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Subject: Re: Haversine formulae for Great Circles
From: Lu Abel (lunav@XXX.XXX)
Date: Mon Nov 19 2001 - 11:46:47 EST
At 10:08 AM 11/18/2001 -0800, Chuck Taylor wrote:
>>Even with this high
>>accuracy, distances of 0.1 nautical miles have started to go a bit
>>inaccurate, and distances which should be .01 miles are calculated as zero.
>>With a computer using single-precision, things would be a lot worse. So the
>>problem Lu has raised is a real one, in these special circumstances of
>>close approach.
>
>
>Why would one be interested in great circle routes for such short distances?
>Mercator Sailing or Mid-Latitude Sailing would seem much more applicable for
>distances under, say, 60 nautical miles.
For sailing (which IS the focus of this group, of course) mid-Lat would be
just fine (Mercator Sailing gives a rhumb line course so it wouldn't be
applicable)
Imagine, however, that you are a GPS vendor and you need to program your
GPS unit to provide distance and bearing. You'd want to make it calculate
GCs for long distances. But then people will also read Distance To Go as
distances get very small approaching a waypoint.
As a computer programmer myself, I'd rather have ONE formula that worked
for all distances than have to go through some complex logic to switch from
one formula to another. Plus any time one does a switch there's always a
chance the answer will jump, and for sure some customer would call and
complain "I moved 10 ft and the DTG jumped 100 ft, your software must have
a bug in it!"
Lu Abel
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