GEMPAK Manual | Programs | Parameters


     IJSKIP is used to control subsetting of the internal grid by
     declaring bounding index values and numbers of points to skip in
     each index direction, I and J.  Input is entered as follows:


     where Iskp and Jskp are the number of grid points to skip in
     the I and J grid index directions, respectively; Istrt and
     Istp are the beginning and ending I index values, respectively;
     and Jstrt and Jstp are the beginning and ending J index values,
     respectively.  GEMPAK grids have (I,J) = (1,1) at the lower
     left corner, with I increasing in the X (horizontal or east-west)
     direction.  J increases in the Y (vertical or north-south)
     direction.  The internal grid navigation is determined from the
     GDFILE entry.

     The defaults for Iskp and Jskp are zero.  The other parameters
     default to missing values so that they will be determined
     automatically, depending on the area of the grid required by
     the map display.

     If only Iskp is given, then Jskp = Iskp, and the bounds are
     determined automatically.  The skip values thin the grid by
     skipping over the number of points specified.

     If IJSKIP = NO, then the results are the same as for IJSKIP set
     to blank or zero.  If IJSKIP = YES, then all the skipping values
     are computed automatically.  When IJSKIP = YES, skipping is only
     done if the number of grid points required to cover the display
     area is too large.  The automatic number of points skipped is
     the same in each index direction, and it will be the minimum
     number required to make the internal computational grid fit in
     allocated memory.

     IJSKIP and SKIP are functionally independent.  If SKIP is also
     specified, IJSKIP operates first to create the subset grid,
     then the SKIP information is applied to that subset grid, which
     has already been thinned by IJSKIP.  Since IJSKIP and SKIP are
     done in series, the combined effect is multiplicative.  IJSKIP
     determines the internal computational grid; therefore, changing
     it changes the results of computations involving derivatives.
     The SKIP parameter is applied to the internal grid after compu-
     tations and affects only the display.  SKIP and IJSKIP results,
     when done separately and compared, will not always yeild coinci-
     dent points for the same skipping values, because IJSKIP creates
     a computational halo of grid points around the display region
     resulting in different initial indexes for IJSKIP vis-a-vis SKIP.

     In programs like GDPLOT2, which allows different specifications
     of IJSKIP for different GDPFUN entries, IJSKIP must be the same
     for all fields if the // grid naming feature is used in GDPFUN.
     This is because IJSKIP determines the navigation of the internal
     grid, which must remain the same if saved grids are to be kept
     for reuse.

     If user-supplied index bounds are not consistent with the given
     number to skip, then the bounds will be reset, taking into
     account any nearby grid boundaries.  The reset values will be
     close to what the user intended.  For best results, provide
     consistent bounding index values.  If the full range of the index
     is required, then the unit (1) value of the index is the starting
     If the grid is a global grid and the "seam" falls within the
     display area, then if Istrt is given, Istp must also be given;
     otherwise, an error message is generated.  If both Istrt and
     Istp are given in the case of the global grid, the grid "seam"
     will not be shifted out of the display area.

     If a start value (strt) exceeds a stop value, an error message

     Some examples follow:

      Example                            Action

     IJSKIP = YES   Skip only if necessary, skipping as few points
                    as possible; if the subset area is reduced fewer
                    points will be skipped

     IJSKIP = 5     Skip 5 points in both index directions; beginning
                    and ending indexes are automatically determined

     IJSKIP = 5/3   Skip 5 points in the I index direction; skip 3
                    points in the J index direction; beginning and
                    ending indexes are automatically determined

     IJSKIP = 5;3/2 Skip 5 points in the I index direction; skip 2
                    points in the J index direction; beginning I
                    index is 3; all other indexes are determined