Introduction

With most grib packing methods, the grid values are usually stored as scaled integers. The grib format allows the scaled integers to have up to 254 bits (simple packing). This is an unreasonably high precision as wgrib2 converts the data to a single-precision floating point number which typically has only 25 bits of precision. To speed up the encoding and decoding processing, wgrib2 uses 32 bit integers which limits the maximum size of scaled integers to 25 after accounting for the limits of the packing and unpacking routines.

Grib fields are usually stored as a scaled integers, and usually you don't need 25 bits of precision. For example, temperature to the nearest 0.1 degree, means that the a temperature range of 50 C can be stored as 500 descrete values and only 7 bits are needed to store the temperature to the nearest 0.1 degree.

The problem is that people can set the scaling factors to inappropriate values, using 25 bits of precision will was (disk) space. Consequently wgrib2 has a resetable limit of the precision of the scaled integers allowed. If you exceed the limit, the scaling factors are change so that the scaled integers are limited.

The -set_grib_max_bits option sets the maximum number of bits that the scaled integers can have when encoding data into grib. The value should never be greater than 25 as that is the limit for wgrib2 encoding. However, some grib packages may not support 25 bit precision in decoding, so you may want to limit the precision to 24 to remain compatible with other software packages.

The -set_grib_max_bits option does not set the binary precision of the grib output but sets the maximum possible precision of the numbers.

Usage

-set_grib_max_bits N
  N = maximum number of bits used to encode data
  the default value is 16, N = 1..25

Comments

The -set_grib_max_bits option does not affect grib data written using the -grib option because the -grib option does not encode data but copies the input grib message.

See alse: -set_scaling -set_bin_prec -grib_max_bits