title: How to compute Aspect angle
body: <p>I have many 3D planes. The thing that I need to know is the way of computing aspect angle. 
  I hope, I can compute the aspect angle by using the projected normal vector of each plane (my plane equation is ax+by-z+c=0; so normal vector of this plane is a,b,-1) to the XY plane. Then, from the Y axis I can compute the aspect angle. But, I don’t know how to get the projected Normal vector after I projected to XY plane. Then, can I apply the equation which gives angle between two vectors to compute angle of my desired vector from the y axis. </p>
  
  <p>On the other hand, I found, aspect angle is defined as the angle between any line which passes along the steepest slope of the plane and north direction (here, Y axis). Does this definition will follow, with my proposed way that is taking normal vectors? I mean, does the projected normal vector always given along the steepest slope of the plane? Also, some one told me, that this problem should consider as a 2D problem. 
  Please comment me and send me the relevant formulae in order to compute aspect angle. Thank you.</p>