How to calculate the angle between two points (according to a reference point)

I need to calculate the angles between two points on ArcGIS. I have ArcInfo License, too. I have 5 points on a diagram. One of them is my reference point. The other four creates angles between each other. I need to calculate each angle automatically. Because I have 7500 of these points. I'm also adding a picture of my points. I need to calculate the angles of point number 1-2, 2-3, 3-4, 4-1. http://i.stack.imgur.com/EGpkp.png

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Will there always be a straight line through the reference point between points 1 and 4 (as shown in the illustration)? If so, angle 4-1 will always be 180 degrees. –  user3461 Mar 11 '13 at 16:45

The Generate Near Table tool (under Analysis Tools > Proximity) will get you started. You can give the tool a search radius around your reference points and tell it to find the 4 nearest other points, it will calculate the distance and angle to each of the 4 points from the reference point. Make sure the Angle box is checked, Find Only Closest Feature is unchecked, and you enter the appropriate number of nearest points to find:

Once that's done you will have angles to each point from the reference point, so you'll need to do a little math to figure out the angles in your screenshot. The Near tool calculates angles relative to the X axis so an angle of 0 would be due east, 90 would be north and -90 would be south.

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Thanks for your help. I already calculated the angles (According to x-axis) all my 7500 points via this tool. However I need the angels between the points, not the x-axis. I'm tri-ying to develop an excel formula to calculate the angels with the angels to x-axis. Still workimng on it. Thanks for your precious help –  Onur Alkan Mar 11 '13 at 20:54
If my answer helped, please click the up arrow next to it to vote it up. Thanks! –  Dan C Mar 11 '13 at 22:09