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

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Re: Position from altitude and azimuth.
From: Sean C
Date: 2020 May 10, 23:52 -0700

Just before Dave W. posted his original question I made two spreadsheets which might be tangentially related to this topic. One is a simple sight reduction calculator and the other uses two or more sights to compute a fix using the least squares formula from the Nautical Almanac. I made the sight reduction sheet to use as a quick check on other methods or just to use when I felt too lazy to punch in all the numbers on my calculator. (Which is pretty lazy, I know.)

I made the other one ("APGuess") specifically for questions like Dave's that pop up here or elsewhere from time to time. I have also been fascinated with the idea (however unlikely) of needing to determine a position with an uncertain/unknown DR or AP. The spreadsheet has sliders which allow you to move the AP around until the intercepts are minimal and it calculates the distance from the AP to the fix. It also has a world map which displays the [approximate] positions of both. I was going to try and work out a way to have Excel scale the plots for a Mercator projection, but then I realized that the equirectangular map I had saved on my laptop should work well enough by itself. Individual sights can be "turned on or off" with the "use" checkboxes above the Ho. It's already set up with some ficticious data, so see if you can find the best fix.

Unfortunately, the APGuess sheet needs at least two sights to compute a fix, so it wouldn't work on Daves question. But, the sight reduction sheet could. I played around a little and found that it's much harder to home in on the likely fix with just one azimuth and intercept - but it's certainly possible. Not unlike the way Frank did with Stellarium.

Anyway, the two spreadsheets are attached if anyone is interested. I made them for my own use, so it might help to know that any grey cells are calculated by Excel. You only need to fill in the white cells. The sight reduction sheet will calculate dip if given HoE and it will calculate the Moon's SD if given HP, but you have to enter these manually in the white cells. (PA is automatically included. Delete the HP cell or enter zero there to omit.) Also, they might be set to manual calculation - I'm not sure right now. If so change it in the "Formulas" tab or hit "F9" to do the calculations.

Regards,

Sean C.

File:
FastReduction.xlsx
File:
APGuess.xlsx
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