12.11.15
Gilwern Hill (SO 098 582) and Pt. 409.9m (SO 083 579)
Gilwern Hill (SO 098 582) |
Two hills that are
listed as Pedwarau with Gilwern Hill also being listed as a Submarilyn, this
higher prominence hill has an interesting summit height as it is currently
listed as 439m high in the Marilyns list and as 441m high in the Pedwar
list. The Marilyn height is based on the
current Ordnance Survey map spot height whilst the Pedwar height is based on
the 441.045m flush bracket height adjoined to the trig pillar that is situated on
the summit area of the hill. And what
better way to sort the height of Gilwern Hill out than with two of the three
hill list authors who put their names to the Marilyns and Pedwarau lists.
Alan had stayed the
night in Welshpool on his way to the Bagger Rambles meet at Plas Tan y Bwlch
in Maentwrog, and although the weather forecast was not good we decided that a
survey needed to take place, with an option being to choose a hill that
appeared in both listings and one where the data did not correspond and Gilwern
Hill fitted the bill perfectly.
As I drove south and
Alan navigated me toward these hills the grey, murky cloud descended and
drizzly rain enveloped the land. Thankfully
the track approaching these hills from the south-west is driveable and we
parked adjacent to a track junction at SO 087 580.
As we set off the cloud
was down and an autumnal dampness pervaded the land, we’d come well prepared with
three lots of surveying equipment, my Trimble GeoXH 6000, Alan’s Leica RX1250
and another attractive little yellow number, looking all coy and relatively new,
this was Alan’s new toy and it matched mine perfectly. We’d also come well prepared as wellies were
de rigueur for the day, these are marvellous inventions and are essential pieces
of hill walking kit for lower heighted autumnal and winter hills.
Somewhere in the gloom is the summit of Gilwern Hill |
We soon left the track
and followed an indistinct path across fields to the murky heights of Gilwern
Hill’s summit, this is crowned by an ancient cairn that is now grassed over and
which has a trig pillar perched on top of it.
The perimeter of the cairn can be relatively easily distinguished but
all now merges into one, with field and cairn and what constitutes a hill’s
summit blending and merging, and we both marched to the top of this grassed
ancient construction and assessed where the high point lay. This was a patch of grass adjacent to a
number of small rocks all of which moved with none being embedded.
Once the high point had
been determined Alan set his Leica up and we stood below the trig on the
leeward side of the grassed cairn until 30 minutes of data were collected. During this Alan and then I took data with our
Trimbles from the top of the trig to compare against the flush bracket height
given in the OS Trig Database with a measured 0.89m offset from top of the
flush bracket to the top of the trig.
The Leica RX1250 set-up position at the summit of Gilwern Hill |
After the Leica had been
dismantled Alan took a data set with his Trimble from the high point and then I
followed with a five minute data set with my Trimble. Happy that we’d got sufficient data we packed
everything away and headed back toward the car and our next summit which was
conveniently positioned no more than five minutes from where the car was
parked.
Gathering data with my Trimble GeoXH 6000 from the summit of Gilwern Hill |
By now it was obvious
that the wet conditions had set in for the day, but the mistiness that had
skimmed the summit of our next hill had risen slightly, and although everything
around was damp the rain was not sufficiently heavy to be uncomfortable.
I quite like symmetry |
Alan beside the Leica RX1250 and the two Trimble GeoXH 6000's |
The summit area of our
next hill proved relatively flat and therefore we set the three pieces of equipment
up within a metre or so of one another and had them all gathering data at the
same time, which made me smile. Alan's Leica and Trimble were set up at approximately the same height so he could compare their data, whilst each was positioned approximately 2-3cm below where I had positioned my Trimble. During
this a vehicle drove down the track from the direction of Upper Gilwern which
is the farm between these two hills, I felt like waving and trying to stop it
and running down to ask about the names of the hills as the one we were
currently gathering data on is listed using the point (Pt.) notation in the
current edition of Y Pedwarau.
Gathering data with the three pieces of equipment from the summit of Pt. 409.9m |
Once data were collected
we packed everything away, walked back to the car and drove to the area of the
bwlch for Gilwern Hill. This is
positioned close to the A44 and I had zoomed around on this road during the
morning in a Google Car and found a parking place close to a gate which gave
access to a field and the area of the bwlch.
By the time we had parked
and walked back up the road toward the gate the murky afternoon was quickly
turning dimmer as dusk set in. We found
a gate and bashed through hawthorns and soon were heading down toward the
bwlch, this proved relatively easy to find its critical point, and once we had
assessed the lay of land from a number of directions Alan set the Leica and his
Trimble to gather data.
The Leica RX1250 set up at the critical bwlch of Gilwern Hill |
As data were gathered we
chatted away as afternoon murk turned to dimmed dusk and by the time I set my
Trimble up to gather its customary five minutes of data it was turning decidedly
darkish.
After all data were
gathered we sloshed our way back up the field with Alan first and me scampering
behind, losing one another in the darkening dimness and re-joining each other
back at the car. All that remained was
the drive back home, a quick change, wash, snack, followed by Montgomery Film
Club and an excellent Locke and a yummy Ginger Chicken in Spice UK afterwards….
Good times.
Survey Result:
Gilwern Hill
Summit Height: 440.7m (converted to OSGM15, and average of two Trimble GeoXH 6000 surveys) 440.7m (converted to OSGM15, Leica RX1250)
Summit Grid Reference: SO 09895 58290
Bwlch Height: 294.0m (converted to OSGM15, and average of two Trimble GeoXH 6000 surveys) 293.9m (converted to OSGM15, Leica RX1250)
Bwlch Grid Reference: SO 14378 60966
Drop: 146.6m (average of two Trimble GeoXH 6000 surveys) 146.8m (Leica RX1250) (Submarilyn status confirmed)
Dominance: 33.30% (using Leica RX1250 results) (non Lesser Dominant status confirmed)
Dominance: 33.30% (using Leica RX1250 results) (non Lesser Dominant status confirmed)
Pt. 409.9m
Summit Height: 409.8m (converted to OSGM15, and average of two Trimble GeoXH 6000 surveys) 409.9m (converted to OSGM15, Leica RX1250)
Summit Grid Reference: SO 08331 57995
Bwlch Height: 379.2m (LIDAR)
Bwlch Grid Reference: SO 08711 58270 (LIDAR)
Bwlch Height: 379.2m (LIDAR)
Bwlch Grid Reference: SO 08711 58270 (LIDAR)
Drop: 30.7m (Leica RX1250 summit and LIDAR bwlch)
Dominance: 7.49% (Leica RX1250 summit and LIDAR bwlch)
Dominance: 7.49% (Leica RX1250 summit and LIDAR bwlch)
The comparisons between the results produced by the
two Trimbles are as follows:
Gilwern Hill (top of Trig) = 440.742m (AD)
440.825m (MP) (all heights converted to OSGM15)
Gilwern Hill (summit) = 440.617m (AD)
440.691m (MP) (all heights converted to OSGM15)
Gilwern Hill (bwlch) = 294.048m (AD)
294.033m (MP) (all heights converted to OSGM15)
Pt. 409.8m SO 083 579 (summit) = 409.879m
(AD) 409.748m (MP) (all heights converted to OSGM15)
For further details please
consult the Trimble survey spreadsheet click {here}
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