NavList:
A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding
Re: Consistency v Accuracy in celestial navigation
From: Bill Noyce
Date: 2004 Jan 6, 11:04 -0500
From: Bill Noyce
Date: 2004 Jan 6, 11:04 -0500
Chuck Taylor's excellent discussion of precision and accuracy included this paragraph: > Three sights can be very useful if they plot in a > straight line. Then you at least can conclude that > they are consistent. If they do not plot in a > straight line, you are stuck. You can make a case > that two of them are consistent and the third one is > not, but you have no way of knowing *which* one is the > odd sight out. In fact, you can do better than this. Compute a couple of predicted altitudes for the earliest and latest times of your sights, using your estimated position. Then either apply corrections for refraction, dip, etc "backwards" to these computed results, or apply them normally to your actual observations, so that the values are commensurate. Plot the computed altitudes against time, and find the slope of the line between them. Draw a line with the same slope through your actual observations; the slope should make it clearer which sights are good and which are bad. This technique also helps when you have more sights. The point is that you don't need to guess about the slope of the "best fit" line; you can compute that from the Almanac if you have a reasonably good EP. I'm pretty sure the same technique applies to lunar distances, though the computations are messier. I'm not sure whether to suggest "un-clearing" computed distances, or clearing all the observations, before plotting, but one or the other is necessary. -- Bill