Draws a two-dimensional velocity field plot on a map, with each directed arrow indicating the magnitude and direction of the field.

u—The X component of the two-dimensional field. Must be a two-dimensional array of the same size as v.

v—The Y component of the two-dimensional field. Must be a two-dimensional array of the same size as u.

x—(optional) The abscissa values. Must be a vector of longitude coordinates. The size of x must equal the first dimension of u and v.

y—(optional) The ordinate values. Must be a vector of latitude coordinates. The size of y must equal the second dimension of u and v.

Color—Sets the color index of vector arrows. If this keyword is omitted, !P.Color specifies the color index.

Dots—If present and nonzero, places a dot at the position of the missing data. Otherwise, nothing is drawn for missing points. Dots is only valid if the Missing keyword is also specified.

Length—A length factor. The default value is 1.0, which makes the longest (u, v) vector have a length equal to the length of a single cell.

Missing—A 2D array with the same size as the u and v arrays. It is used to specify that specific points have missing data.

If the magnitude of the vector at (i, j) is less than the corresponding value in Missing, then the data is considered to be valid. Otherwise, the data is considered to be missing.

Thus, one way to set up a Missing array is to initialize all elements to some large value:

missing_array = FLTARR(n, m) + 1.0E30

Then, if point (i, j) is a missing point, set the corresponding element to a negative value:

missing_array(i, j) = -missing_array(i, j)

MAP_VELOVECT draws a two-dimensional velocity field plot. The arrows indicate the magnitude and the direction of the field.

If missing values are present, you can use the Missing keyword to specify that they be ignored during the plotting, or the Dots keyword to specify that they be marked with a dot.

For examples of MAP_VELOVECT, refer to the following program: