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Avelines Hole Survey

By R.D. Stenner

The survey was made in six hours on the 2nd and 4th July 1968 with a party from Hartcliffe School, Bristol.

The instruments used were a Mark 4 liquid filled prismatic compass, an Abney level (Japanese manufacture) and a 50 x 0.05ft. fibron tape.  The instruments were tripod mounted, and the compass and clinometer were read to the nearest 0.05ft.  The target (a carbide lamp) on convenient ledges and projections.  Special care was taken in the choice of survey stations so as to minimise errors due to reading the compass while sighting downwards. Passage details were measured at all stations, and wherever necessary elsewhere by ‘raying’ from a station. Inaccessible details were measured by triangulation.

The centreline of the survey, including the side passages, was made of 15 legs, totalling 223.5ft.

The zero error of the clinometer was checked after use, and appropriate correction made to the measurements.  The compass was calibrated by taking bearings along a nearby straight stretch of the road. Six readings with a maximum spread of ±½0 were made, and the mean was recorded.  The bearing of the road was found from a 6” O.S. map, and all readings corrected to give the Grid North bearings.

The changes of co-ordinates were worked out using four figure tables and a calculating machine. The co-ordinates of a trig-mark carved at the top of a prominent boulder at the cave entrance was assigned the co-ordinates N= 50.00ft; E= 0.00ft; Ht= 324.82ft. above O.D.  Because of the small size of the cave no other fixed stations were left.

The co-ordinates of the centreline were plotted on graph paper checked for accuracy, and the passage details plotted and the drawing made.  The survey was traced onto stable detail paper.

The survey contained no traverse closures, but it is possible to check the precision of the survey. On 19th February 1967 a Grade 5 line survey of the cave was made by the author with Joan and Roy Bennett, and Eddy Welch, during a surveying course organised by the B.E.C.  The instruments were the same as used in the survey published here, used in a similar way, hand-held instead of tripod mounted.  On 30th August 1968 a further check was made by Brian Britton and the author using the same instruments mounted in a survey unit. The three surveys were compared, and the results are tabulated below.

RESULTS.

SURVEY 1:  Survey published inn this report, 2-4th July 1968.

SURVEY 2:  C.R.G. Grade 5 survey 19 February 1967.

SURVEY 3:  Survey made with Surveying unlit, 30th August 1968.

Co-ordinates of a point at the end of the cave~:

No.

Length

Legs

Ht.

N

E

1

157.5

8

277.19

37.64

134.46

2

173.2

9

279.12

38.03

133.63

3

161.6

7

278.17

39.10

136.47

From the three close traverses were constructed: -

Surveys

Slope Misclosure

Percentage Error

1,3

2.67ft.

0.84

1,2

2.12ft.

0.64

2,3

3.18ft.

0.95

The conclusion is that the three surveys are all within the accuracy to be expected from an accurate magnetic survey of this length.  As the survey published within this report gave co-ordinates for the end of the cave which lay between those of the other two surveys, it was probably the most accurate of the three and no corrections were made based on the closure errors that were determined.

AVELINES HOLE based on C.R.G. Grade 6 survey by R.D. Stenner et. al.

(Ed. note:  This survey will be available through the Mendip Survey Scheme within the next few months).