Introduction

In grib2, the grid points can have undefined values. Grib doesn't doesn't use a special value to indicate an undefined but uses one of 3 methods.

1. a bitmap (used by jpeg2000, simple, ieee and by complex packing)
2. the IEEE 754 format undefined (ieee packing)
3. a special bit pattern by complex packing (prefered)

However, users usually find a special value the easiest method to indicate an undefined value. Wgrib2api, like wgrib2, uses the value 9.999e20 to indicate a special value. There are one symbolic values and two functions in wgrib2api (v2.0.8) to handle undefined values.

• grb2_UNDEFINED: symbolic value for 9.999e20
• grb2_UNDEFINED_VAL(x): returns true if x is the undefined values otherwise false
• grb2_DEFINED_VAL(x): returns true if x is a defined values otherwise false

Usage

    grid(i,j) = grb2_UNDEFINED
       set grid(i,j) to the special undefined values 9.999e20
    if (grb2_UNDEFINED(grid(i,j)) then
       if grid(i,j) is undefined than
    if (grb2_DEFINED(grid(i,j)) then
       if grid(i,j) is defined than