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Notes on the survey of Tyning's Barrows Swallet

by D.J. Irwin

By now the reader will have read one of the several accounts dealing with the breakthrough and exploration of this new Mendip system.  As an aid to exploration a BCRA grade 5c survey commenced on the 26th February, 1977 and during the course of the next few weeks the survey was completed except for Aardvark and Bertie's Paradise.

The equipment used is of some interest to surveyors.  Basically it consisted of the Suunto compass (KB14/360) and clinometer (PM5/360) coupled together in the form of a handheld surveying unit.  This eliminated the problem of handling the separate instruments and in low, awkward passages this was a great advantage.  Details of the connecting bracket is being prepared by Chris Batstone. In use this combination of instruments enabled rapid readings to be made.  The tape was a Chesterman 100 ft. fibron.  Due to the rather constricted nature of the passages and the urgency of getting the main line surveyed it was decided to produce a grade 5c survey.  At each station care was taken to minimise the accumulation of station error by pivoting around the instruments and on occasion using rock features to hold the instruments.  Because the scale at which the drawing was produced (1/480) the drawing error would be considerable greater than the survey random errors.

The instruments were read to the nearest 1O and the tape to the nearest 0.1 ft.  The leap-frog method was adopted.  The data assembled was reduced to co-ordinates using four-figure logs and the survey, plotted and checked on graph paper.  Due to the scale (1/480) no detail of the deposits could be shown on the drawing.

The instruments were not calibrated in the conventional manner due to influences from steel wire and half buried farm implements in the area.  Instead, a fixed bearing was obtained from the compass station near the farm entrance at the start of each trip; any variation in magnetic deviation enabled each section of the survey to be so corrected.  Thus, the survey figures were corrected to ‘compass north’. With the help of Brian Prewer et al, a radio transmission was made on March 12th 1977 from the base of Pyramid Pot and the point located in the field above.  A surface survey then commenced radiating lines to the cave entrance and the corners of the field.  By checking the 6” O.S. map of the area it was then possible to rotate the survey to grid north.  The ‘mismatch’ of the surface point located from the survey line and the signal point was less than 20 feet giving a closure error of approximately 1%.

Due to the complex plan form of the upper series and the general pattern of the main passage the idea of a projected elevation was abandoned and an extended elevation produced. The drawings were then transferred onto a nylon drawing sheet to produce the master original.  This is to be sent t8 B.M. Ellis for inclusion in the Mendip Cave Survey Scheme.

The surveyed length is 4,000ft + 200ft. un-surveyed (1,335 metres + 60 metres) and 433ift. (131.97 metres) deep.

The following table records the dates and personnel involved with whom this survey could not have been produced: -

?.8.76

Entrance to dig. (1 hour) D. Irwin & C. Batstone.

26.2.77

'A Day' to Pyramid Pot (3 hours) D. Irwin, G. Wilton-Jones, N. Halstead and C. Batstone.

5.3.77

Pyramid Pot to Breakthrough and Paton Place. (4 hours) D. Irwin, G. W-Jones and G. Price (C.S.S.).

11.3.77

Transporting radio locating gear to Pyramid Pot C. Hawkes; B. Prewer, T. Reynolds (W.C.C.) and P. Smart (U.B.S.S.).

 

Drunken Horse Inlet: D. Irwin and T. Large.

12.3.77

Surface survey: D. Irwin, J. Batstone, B. Prewer and R. White

20.3.77

Paton Place & White Dog Passage, D. Irwin, G. Wilton-Jones, J. Dukes, R. Mansfield.

20.3.77

Velcro Passage: P. MacNab & D. Turner.

14.4.77

Sheep's Jaw and misc. side passages: D. Irwin, P. MacNab and R. Halliwell.

The cave became choked with mud below the second pitch during May, 1977 leaving Aardvark and two small side extensions un-surveyed.  These have been sketched in on the plan.

 

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The views expressed by contributors to the Belfry Bulletin, including those of club officers, are not necessarily the views of the committee of the Bristol Exploration Club or the Editor, unless so stated.  The Editor cannot guarantee that the accuracy of information contained in the contributed matter, as it cannot normally be checked in the time at his disposal.