[GRASS5] Angle of Line Function?

[Eric G. Miller]

On Sat, Mar 10, 2001 at 04:25:20PM -0400, Bob Covill wrote:
> Jeshua Lacock wrote:
> 
> > Greetings,
> >
> > I would greatly appreciate any help with developing a simple function
> > that returns the angle (in degrees) of a line when given two points
> > (x1, y1 and x2, y2).
> >
> > Embarrassingly I have spent days trying to figure this out (that's
> > what I get for using the teachers edition of geometry in high
> > school!!).  ;->
> >
> > I have ran across a function on the web, but somehow it seems really off:
> >      angle = (((Atan((northingDifference/eastingDifference)))*180)/PI)

Arctan() will give you an acute angle. You would need to know the
quadrant the line is in (assuming the first point as the origin) to add
a correction factor.

Assuming (0 == North, 90 == East, etc...):

// Sorta code...

dy = (y2 - y1);
dx = (x2 - x1);

if (dy > 0.0 && dx > 0.0) {quadadjust = 0.0;}
else if (dy > 0.0 && dx < 0.0) {quadadjust = 360.0;}
else if (dy < 0.0 && dx > 0.0) {quadadjust = 180.0;}
else if (dy < 0.0 && dx < 0.0) {quadadjust = 180.0;}
else {
  if (dx == 0.0) {slope = (dy > 0.0) ? 0.0 : 180.0;}
  else if (dy == 0.0) {slope = (dx > 0.0) ? 90.0 : 270.0;}
  else {/* huh? */}
  return slope;
}

slope = ((atan(dy/dx) * 180) / M_PI) + quadadjust;
return slope;

// End.

Note, I haven't really checked the above...  And floating point numbers
can be weird in comparisons, so maybe an EPSILON is needed...

I understand there are better ways to do this than using atan() which
can be expensive...


> Jeshua,
> 
> If you take a look at the new version of r.profile, it has a routine that returns
> the azimuth between two points. Hopefully this helps with what you are looking
> for.
> 
> A library routine that returns the azimuth between two points would be a nice
> feature.
> 
> --
> Bob Covill
> 
> Tekmap Consulting
> P.O. Box 2016
> Fall River, NS
> Canada
> B2T 1K6
> 
> 
> 
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> 

-- 
Eric G. Miller <egm2 at jps.net>

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