# NavList:

## A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding

**Re: Systematic Error (LOPs revisited)**

**From:**Trevor Kenchington

**Date:**2003 May 28, 20:52 -0300

Peter Fogg wrote: > I was intrigued by this 3 to 1 claim and followed the discussion with great > interest and not a little scepticism. After a while I stated that it is an > irrelevance. My reasoning was that if you take any triangle of LOPs or > 'cocket hat' there is only one fix position that can be found by the > doctrine of least squares. That would be (as George Huxtable has noted) the most probable position but it would not necessarily be the true position. In navigation, as in many other situations in life, the best policy is _not_ to treat the best point estimate as though it is necessarily correct. You have to remain aware that your true position might be in less probable points on your plot, particularly those that would mean that you were heading straight for some nasty hazard. > Lets start with those 2 intersecting LOPs. If there is a systematic error > then it will be either towards the direction of the 2 azimuths or away. So > instead of a 25% chance of the fix lying in 4 quadrants there is a 0% chance > of it lying in 2 opposing directions and 50% each for the other 2. In this > case the fix position lies along the line bisecting the two 50% quadrants, > passing through the intersection. If there is only a fixed systematic error, then there is no point in dealing with probabilities where the radii around the intersection are concerned. The bisector would be a locus on which your true position must lie -- an absolutely certain LOP, in effect. (Probabilities would still be relevant to a consideration of how far from the intersection of the LOPs you might be, unless you knew the magnitude of the systematic error.) If there is a combination of random and systematic errors, then the above analysis does not hold true. To the extent that the systematic errors were large relative to the random ones, the probability that your true position lay in two of the quadrants would be higher than the probability that it lay in the other two, but the probabilities would only approach 0.5, 0, 0.5 0 as the random error dropped to being negligible compared to the systematic error. George Huxtable then contributed to this thread: > True for four QUADRANTS, but is it this relevant? The two lines crossing at > a point divide the plane they lie in into four triangles which snug > together at their intersection. Only if the lines are at right-angles are > the triangles quadrants. Otherwise they are two wide triangles and two > narrow ones, and the chances of the true position being in any one is the > angle at its apex divided by 360 degrees, if only random errors exist. I think that is a mistake. The probability contours around the intersection of two LOPs which meet at an acute angle will not be circular but elliptical -- assuming simple random errors in both LOPs. [If we had two closely aligned LOPs, we could be relatively confident that we lay somewhere along their combined alignment but we would have almost no idea where along that line we were.] Thus the probability of being in either of the "narrow" sectors is higher than the proportion of the circle that they occupy. On the other hand, I doubt that the probability of being in each sector remains 25% as they narrow. I think they must drop but not as fast as their apex angles do. Perhaps some mathematician on the list can provide an exact equation for the probability. Or did we already get that far last time we discussed this? Trevor Kenchington -- Trevor J. Kenchington PhD Gadus@iStar.ca Gadus Associates, Office(902) 889-9250 R.R.#1, Musquodoboit Harbour, Fax (902) 889-9251 Nova Scotia B0J 2L0, CANADA Home (902) 889-3555 Science Serving the Fisheries http://home.istar.ca/~gadus