How to calculate an angle from points?

Question

I want to get a simple solution to calculate the angle of a line (like a pointer of a clock).

I have 2 points:

cX, cY - the center of the line.
eX, e         


        

Answer 1

One of the issue with getting the angle between two points or any angle is the reference you use.

In maths we use a trigonometric circle with the origin to the right of the circle (a point in x=radius, y=0) and count the angle counter clockwise from 0 to 2PI.

In geography the origin is the North at 0 degrees and we go clockwise from to 360 degrees.

The code below (in C#) gets the angle in radians then converts to a geographic angle:

    public double GetAngle()
    {
        var a = Math.Atan2(YEnd - YStart, XEnd - XStart);
        if (a < 0) a += 2*Math.PI; //angle is now in radians

        a -= (Math.PI/2); //shift by 90deg
        //restore value in range 0-2pi instead of -pi/2-3pi/2
        if (a < 0) a += 2*Math.PI;
        if (a < 0) a += 2*Math.PI;
        a = Math.Abs((Math.PI*2) - a); //invert rotation
        a = a*180/Math.PI; //convert to deg

        return a;
    }