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Re: sight reduction with GPS receiver
From: Frank Reed CT
Date: 2005 Mar 21, 20:33 EST
And:
From: Frank Reed CT
Date: 2005 Mar 21, 20:33 EST
Bill you wrote:
"This is where I get lost in the jargon. Does "calculate geodesics on
the
Earth as shortest distance arcs on the ellipsoid" mean that corrections for
the fact the Earth is not a perfect sphere are included?"
Earth as shortest distance arcs on the ellipsoid" mean that corrections for
the fact the Earth is not a perfect sphere are included?"
Yes. A geodesic, by the definitions of geometry, is the path with shortest
distance between any two points (technically "extremal" length). On a spherical
Earth, the geodesics are great circle arcs. Because the Earth is ellipsoidal,
there is an advantage in distance gained by staying away from the equator (where
the Earth is fat). This means that the geodesics are slightly curved. If you are
directly above the middle of a great circle arc, it will appear exactly straight
to you, unlike the geodesics on the ellipsoid.
And:
" Is this "GCD?" This would be more accurate than calculations that
assume the Earth is a
sphere, correct?"
sphere, correct?"
I think HP was using GCD just to save the time of typing out "Great Circle
Distance". Calculating the distance based on ellipsoidal geodesics is "more
accurate", yes, but not in any practically useful way that I can think of.
Anybody know of a practical use?
And:
"I also have not resolved the question posted earlier: "My confusion is
that
132d is the starting course of a great-circle route. I am having trouble
understanding why that would be my azimuth. (spherical trig is still a
plug-and-chug function for me)."
132d is the starting course of a great-circle route. I am having trouble
understanding why that would be my azimuth. (spherical trig is still a
plug-and-chug function for me)."
Try drawing the two spherical triangles involved - one for the calculation
of the great circle distance between two places on the Earth (assumed spherical
for this), the other for the calculation of star's altitude. Does this
help?
-FER
42.0N 87.7W, or 41.4N 72.1W.
www.HistoricalAtlas.com/lunars
42.0N 87.7W, or 41.4N 72.1W.
www.HistoricalAtlas.com/lunars