title: Calculate angle between three GPS coordinates
body: <p>perhaps this is a simple question.
  I have 3 GPS coordinates (one is the current user location). What I want now is to calculate the angle between the user location and the two GPS coordinates. Imagine the user location in the center of the two other points, the three points can be seen as a triangle. And I want to calculate the angle at the user location.
  I hope someone can help me because I have no idea how to do this with spherical coordinates like the GPS coordinates I have.</p>
  
  <p>THX - nekro</p>