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Re: What is the problem ? [message #68407 is a reply to message #68404]

Tue, 27 October 2009 23:58

Chris[6]
Messages: 84
Registered: July 2008

Member

On Oct 27, 8:28 pm, Ruby <wuqiao...@gmail.com> wrote:
> I wrote a simple program to caculate the distance between two location
> (lon1,lat1), (lon2,lat2)
>
> function earth_dis ,lon1,lon2,lat1,lat2
>
> b1 = !pi*lat1/180.0
> b2 = !pi*lat2/180.0
> a1 = !pi*lon1/180.0
> a2 = !pi*lon2/180.0
>
> dis = 6378.1*acos( cos(b1)*cos(b2)*cos(a1-a2)+sin(b1)*sin(b2))
>
> RETURN,dis
> END
>
> But When I tried to test the program, the results turned to be like
> IDL> print,earth_dis(4.0,4.0,4.0,4.0)
> -NaN
> IDL> print,earth_dis(8.0,8.0,8.0,8.0)
> 2.20215
>
> In both cases, the result should be straightforwardly equal 0. Then
> what is the problem with my program or IDL ?

Check out the wikipedia entry on great circle distances:
http://en.wikipedia.org/wiki/Great-circle_distance

The formula you use doesn't work well on a computer with the distance
is small (roundoff errors become large). They reference a better
formula. Alternatively, use the GCIRC function in the IDL Astronomy
user's library.

Chris

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[Message index]

		Re: What is the problem ? By: Ruby on Wed, 28 October 2009 18:56
		Re: What is the problem ? By: penteado on Wed, 28 October 2009 04:22
		Re: What is the problem ? By: penteado on Wed, 28 October 2009 04:20
		Re: What is the problem ? By: Chris[6] on Tue, 27 October 2009 23:58

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