Monday, 17 April 2017

Mapping Mountains – Trimble Surveys – Cefn Digoll



24.03.17 & 28.03.17  Cefn Digoll (SJ 264 058)

Cefn Digoll (SJ 264 058)

The high point of Cefn Digoll (known as the Long Mountain in English) is a copse of trees in the centre of a well preserved ancient hill fort.  Immersed in the trees is a triangulation pillar and it is land 3-4 metres from its base which is the high point of the hill.

The hill fort encircles the copse of trees, and its occupation is dated to the later part of the Bronze Age and the early part of the Iron Age.  The hill fort is known as Caer Digoll in Welsh and Beacon Ring in English.

Information board at the entrance to the wood at the summit of Cefn Digoll

On a chilled crisp day in December 2009 I visited the summit of Cefn Digoll with John Barnard and Graham Jackson with our aim to ascertain where the high point lay.  We used a level and staff and took readings on each high point of the ancient earthen embankment and proceeded in to the wood to the ground beside the trig pillar.  We found that the ground 3-4 metres from the base of the trig is 0.29m higher than the high point of the ancient embankment.  Today I wanted to re-visit Cefn Digoll and take a Trimble reading from the embankment high point which in affect would also then give the summit height.

The western side of the ancient earthen embankment

Access to the summit is relatively easy and I parked in a passing place near to the track that leads toward the two masts that adorn the summit area of the hill.  As I walked up the track the sun shone from a deep blue sky with a brisk easterly breeze adding a spring chill to proceedings.

I walked round the northern part of the ancient embankment before entering the wood from its west, the trig pillar is situated in the centre of the wood and I waited a few minutes with the Trimble positioned on the high point to note a ten figure grid reference for the summit.  During this dabbled light cascaded through the trees, colouring their bark and lighting an otherwise dark place.

Somewhere amongst the trees is a trig pillar


The Trimble marks the high point of Cefn Digoll


The trig pillar with the high point of Cefn Digoll about 3-4 metres to the right and where the Trimble is just visible at the base of a tree

Leaving the wood to its east I gained the top of the embankment and walked to its high point, I spent a couple of minutes stomping down a number of brambles and then positioned the Trimble on top of my rucksack to give it elevation above its immediate surrounds, I then measured a 0.41m offset between its internal antenna and the ground at the base of the ruck sack and waited for the 0.1m accuracy level to be attained before data should be logged.  As this high point of the ancient embankment is 0.29m lower than the summit beside the trig pillar it meant that with the 0.41m offset, that the Trimble was positioned 0.12m above the height of the hill’s summit.

Gathering data at the high point of the earthen embankment

Once five minutes of data were collected I switched the equipment off, took a few photographs, packed it away and walked back to my car and headed down into deepest darkest England to investigate the bwlch connecting this hill to the next higher summit.

This bwlch is positioned in the vicinity of Marton, which is a village situated a couple of miles across the border in England, with the bwlch either being beside Marton Pool, or to its east or to its south-west, dependent upon whether the water flowed in to or out of the lake.  Marton Pool is approximately 25 - 30 acres in area, having been upwards of 70 acres in area many years ago, its drainage and the flat bottomed nature of its immediate land meant that the adjacent fields would probably be wet, I didn’t have concern as I’d come prepared and wore my wellies.

Marton Pool

I wanted to take a series of data sets from the vicinity of the lake, but my priority was the find which direction water flowed on the eastern and the south-western sides of the lake as Ordnance Survey maps imply that an outflow on the east formed the infant Rea Brook, whilst the water on the south-west formed the infant Aylesford Brook.

Parking my car close to where a 103m spot height appears on the map at SJ 288 021 I took a data set with the Trimble positioned on the car roof with a 1.44m measurement offset to the road below, and then another data set in the adjacent field to the south beside a culvert pipe feeding a dried up dyke.  The connection of this pipe ran under the road to another drainage ditch in the adjacent field to the north.  Once data were stored I drove a mile or so north-east to the Marton Pool Caravan Park.

Gathering data on the southern side of the lane

I hoped that I could walk to the eastern part of the lake from the caravan park and as I drove down its access lane I waved to a man on a small pick-up vehicle, we stopped and chatted and I explained what I hoped to do, he explained that the water on the eastern side of the lake flowed out from a sluice gate and then offered to take me through the caravan park to the water’s edge.  I left my car in the small car park and we then chatted for about 15 minutes, beside us was an inflow from the Lowerfield Brook and he confirmed that the water drained out of the lake to the east, this meant that the bwlch for Cefn Digoll must either be on the south-western part of the lake or immediately beside the lake if the water also drained out on the south-western side.  He advised me to call at the pub in Marton as the fields behind led to the remains of an old boat house beside a part of the lake.  Before leaving I took a data set to ascertain the height of the water level.

On occasion life waiting for the Trimble to gather data can be a beautiful experience

During taking this data set I stood and looked out across the water to Corndon beyond, a subline image with breeze blown movement of bull rushes adding depth to the scene.

Gathering data to ascertain the height of the water level in Marton Pool with Corndon in the background

My next objective was the south-eastern side of the lake; I called at the pub and was invited in although it was locked for an afternoon siesta.  The man at the pub told me the route down to the lake which I followed across a couple of fields.

Between the lake side on its south-eastern side and the lane where I took the first data set are a series of water channels, I investigated two close to the lakeside and threw clumps of grass in to one and the water trickled in to the lake, this meant that in all likelihood the bwlch was positioned between the south-eastern side of the lake and the lane.  I proceeded to take a further three data sets in the fields following the series of water channels as they headed toward the lane.  I hoped to follow the main drainage channel to the lane but gave up when confronted with a field of penned in horses, happy that I’d taken sufficient data to answer where the bwlch was roughly positioned I headed up a field toward the houses in Marton, only to be confronted with a number of back gardens, thankfully a woman had spotted me and kindly directed me through her goat pen, past her chickens and rabbits and out toward the road next to her house.  I thanked her and explained what I was attempting to fathom, we talked about the drainage from the lake, and the natural spring emanating from her field and a multitude of other things.  She informed me that the culvert under the lane is blocked but the water to its north flowed in to the lake and the water to its south flowed down the valley, this meant that the position of the 103m spot height on the lane would be roughly where the bwlch for Cefn Digoll is positioned.

Gathering data beside one of the water ditches feeding into Marton Pool which is shielded by the trees in the background

I left Marton as the sun was sinking and the chilled air heralded a spring evening happy in the knowledge that a number of data sets were ready to process and their results would hopefully confirm that the land north of the lane increased in height as it neared the lake.

Postscript:

Having processed the data the results confirmed that the land headed down toward the lake on the northern side of the lane, however once the results of this Pedwar survey were forwarded to Aled he checked the area of the bwlch against LIDAR data and sent me the height and position of what LIDAR determined to be the remains of the natural bwlch, this was positioned close to the drainage ditch feeding the blocked culvert on the northern side of the lane, and therefore four days after surveying the summit and various points in the area of the bwlch of Cefn Digoll in glorious sunshine I headed back to take a series of data sets at the point where LIDAR data positioned the bwlch.

It was early morning when I pulled my car up beside a gate entrance on the lane, and a few minutes later the Trimble was positioned on top of my rucksack gathering data.  Thankfully the lay of land was not difficult to assess, and having dismissed the drainage ditch as having been tampered with by man and the earthen embankment above it as man-made, the Trimble was gathering data at the point where I judged the valley to valley traverse was at its highest and the hill to hill traverse at its lowest.

Gathering data at the critical bwlch of Cefn Digoll

It was a beautiful morning with a slight breath of breeze whisping the land as mist enveloped the early morning scene.  I was so at peace standing in the field waiting for the Trimble to gather its allotted five minutes of data that I collected three data sets from the same position with the aim to average the results and use the figure attained as that for the bwlch of Cefn Digoll.        



Survey Result:



Summit Height:  408.3m (converted to OSGM15)

Summit Grid Reference:  SJ 26476 05824

Bwlch Height:  103.3m (converted to OSGM15, and average of three Trimble surveys)

Bwlch Grid Reference:  SJ 28827 02177

Drop:  305.0m

Dominance:  74.69% 
 






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