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From: James H. Maynard (no email)
Date: Sun Aug 22 1999 - 20:25:53 EDT
You bet, there are several formulas. Choose one according to the degree
of accuracy you want, and also according to how close together the
points in question are.
The law of cosines for spherical triangles is a convenient one. It has
two disadvantages, though.
1) If the points in question are too close together, you end of
finding the inverse cosine of a number that is very close to unity. The
number of iterations your inverse cosine routine goes through, and the
accuracy of the result, degrade the closer the points are together. Not
recommended for high accuracy if the points are only a mile or so apart.
2) It assumes that the earth is spherical in shape. More accurate
formulas assume that the earth is an ellipsoid ("oblate spheroid").
If you are willing to live with these limitations, then assume that a
minute of arc along a great circle is one nautical mile (1852 metres),
and plug into the law of cosines for spherical triangles. Now, let's see
if I actually remember it:
cos D = sin(LatA)*sin(LatB) + cos (LonB-LonA)*cos(LatA)*cos(LatB)
D is the central angle subtended by the great circle arc in question.
Compute its cosine by the formula, take the inverse cosine and express
that inverse cosine in degrees (rather than radians). Multiply by 60 to
convert degrees of arc to minutes of arc, and assume that each minute of
arc is a nautical mile.
It's an easy formula, and is the usual one used in celestial navigation.
But it does have the disadvantages that I mentioned. There are other,
more elaborate, formulas used by mapmakers and geodesists. But I expect
that this one will do the trick for you.
Paul Saltzman wrote:
>
> I started to get organized and created a Excel based worksheet for way
> points. Now I was wondering if there's a mathmatical formula that based on
> Lat. & Lon. of two points can determne the distance between those points.
> If so does anyone know it? And agian if so could they pass it on to me?
>
> thanks...
> SMOOTH SAILING:
>
> Paul Saltzman
> S/V EVENING STAR
> Kingston, NY
>
> Photo of Evening Star.
> <http://sites.netscape.net/paulsaltzman/homepage>http://sites.netscape.net/
> paulsaltzman
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