Calculate angle between two Latitude/Longitude points

Question

Is there a way to calculate angle between two Latitude/Longitude points?

What I am trying to achieve is to know where the user is heading. For example, user is head

Answer 1

To provide heading you have to compute bearing.

To understand bearing read this article.

According to this article (section bearing) the formula is :

θ = atan2( sin Δλ ⋅ cos φ2 , cos φ1 ⋅ sin φ2 − sin φ1 ⋅ cos φ2 ⋅ cos Δλ )
where φ1, λ1 is the start point, 
      φ2, λ2 the end point,
      Δλ is the difference in longitude`

Here's a sample on how to compute the angle (in degrees) between two points expressed in Lat/Lon. (done in C#)

Let's say Point is a simple class with two double attributes X (for longitude) and Y (for latitude).

public double ComputeBearing(Point start,Point end)
{
     var φ1 = start.Y; //latitude 1
     var λ1 = start.X; //longitude 1
     var φ2 = end.Y; //latitude 2
     var λ2 = end.X; //longitude 2

     var y = Math.Sin(this.degreeToRadian(λ2 - λ1)) * Math.Cos(this.degreeToRadian(φ2));
     var x = Math.Cos(this.degreeToRadian(φ1)) * Math.Sin(this.degreeToRadian(φ2)) - Math.Sin(this.degreeToRadian(φ1)) * Math.Cos(this.degreeToRadian(φ2)) * Math.Cos(this.degreeToRadian(λ2 - λ1));

     var θ = Math.Atan2(y, x);
     θ = this.radianToDegree(θ);

     return θ;
}

Using the following methods :

public double degreeToRadian(double angle)
{
    return Math.PI * angle / 180.0;
}

public double radianToDegree(double angle)
{
    return angle * (180.0 / Math.PI);
}

By using ComputeBearing you will easily get an angle expressed in degrees easily usable as heading