The G4GRID Procedure

The GRID Statement

GRID x*y*z=variable< ...variable-n> </ <grid-options>>;

The GRID statement specifies the variables for interpolation. These include the volume variables to be used as coordinates for the output grid, as well as one or more response variables to be interpolated. You can specify multiple response variables for the three volume variables that you specify. Options for the GRID statement control the observations produced in the output data set. You can specify only one GRID statement.

Options

Options affect all output produced by the GRID statement. If you use any of these options, separate them from the grid request with a slash (/). If you do not use any options, omit the slash.

XAXIS=list-of-coordinates
YAXIS=list-of-coordinates
ZAXIS=list-of-coordinates

specifies a list of numeric values to be assigned to the respective axis. The list does not have to be in order; it will be arranged in ascending numeric order. The list should not contain duplicates. The list overrides the number of values defined by the NXAXIS=, NYAXIS=, or NZAXIS= options. The list of coordinates may be specified in the following ways:

n n ...n 
n,n, ... ,n 
n TO n <BY increment>
<n ... n> n,n ... ,n TO n <BY increment> <n, ... ,n>

NXAXIS=n NYAXIS=n NZAXIS=n

specifies the number of coordinates along the respective axis. The default is 11. The minimum and maximum values along each axis are used as its starting and ending points. Each axis is divided into n-1 equally spaced sections. These options are ignored if the corresponding XAXIS=, YAXIS=, or ZAXIS= options are specified.

XEXTRA=n YEXTRA=n ZEXTRA=n

specifies the number of temporary grid cells to be added to the interpolation along the respective axis. Normally, only points within the grid volume are included in the interpolation. However, this can result in some edge effect errors, since no data outside this volume are used. To avoid such conditions, you can set the option to a positive non-zero value.

The distance of the new points from the end, which is the size of each "border," is the width of the first and last coordinate pair, respectively. All points that fall within the new coordinate boundaries are used in the interpolation. The size of the output grid is not affected.

UNIFORM

instructs the G4GRID procedure to assume that all coordinates along each axis are equally spaced. This allows the procedure to take some short cuts to slightly speed processing. The UNIFORM option is not necessary unless XAXIS=, YAXIS=, or ZAXIS= options are specified.

If XAXIS=, YAXIS=, or ZAXIS= is specified, PROC G4GRID must assume that the spacing between output grid points is not equal. As a result, special processing must occur for each correction pass.

If none of the above options are specified or the UNIFORM option is specified, the procedure assumes all points in the output grid are equally spaced and omits the special processing. If you specify one of the above options and the output grid points are known to be equally spaced, specifying the UNIFORM option may slightly speed processing. If, however, the output grid points are unevenly spaced to a significant extent, and the UNIFORM option is specified, erroneous results can occur. UNIFORM is the default unless XAXIS=, YAXIS=, or ZAXIS= is specified.

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