A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding
From: Frank Reed
Date: 2016 Feb 9, 12:14 -0800
Jim Rives, you wrote:
"One of my future (ie, ain't gonna ever happen) projects would be to run multiple iterations of a sight reduction program, a round of say 4 or 5 well placed objects, and just increment the dip by 1 foot each round to see if the fix closes in on the real height of eye."
Yes, this definitely works. You can treat dip and index error as a combined unknown. If you have enough sights taken in a short enough period of time, such that those two sources of error would not be expected to change, then you can do a least-squares-style fix solution and extract the fixed error (dip + IE) along with the latitude and longitude. For a visual solution, you just fiddle the dip up and down until the lines of position cross in the smallest possible "knot" of lines. For example, suppose I am a hundred feet above the water out in the ocean, and I am able to shoot eight stars on azimuths evenly spaced around the horizon. If I treat dip as unknown, setting it to zero on the first pass, then my lines of position will cross in a large open octagon with a diameter of about 20 nautical miles. By increasing the dip a minute of arc at a time, I would quickly find that the lines cross in the tightest bundle with aabout ten minutes of arc of dip.