if it did, it meant that the ground to the east of the pool is lower than the surface water.  I followed the water course from the pool and it remained still for a number of metres, until I spotted a tranquil trickle as it glistened in the early morning sun, I continued following it until the ground started going downhill, working my way back toward the pool I looked for any sign that the water course was not continuous, there was none, therefore the surface water of the pool was lower than the height of the vehicle track.

With all bwlch surveys now complete I gathered the Trimble up, packed it away and walked to the summit following the continuation of the track and finally across a field to where the rounded dome of the high point of the hill is situated.  This rounded dome is a slight ridge; whale backed in shape and aligned in a north to south direction, to its east is a visually lower ridge of same formation.

The first data set I took was from where the 437m spot height appears on the ground, this point was on a downward slope and although not necessary to conduct, it would at least give another accurate height for this particular point.  The second data set was taken from where I had placed the Trimble during last summer’s survey.  I now wanted to take a series of data sets following the flattened ridge crest working from north to south, each Trimble placement was approximately seven metres apart, with eight taken in all on the whale backed summit ridge.

As I closed the Trimble off at each of these points I placed a piece of paper on the ground with a nail inserted in it to indicate where the internal antenna had been aligned.  Leaving the paper trail in situ I then visited the visually lower ridge and took a data set from what I deemed to be its highest point.

The position where the Trimble is placed proved to be the highest point surveyed and was the last of ten data sets taken at the summit, with three placements indicated with paper running the length of the whale backed ridge in a southerly direction

Looking northward with the Trimble placed on the highest point surveyed and with the paper indicating a further four placements where data had been gathered

Only one survey remained for me to conduct and this was to the point of the hill that I now thought to be the highest.  I assessed the summit ridge from the vantage point of the visually lower ridge and decided to place the Trimble between what I deemed to be the two highest previous placements.  As the Trimble beeped away gathering its allotted five minutes of data during the last of 19 data sets I stood a safe distance away, scribbling all detail that would form a part of the spreadsheet documenting all of my Trimble surveys, and happy in the knowledge that I had done as much as I could with the Trimble to decide the fate of this hill, I gathered it up, packed it away and happily sauntered off down the hill in continuing glorious sunshine to my awaiting car below.

  
Survey Result:


Pt. 436.3m

Summit Height:  436.3m (converted to OSGM15)

Summit Grid Reference:  SO 06252 85015

Bwlch Height:  406.2m (converted to OSGM15)

Bwlch Grid Reference:  SO 06027 84366

Drop:  30.2m (Pedwar status confirmed)

Dominance:  6.92%




For the post relating to the first survey of this hill please click {here}


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


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