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From: Frank Reed (no email)
Date: Thu Apr 28 2005 - 17:55:25 EDT
Bill wrote:
"How the heck do I correct for parallax error for two points at varying
distance? "
I don't think anyone bothers with this. It might be an interesting
mathematical puzzle, but I don't think it would ever matter in practice.
And:
"and a three-arm protractor is precise to 0.1'."
But is it that accurate? Let's suppose the arms are 50 cm long. Placing the
end points at 0.1 arcminute accuracy would be equivalent to placing the tips
of the arms with a linear accuracy of 0.015 millimeters while the width of a
line on a chart is probably around 0.5 mm in most cases --that's over 30
times larger. I wouldn't count on angles laid out with a three-arm protractor to
be any better than 0.1 degree accuracy. Then again, maybe I've missed
something.
As an aside, here's a little mantra: Angles *ARE* Ratios.
An angle of one arcsecond is a ratio of 1:206,265. An angle of one arcminute
is a ratio of 1:3438. And an angle of one degree is a ratio of 1:57.3.
Memorize any one of these and you never need trig for small angle calculations.
-FER
42.0N 87.7W, or 41.4N 72.1W.
www.HistoricalAtlas.com/lunars
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