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Suppose I am standing at latitude, longitude $(-33, 151)$ and I want to calculate the angle between two points $(-32, 150)$ and $(-34, 152)$ from my point of view. Can someone please tell me how can I do that ?

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Are you standing in Tarnow? – copper.hat Mar 15 '13 at 3:17
up vote 1 down vote accepted

If a spherical earth is good enough, you can convert the three points to Cartesian coordinates: $x=R \cos \phi \cos \lambda, y=R \cos \phi \sin \lambda, z=R \sin \phi$. Then subtract to get the two vectors from where you are to the other two points and use the dot product formula. This will give the angle in space between the vectors.

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Thanks for the answer Ross. Does the ϕ and λ represents latitude, longitude respectively ? – user975027 Mar 15 '13 at 2:19
The cross product might be better for small angles, $\arccos$ is insensitive for small angles. Also, increasing $\phi$ corresponds to north, increasing $\lambda$ corresponds to east. – copper.hat Mar 15 '13 at 2:27
@user975027: yes. These are standard. – Ross Millikan Mar 15 '13 at 2:28
@RossMillikan -is there a way to get the angle without converting to Cartesian ? Will calculating the great circle bearing give the same answer in terms of the arctan2 function? – gansub Jun 2 at 11:44
@gansub: It depends what you want. The Cartesian conversion will give you vectors that go through the earth to the other points. If you are standing at a point and the two destinations are almost opposite you on the earth, the bearings of the great circles can be quite different but the angles through the earth are almost the same. – Ross Millikan Jun 2 at 14:39

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