Cambridge Underground 1986-7, pp 13-14

Computer Drawn Passage Walls

by A. Waddington

When computerised processing of cave surveys first became popular a decade or so ago, several programs were written which attempted to provide more than just a centre line plot. The number of extra measurements that needed to be taken, combined with the disappointing results obtained compared with even a mediocre hand-drawn survey, led to a general belief that computer drafting of complete surveys would never be practical. That this belief still exists is evidenced by recent correspondence in BCRA Caves and Caving.

The aims of this article are:

Why computer aided drafting ?

The advantages of computerised data processing to calculate coordinates and plot a survey centre line are well-known: The calculations are faster, easier and less prone to error than calculations by hand. Survey data can be amended, added to, or linked in to other surveys more easily, and the business of distributing loop closure errors is greatly simplified.

None of these advantages at first sight apply to computer-drawn passage walls, and the age-old technique of copying from survey-book sketch to final survey seems at once to be both easy and reliable, if time-consuming.

Consider however, a complex cave which is partly surveyed, but still not fully explored. Surveys will have been drawn and published, and perhaps the original notes lost. Somewhere, there is a master-drawing with as much detail as is currently available. Team explorer, meanwhile, find a major new passage linking together two hitherto distant (in terms of survey length) parts of the system. The link is surveyed, and loop closure error redistributed around the system. A new survey is to be published, but the new passage won't fit into the space on the old survey. The centre-line has altered with the new data so that the whole survey needs to be re-drawn. The original sketches are lost, and much of the drawing is merely a copy of the old survey, bent a bit, and with a few new errors introduced. Sounds familiar ? Those who have been involved in surveying systems such as Easegill or Gaping Gill will immediately recognise the problem, which explains why so many surveys appear showing just the new passage, and not the rest of the system nearby. If the passage detail had been stored away with the centre line data, the whole survey could be redrawn easily, with no newly introduced draughting errors, and various parts of the cave stretched or rotated slightly to fit the new centre line.

So How is it done ?

How then to achieve this without an impossible number of extra measurements ? Here we must look at how the traditional surveyor achieves his immaculate results without similar measurements. The answer of course is that he sketches the passage detail in his survey notebook while underground, and then assembles this information around the centre line of the final survey. If an 'Honest' grade 4c is surveyed, then all necessary passage detail is recorded in the cave. If the surveyor is doing his job properly, then his notebook contains all the information needed to draw the final survey, which could be (and often is) drawn by a different person without reference to the surveyor's memory. The advent of cheap digitising tablets means that passage detail sketched in the notebook can be digitised, and related to the two endpoints of the survey leg. Then if the survey leg is stretched or rotated, the passage detail remains with it. The knowledge that one's sketching will be used in this way has the further advantage of enforcing a stricter discipline on the surveyor: the temptation to 'remember' (or invent) bits of detail which were not recorded in the cave is much reduced. The reduction in detail on the final survey which this may entail when the technique is first used is compensated by the knowledge that what is shown is actually there in the cave. With practice, all the original detail needed for a polished survey is recorded in the cave, and overall survey quality improves substantially for disproportionately little effort. Indeed, the effort involved may be less than with traditional methods since the survey is only drawn once.

I know of no-one actually using the techniques outlined above in this country as yet, but similar techniques have been in use in the USA for five years (McKenzie,1980), chiefly for maze caves where the centre line can change marginally with each surveying trip. The quality of sketching recorded by AMCS surveyors in Mexico is much higher than is common in Europe in my experience, and perhaps this is why the techniques have been adopted so readily. I am grateful to Peter Sprouse and his Proyecto Espeleologia Purificación for introducing me to their high quality surveying techniques, which seem to take no longer than the less detailed surveys made on CUCC expeditions (albeit with three or four persons to a survey team).

For the precise mathematical details of the data transformations required to ensure that passage walls remain continuous when adjacent survey legs are rotated, I can do no better than to refer the reader to McKenzie's original article on the subject (cited above). The amount of data involved currently precludes the use of a 'home' micro to draw the survey, but the original digitisation process and survey data entry is probably best handled by such a machine. CUCC had a BBC micro in the field in 1984, capable of calculating coordinates for up to 1000 survey stations in about 20 minutes. (Waddington, 1984) The data were able to be downloaded to a mainframe machine on the first day back in England. By using the techniques above, a complete draft survey could be drawn on large (eg. A0) paper, and be in the surveyor's hands within a day of arrival home.

A.E.R.Waddington, June 1985


McKenzie, David, December 1980
'Computer-Drawn Passage Walls'
Association for Mexican Cave Studies Activities Newsletter #11 pp86-89
Waddington, Andrew, October 1984
'SU - A general purpose cave survey program :
BBC Micro version 1.00 User Reference' (40 pp including listings)

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