SUAZIMUTH - compute trace AZIMUTH given the sx,sy,gx,gy header fields	
		and set a user-specified header field to this value	

  suazimuth <stdin >stdout [optional parameters]			

 Required parameters:							
 none									

 Optional parameters:							
 key=otrav		header field to store computed azimuths in	
 scale=1.0		value(key) = scale * azimuth			
 az=0			0-179.9999 deg convention, reciprocity assumed	
			=1 0-359.999 deg convention, points from source 
			to receiver					
 sector=1.0		if set, defines output in sectors of size	
			sector=degrees_per_sector, the default mode is  
			the full range of angles specified by az	
 offset=0		if offset=1 then set offset header field 	
			based on (sx,sy,gx,gy).  Since this is a 3-D    
			offset it is unsigned.  For 2-D use sushw.	
 cmp=0			if cmp=1 then set header fields for (cmpx,cmpy)	
			based on (sx,sy,gx,gy).  Header assignment is   
			cmpx=tr.ep cmpy=tr.cdp				
 Notes:								
 The value of header word "key" is computed from the values of	
 sx,sy,gx,gy. The output field "otrav" was chosen arbitrarily as an	
 example of a little-used header field, however, the user may choose 	
 any field that is convenient for his or her application.		

 Setting the sector=number_of_degrees_per_sector sets key field to	
 sector number rather than an angle in degrees.			

 For az=0, azimuths are measured from the North, however, reciprocity  
 is assumed, so azimuths go from 0 to 179.9999 degrees. If sector option
 is set, then the range is from 0 to 180/sector. 			

 For az=1, azimuths are measured from the North, with the assumption that
 the direction vector points from the receiver to the source, no	
 reciprocity is assumed, so the angles go from 0 to 359.999 degrees.	
 If the sector option is set, then the range is from 0 to 360/sector.	

 Type: sukeyword -o    to see the keywords and descriptions of all	
		       header fields.					

 To plot midpoints, use:     su3dchart					


 Credits:
	based on suchw, su3dchart
      CWP: John Stockwell and  UTulsa: Chris Liner, Oct 1998
      UTulsa: Chris Liner added offset option, Feb. 2002
 		cll: fixed offset option May 2003
		cll: added cmp option May 2003

  Algorithm:
    midpoint x  value  xm = (sx + gx)/2
    midpoint y  value  ym = (sy + gy)/2
 
   Azimuth will be defined as the angle, measured in degrees,
   turned from North, of a vector pointing to the source from the midpoint, 
   or from the midpoint to the source. Azimuths go from 0-179.000 degrees
   or from 0-180.0 degrees.
   
   value(key) = scale*[90.0 - (180.0/PI)*(atan((sy - ym)/(sx - xm))) ]
     or
   value(key) = scale*[180.0 - (180.0/PI)*(atan2((ym - sy),(xm - sx)) ]
 
   Trace header fields accessed: sx, sy, gx, gy, scalco.