NavList:
A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding
Good data from bad
From: Peter Fogg
Date: 2001 Dec 06, 8:48 AM
From: Peter Fogg
Date: 2001 Dec 06, 8:48 AM
How to Turn a Series of Doubtful Observations Into an Excellent One. Sounds too good to be true, doesn't it. Let's start at the beginning. Making observations of celestial objects from the deck of a small boat out at sea, which never stops moving, using a horizon that is bobbing up and down, often as it is quickly getting too dark or too light, can be difficult. The conventional wisdom is to: 1. Make a series of observations of the same body and average them out. But if one or two of them are wildly out (and we'll go into why that might be a little further on) then those wrong ones will ensure that the average will not be correct, even if most of the others are not bad. 2. Make observations of many bodies. But this gives you lots of sight reduction to do, and I find it confusing to have lots of lines on the plotting sheet. For me, 3 lines of position is the right number. There is another way, one I've never come across anywhere else. It does add another step to the process, and some simple plotting on a graph sheet. First you make as many observations of the same body as you can during a 5 minute period, or less, noting the changing Hs and times. Repeat this with your other bodies. Back at your Nav desk, calculate the change in altitude of your body in 5 minutes, according to its azimuth and your Latitude. This gives you a figure, say 30', either ascending or descending. This you draw on the graph paper as a straight line. Its actually an arc, which is why you shouldn't extend it beyond 5 minutes. The vertical side is altitude, the horizontal is time, the 5 minute period. Plot your observations as points.Then you draw a line parallel to your 'change of altitude' line which best fits your observations. Hopefully most of them will be fairly close, just above and below the line. You might have one or two that are wildly out. These you can ignore, so they won't pollute the others - but take another look, first. Are they exactly 1 minute of time out, or maybe 1 degree ? Its easy to write down the wrong minute, while you're concentrating on the seconds, or read the wrong degree, while you're peering at the micrometer drum. It may be you can adjust these to join the others. You can't do this by just looking at the numbers written down, it only becomes clear when its plotted, and becomes a picture. Then you take any point along this observation line and read off to the left the altitude, your Hs, and down to the time. For convenience you might choose a whole degree or minute of time, or any other point you like along the line. Then do your sight reduction and plotting as usual. I find it works well, and gives me feedback on how good or bad my observations are. You can end up with an observation which is better than any of the ones you actually made. I didn't invent this; it comes, complete with the required forms and graphs ready to be photo-copied, from a book 'The Complete On-Board Celestial Navigator', by George C Bennett. Its not a big book, but also includes a 5 year almanac and sight reduction tables, and much else besides - its supposed to be 'Everything But the Sextant' ! Published by International Marine, ISBN 0-07-007110-1. Its available in the USA and in Australia, and I don't know where else. There is also a web site, www.netspace.net.au/~gbennett/ Peter Fogg