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NAME: VELOCITY_FIELD PURPOSE: Overplots a 2-D velocity field atop a map or plot window. CATEGORY: Plotting CALLING SEQUENCE: VELOCITY_FIELD, U, V, X, Y [, keywords ] 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. V must have the same dimensions as U. 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) ). X -> Abcissae values. X must be a vector with a length equal to the first dimension of U and V. Y -> Ordinate values. Y must be a vector with a length equal to the second dimension of U and V. KEYWORD PARAMETERS: MISSING -> Missing data high-cutoff value. Vectors with a magnitude greater than MISSING are ignored. /DOTS -> If set, will place a dot at each missing data point. Otherwise, will draw nothing for missing data points. /DOTS only has effect if MISSING is specified. X_STEP -> The X-extent of a cell in data coordinates. If X_STEP is not specified, then a VELOCITY_FIELD will compute it as: X_STEP = ( Max(X) - Min(X) ) / ( N_Elements(X) - 1 ) Where X is the array of abscissae as described above. Y_STEP -> The X-extent of a cell in data coordinates. If X_STEP is not specified, then a VELOCITY_FIELD will compute it as: Y_STEP = ( Max(Y) - Min(Y) ) / ( N_Elements(Y) - 1 ) Where X is the array of ordinates as described above. COLOR -> Specifies the color of the arrows. Default is black. /DATELINE -> Will skip points for which an arrow cannot be drawn (usually across the date line). HSIZE -> The length of the lines used to draw the arrowhead. If HSIZE is positive, then the arrow head will be the same regardless of the length of each vector. (Default size is !D.X_SIZE / 100). If HSIZE is negative, then the arrowhead will be 1/HSIZEth of the total arrow length. LEGENDLEN -> Specify an arrow of a given length in DATA coordinates displayed as a legend in the calling routine. Default is LONGEST. LEGENDNORM -> Returns to the calling program the length of LEGENDLEN in NORMAL coordinates. This is needed in order to plot a legend arrow in the calling program. LEGENDMAG -> Returns to the calling program the magnitude of the vector of size LEGENDLEN. Default is LONGEST. _EXTRA=e -> Picks up all other keywords for PLOT, PLOTS, etc. OUTPUTS: None. SUBROUTINES: Internal Subroutines: ===================== VF_MAGNITUDE (function) NOTES: (1) You need to call MAP_SET or PLOT first, to establish the coordinate system. VELOCITY_FIELD can only overplot vectors atop of an existing map or plot window. (2) If you are calling VELOCITY_FIELD to overplot vectors atop a world map, then in the calling program you must make sure that the longitude values contained in the X vector are in the range 0 - 360. (3) If you do not explicitly specify Y_STEP, and your grid has half-size boxes at the poles, then the value of Y_STEP computed by VELOCITY_FIELD might be different from the actual latitude interval. (4) VELOCITY_FIELD assumes that U, V, X, and Y are on a regularly-spaced grid (e.g. longitude & latitude). (5) Now uses the IDL ARROW command to draw the arrow heads. RESTRICTIONS: None. CALLING SEQUENCE: VELOCITY_FIELD, U, V, X, Y, Thick=3, HSize=0.1 ; produces a velocity field plot with an arrow ; thickness of 3 and a arrow head size of 10% of ; the arrow body size. 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. bmy, 03 Dec 1999: GAMAP VERSION 1.44 - renamed to VELOCITY_FIELD - added ARRLEN, HSIZE, HANGLE, THICK keywords - cleaned up some things bmy, 26 May 2000: GAMAP VERSION 1.45 - updated comments, minor cleanup bey, bmy, 24 Jul 2000: GAMAP VERSION 1.46 - several bug fixes - added internal routine MAGNITUDE - added X_STEP, Y_STEP, MAXMAG keywords sjg, bmy, 01 Aug 2000: - added error check on index array GOOD - now compare magnitudes to abs( MISSING ) - now error check for MAXLEN: prevent div by 0 - updated comments bmy, 23 Jul 2002: GAMAP VERSION 1.51 - now use IDL ARROW procedure to draw arrows - HSIZE is now defaulted to device coordinates - removed HANGLE keyword -- it's obsolete - now specify legend vector w/ LEGENDLEN - renamed ARRLEN to LEGENDNORM bmy, 02 Jun 2005: GAMAP VERSION 2.04 - Added /DATELINE keyword to perform a quick test to skip over points which will choke at the date line bmy & phs, 13 Jul 2007: GAMAP VERSION 2.10 phs, 20 Mar 2008: GAMAP VERSION 2.12 - Now test if the vector position is in the plot window - Added the PLOTPOSITION keyword

**(See /n/home09/ryantosca/IDL/gamap2/plotting/velocity_field.pro)**