Friday, 2 October 2015

Mapping Mountains – Trimble Surveys – Y Berwyn

29.08.15  Winllan Hill (SJ 217 214)  

Winllan Hill (SJ 217 214)

Having waxed lyrical on yesterday evening’s walk to Winllan Hill I re-visited its summit this morning and took a further five data sets as yesterday’s data, when processed, showed the Estimated Accuracies to be 0.5-1m: 57.49% and the Standard Deviation to be 0.9m.  The detail within this data set is probably the worst the Trimble has produced during the last 21 months.  Perhaps the large overhanging tree at the summit obstructed satellite coverage to a greater degree than I had imagined, or perhaps it was getting too late and the Trimble just needed to go to bed?

As the afternoon’s forecast was for heavy localised showers I wanted a relatively early start and by 9.30am I was back on top of the hill.  On our previous evening’s walk Mark and I had ascended this hill from the west, this morning I parked in a pull-in spot and walked up the paved road / track that leads to Plas Onn and approached this hill from the north.

I knew exactly what I wanted to do and had come well prepared with every available piece of surveying equipment that I possess.  My aim was to gather a number of data sets and to do so in a variety of ways; this at least would give a comprehensive comparison between styles and the heights produced.  At the very least this would give an opportunity for the average of all data sets to be taken as the height for this hill.

When I arrived at the summit I stored the majority of gear beside the base of the large overhanging tree and headed down to where the 216m spot height on current Ordnance Survey maps appears on the ground.  This is situated at or near to a ringed and grassed natural embankment which resembles the remains of an earthen embankment that one would usually associate with an ancient hill fort, but there is no map evidence that one ever existed on this hill. 

Once this point had been identified I set the Trimble up and as it gathered its five minutes of data I assembled my old measuring staff.  Once the data set was complete I closed the Trimble down and left it in situ and proceeded to take a reading at a 4½ ft height level from the land between where the Trimble was placed and where the high point of the hill is situated under the large tree.  By aligning the fixed spirit level on my measuring staff with the base of the Trimble I then turned 180° and took a reading toward the high point of the hill.  Once re-positioned at this point it meant that this ground was approximately at the same height as the position of the Trimble, I then took readings to the summit with the conclusion being that the summit of the hill is approximately 9ft 2higher than where the Trimble was placed, which was at or near to where the 216m spot height appears on the ground.

The 1st of the morning's data sets - gathering data from near to where the 216m spot height appears on the ground

I then took a second five minute data set with the Trimble from the same spot, and once complete I repeated the measurement with my old staff but started from the position of the Trimble, this meant a greater horizontal distance for the first 4½ ft vertical height measure, by doing this I would have a comparison of measurements taken with the old staff.  This second measure gave a vertical height gain between the Trimble position and the summit of approximately 8ft 10with the average of the two measurements with the old staff being 9ft.

The 2nd of the morning's data sets - with the Trimble just evident on the top of the grassy mound to the right and the much higher summit of the hill on the left

I now wanted to try and get another data set from the high point, whereas the Trimble was set up on top of my rucksack the previous evening with a 0.43m measurement offset recorded, I now positioned the Trimble on the improvised Tupperware tripod which elevated its internal antenna by 0.18m above the ground.

During this data collection I stood behind the large tree out of view of the Trimble and only went out into the field to encourage sheep to graze in another part of the field, as they were slowly encroaching westward toward where the Trimble was now gathering data. 

The 3rd of the morning's data sets - with the Trimble at the summit set up on its improvised Tupperware tripod

Once this third morning’s data set was complete I switched the equipment off and grabbed my measuring staff as this time I wanted a data set with the Trimble positioned 4½ ft below the summit of the hill which would probably give it a greater chance of achieving a good data set.  I proceeded to take readings at a 4½ ft height level until I had aligned the spirit level with the high point of the hill, I then positioned the Trimble on top of my rucksack and measured a 0.42m offset.

By now the sky towards The Wrekin away to the east was breaking out into heavy showers, thankfully these were forming behind Cefn Digoll (the Long Moutnain) and were pushed northward into Shropshire, but as I started to sort out the gear for the fifth and last of the morning’s data sets I noticed a large grey patch of sky forming in the Dyffryn Meifod and as this was closer to Winllan Hill, I did not want to linger.

To the east showers were breaking out over The Wrekin

As the Trimble gathered data positioned 4½ ft below the summit and 0.42m above the ground on top of my rucksack I assembled the tripod and fixed the external antenna to its top.  I haven’t used this tripod for a number of months, as it is always more convenient to use the Trimble operating with its internal antenna, but necessity dictated that a variety of surveying methods would give a comprehensive coverage in determining the height of this hill.

The 4th of the morning's data sets - with the Trimble positioned 4½ ft below the summit and on top of my rucksack

Once the fourth morning’s data set was completed I assembled the tripod and Trimble over the high point of the hill and set it to gather data.  The Trimble always looks good when on its tripod and the external antenna was now positioned above the fence which runs east – west adjacent to the high point of Winllan Hill.  However, the large tree was overhanging the position of the tripod so it’ll be interesting to compare this data set with the others.

The Trimble connected to its external antenna at the summit of Winllan Hill

The 5th and last of the morning's data sets - with the Trimble attached to its external antenna and positioned on its tripod

As I closed the Trimble down after it had stored the last of its data the showers that had started to appear were still being pushed down into Shropshire away to my east, and patches of sunshine were now breaking out around me.  Once all my gear had been checked and packed away I retraced my steps back down the hill and headed home.

The view south-west from the summit of Winllan Hill

Survey Result:

Winllan Hill

Summit Height:  220.3m (average of six surveys and converted to OSGM15) (significant height revision)

Summit Grid Reference:  SJ 21709 21460 (summit relocation confirmed)

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

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