>

MSVELOVECT

 NAME:                                                                          
  MSVELOVECT                                                                   
                                                                               
PURPOSE:                                                                       
  Produce a two-dimensional velocity field plot.                               
                                                                               
  A directed arrow is drawn at each point showing the direction and            
  magnitude of the field.                                                      
                                                                               
      This routine was developed from the original VELOVECT routine            
      and now allows two dimensional arrays for X and Y for irregularily       
      spaced data.                                                             
                                                                               
CATEGORY:                                                                      
  Plotting, two-dimensional.                                                   
                                                                               
CALLING SEQUENCE:                                                              
  MSVELOVECT, U, V [, X, Y]                                                    
                                                                               
INPUTS:                                                                        
  U:  The X component of the two-dimensional field.                            
      U must be a two-dimensional array.                                       
                                                                               
  V:  The Y component of the two dimensional field.  Y must have               
      the same dimensions as X.  The vector at point [i,j] has a               
      magnitude of:                                                            
                                                                               
          (U[i,j]^2 + V[i,j]^2)^0.5                                            
                                                                               
      and a direction of:                                                      
                                                                               
          ATAN2(V[i,j],U[i,j]).                                                
                                                                               
OPTIONAL INPUT PARAMETERS:                                                     
  X:  Optional abcissae values.  X must be a vector with a length              
      equal to the first dimension of U and V *OR* a 2-dimensional             
              array with the same dimensions as U and V.                       
                                                                               
  Y:  Optional ordinate values.  Y must be a vector with a length              
      equal to the first dimension of U and V *OR* a 2-dimensional             
              array with the same dimensions as U and V.                       
                                                                               
KEYWORD INPUT PARAMETERS:                                                      
  COLOR:  The color index used for the plot.                                   
                                                                               
  DOTS:   Set this keyword to 1 to place a dot at each missing point.          
      Set this keyword to 0 or omit it to draw nothing for missing             
      points.  Has effect only if MISSING is specified.                        
                                                                               
  LENGTH: Length factor.  The default of 1.0 makes the longest (U,V)           
      vector the length of a cell.                                             
             ### Franz Rohrer's modification of the LENGTH keyword:            
             LENGTH now applies a scale factor relative to the data values,    
             not just a relative scaling. If you don't specify LENGTH it acts  
             as before.                                                        
                                                                               
      MISSING: Missing data value.  Vectors with a LENGTH greater              
      than MISSING are ignored.                                                
                                                                               
      OVERPLOT: Set this keyword to make VELOVECT "overplot".  That is, the    
              current graphics screen is not erased, no axes are drawn, and    
              the previously established scaling remains in effect.            
                                                                               
      NOZERO: Do not plot zero vectors as dots.                                
                                                                               
  Note:   All other keywords are passed directly to the PLOT procedure         
      and may be used to set option such as TITLE, POSITION,                   
      NOERASE, etc.                                                            
OUTPUTS:                                                                       
  None.                                                                        
                                                                               
COMMON BLOCKS:                                                                 
  None.                                                                        
                                                                               
SIDE EFFECTS:                                                                  
  Plotting on the selected device is performed.  System                        
  variables concerning plotting are changed.                                   
                                                                               
RESTRICTIONS:                                                                  
  None.                                                                        
                                                                               
PROCEDURE:                                                                     
  Straightforward.  Unrecognized keywords are passed to the PLOT               
  procedure.                                                                   
                                                                               
MODIFICATION HISTORY:                                                          
  DMS, RSI, Oct., 1983.                                                        
  For Sun, DMS, RSI, April, 1989.                                              
  Added TITLE, Oct, 1990.                                                      
  Added POSITION, NOERASE, COLOR, Feb 91, RES.                                 
  August, 1993.  Vince Patrick, Adv. Visualization Lab, U. of Maryland,        
      fixed errors in math.                                                    
  August, 1993. DMS, Added _EXTRA keyword inheritance.                         
  January, 1994, KDB. Fixed integer math which produced 0 and caused           
                  divide by zero errors.                                       
  December, 1994, MWR. Added _EXTRA inheritance for PLOTS and OPLOT.           
  June, 1995, MWR. Removed _EXTRA inheritance for PLOTS and changed            
           OPLOT to PLOTS.                                                     
      September, 1996, GGS. Changed denominator of x_step and y_step vars.     
      February, 1998, DLD.  Add support for CLIP and NO_CLIP keywords.         
      June, 1998, DLD.  Add support for OVERPLOT keyword.                      
      


Modified:  16 Sep 1999: Martin Schultz added support for 2D U and V arrays          
             cleaned up the routine some and added the NOZERO keyword.         
             (renamed as msvelovect.pro on Sep 23)                             
             Also included Franz Rohrer's modification of the LENGTH keyword:  
             LENGTH now applies a scale factor relative to the data values,    
             not just a relative scaling. If you don't specify LENGTH it acts as before.  

 DISCLAIMER:  This routine has been modified from its original form as it was
              supplied by Research Systems, Inc (RSI).  As such, RSI is not responsible
              for any errors existing in this code.