NavList:
A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding
Accuracy, to half a metre?
From: Peter Fogg
Date: 2003 Sep 8, 16:48 +1000
From: Peter Fogg
Date: 2003 Sep 8, 16:48 +1000
Some contributers to this list, who are land based, have detailed their endeavours, using sextants and reflective devices, to obtain fixes as close as possible to their known position. In the recent discussions which have centred around appropriate methods for amateur navigators in small boats using nav. as a practical tool, I hope I haven't appeared to criticize or disparage their efforts. To them I dedicate the following anecdote. Recently I needed to have some survey work done here at home, local council nonsense, and found that the surveyor was old enough to have been taught and to have used field astronomy, meaning taking sights with a theodolite mounted on a tripod. He told me that surveying outback properties, which here in Australia may be measured in thousands of square miles, used to be good business for surveyors. The team would go out in 4-wheel-drive vehicles and set up camp then take multiple sun and then star sights to establish, for example, a corner of a property. Then break camp, pack away the tents and drive, sometimes for hundreds of miles across trackless semi-desert, to another site. These days they arrive by helicopter, set up the GPS, and lift off shortly afterwards to arrive in no time at all at the next location, already fairly accurately determined in advance. On accuracy, he said that the ultimate accuracy using field astronomy was constrained by a number of factors: the accuracy of the time was important (he used a stop watch) but also the precision of the astronomical data available. We use Declinations and Hour Angles stated to the nearest minute of arc or tenth of a minute of arc but they required far more detailed precision. They would take multiple sights and then work on them; plotting the pattern of sights against the rise/fall of the body; also using reiteration, meaning reworking sights using the best position as a new DR, and also using the techniques discussed on the Nav. List in May this year (Bisectors and MPP, etc) to identify and overcome systematic error. He said that these methods combined to increase the final accuracy by about a factor of 10. He said it was possible to get within about half a metre of the real position, which I found fairly astounding, given that a second of arc (measured on the earth's surface) is nominally 30.87 metres. Half a metre is about a sixtieth part of that - is there a term for this in the hexigesmal system? The brightest young men (and they were all men then) of his generation (I'm guessing 30 odd years ago) went to NASA in the United States to work on the space program, but at that time he was more interested in Ironman competitions and surfing than studying. He said he had a bad habit of turning up for classes with wet hair and excuses instead of completed assignments. One of the perks of his surveying course was that during the holidays the students found it easy to obtain berths on yachts as navigators, and so went off sailing in summer to Hobart or even to Tahiti. Then as now there were plenty of sailors who were reluctant navigators. This is an important fact to bear in mind, appropriate systems for them are not necessarily those of surveyors. Its a matter of horses for courses. Now don't get me wrong, I'm not claiming this half metre accuracy as fact. Far from it. I'm reliably informed that accuracy stops, even with a theodolite mounted on a tripod used by an experienced surveyor, at a second of arc, about 30 metres. I guess the point I'm making is that for those interested in squeezing as much accuracy as possible from their circumstances and equipment, the practical tools that can be used are readily available and have been discussed here on the Nav. List from time to time.