NavList:
A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding
Re: Taking four stars for checking accuracy of fix - and "Cocked Hats"
From: Peter Fogg
Date: 2008 Aug 4, 03:58 +1000
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From: Peter Fogg
Date: 2008 Aug 4, 03:58 +1000
Geoffrey wrote:
The slope is plotted with reference to latitude and azimuth and is relatively uncritical - whole degrees are fine. It is impractical to draw the slope with much better than a 1-minute of arc accuracy. Although I imagine it could be scaled up, if you needed to be more precise.
Now I'm the one who is struggling to understand this. The slope is not tethered to any place on the graph with its altitude and time axes (the angle of slope changes with latitude and azimuth). Once the points are plotted then they have to fit that slope as best they can (NOT the other way around). Its to some extent an intuitive process, which is another reason not to confuse it with averaging, which is a purely mathematical approach.
This best fit obtained, any point along the slope can then be adopted as the altitude/time set, then that set reduced by whichever means to derive a line of position. The azimuth is not affected by any of this. Nor is the relative altitude, really; it does remain tethered to time.
What you are saying is that you can calculate how an altitude will vary with time and so you now have a line with the correct slope to fit to your data.
Exactemente!
This is an additional piece of information that simple averaging or statistical fitting by method of least squares, say, does not give you. Having got your line with the correct slope, you can draw a line parallel to this through your data points for a best fit. Yes, no doubt about it, this is superior to simple averaging as an extra piece of information is being included that was not there before.
Having plotted your data points on an altitude vs time graph, and also plotted the calculated line for your estimated position,
The slope is plotted with reference to latitude and azimuth and is relatively uncritical - whole degrees are fine. It is impractical to draw the slope with much better than a 1-minute of arc accuracy. Although I imagine it could be scaled up, if you needed to be more precise.
the distance you have to move the line in the altitude axis direction to get a good fit with your sightings will be just Ho - Hc, the intercept distance along the azimuth.
Now I'm the one who is struggling to understand this. The slope is not tethered to any place on the graph with its altitude and time axes (the angle of slope changes with latitude and azimuth). Once the points are plotted then they have to fit that slope as best they can (NOT the other way around). Its to some extent an intuitive process, which is another reason not to confuse it with averaging, which is a purely mathematical approach.
This best fit obtained, any point along the slope can then be adopted as the altitude/time set, then that set reduced by whichever means to derive a line of position. The azimuth is not affected by any of this. Nor is the relative altitude, really; it does remain tethered to time.
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Navigation List archive: www.fer3.com/arc
To post, email NavList@fer3.com
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