PRO VELOV_Fr,U,V,X,Y, Missing = Missing, Length = length, Dots = dots,  $             
        Color=color, _EXTRA = extra             
;             
;+             
; NAME:             
;       VELOVECT             
;             
; PURPOSE:             
;       Produce a two-dimensional velocity field plot.             
;             
;       A directed arrow is drawn at each point showing the direction and              
;       magnitude of the field.             
;                            
; CATEGORY:             
;       Plotting, two-dimensional.             
;             
; CALLING SEQUENCE:             
;       VELOVECT, 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.             
;             
;       Y:      Optional ordinate values.  Y must be a vector with a length             
;               equal to the first dimension of U and V.             
;             
; KEYWORD INPUT PARAMETERS:             
;      MISSING: Missing data value.  Vectors with a LENGTH greater             
;               than MISSING are ignored.             
;             
;       LENGTH: Length factor.  The default of 1.0 makes the longest (U,V)             
;               vector the length of a cell.             
;             
;       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.             
;             
;       COLOR:  The color index used for the plot.             
;             
;       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.             
;-             
;             
        on_error,2                      ;Return to caller if an error occurs             
        s = size(u)             
        t = size(v)             
        if s(0) ne 2 then begin              
baduv:   message, 'U and V parameters must be 2D and same size.'             
                endif             
        if total(abs(s(0:2)-t(0:2))) ne 0 then goto,baduv             
;             
        if n_params(0) lt 3 then x = findgen(s(1)) else $             
                if n_elements(x) ne s(1) then begin             
badxy:                  message, 'X and Y arrays have incorrect size.'             
                        endif             
        if n_params(1) lt 4 then y = findgen(s(2)) else $             
                if n_elements(y) ne s(2) then goto,badxy             
;             
        if n_elements(missing) le 0 then missing = 1.0e30             
; FR's    
; alt      if n_elements(length) le 0 then length = 1.0             
             
        mag = sqrt(u^2+v^2)             ;magnitude.             
                ;Subscripts of good elements             
        nbad = 0                        ;# of missing points             
        if n_elements(missing) gt 0 then begin             
                good = where(mag lt missing)              
                if keyword_set(dots) then bad = where(mag ge missing, nbad)             
        endif else begin             
                good = lindgen(n_elements(mag))             
        endelse             
             
        ugood = u(good)             
        vgood = v(good)             
        x0 = min(x)                     ;get scaling             
        x1 = max(x)             
        y0 = min(y)             
        y1 = max(y)             
        x_step=(x1-x0)/s(1)             
        y_step=(y1-y0)/s(2)     
;FR's    
        maxmag=max([max(ugood/x_step),max(vgood/y_step)])             
        if n_elements(length) gt 0 then maxmag=length/x_step                    
        sina = (ugood/maxmag)             
        cosa = (vgood/maxmag)    
print,maxmag    
; alt       sina = length * (ugood/maxmag)             
; alt       cosa = length * (vgood/maxmag)             
    
;             
        if n_elements(title) le 0 then title = ''             
        ;--------------  plot to get axes  ---------------             
        if n_elements(color) eq 0 then color = !p.color             
        x_b0=x0-x_step             
        x_b1=x1+x_step             
        y_b0=y0-y_step             
        y_b1=y1+y_step             
        if n_elements(position) eq 0 then begin             
;          plots,[x_b0,x_b1],[y_b1,y_b0],/nodata,/xst,/yst, $             
;            color=color, _EXTRA = extra             
        endif else begin             
;          plots,[x_b0,x_b1],[y_b1,y_b0],/nodata,/xst,/yst, $             
;            color=color, _EXTRA = extra             
        endelse             
;             
        r = .3                          ;len of arrow head             
        angle = 22.5 * !dtor            ;Angle of arrowhead             
        st = r * sin(angle)             ;sin 22.5 degs * length of head             
        ct = r * cos(angle)             
;             
        for i=0,n_elements(good)-1 do begin     ;Each point             
                x0 = x(good(i) mod s(1))        ;get coords of start & end             
                dx = sina(i)             
                x1 = x0 + dx             
                y0 = y(good(i) / s(1))             
                dy = cosa(i)             
                y1 = y0 + dy             
                xd=x_step             
                yd=y_step             
                plots,[x0,x1,x1-(ct*dx/xd-st*dy/yd)*xd, $             
                        x1,x1-(ct*dx/xd+st*dy/yd)*xd], $             
                      [y0,y1,y1-(ct*dy/yd+st*dx/xd)*yd, $             
                        y1,y1-(ct*dy/yd-st*dx/xd)*yd], $             
                      color=color             
                endfor             
;        if nbad gt 0 then $             ;Dots for missing?             
;                oplot, x(bad mod s(1)), y(bad / s(1)), psym=3, color=color             
end