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: 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. 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. 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.