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What is the problem ? [message #68408]

Tue, 27 October 2009 23:28

Ruby
Messages: 2
Registered: October 2009

Junior Member

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 ?

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