wgrib2: set_scaling
Introduction
The values at the grid points points are usually stored in in this format,
Y = (R + i*2**B)*(10**D)
R = reference value (32bit IEEE floating point number)
i = integer, 0..2**N1
N = binary bit precision
B = binary scaling, 127..127
D = decimal scaling, 127..127
There are 3 sytems for storing the number which I call
ECMWF convention: D = 0, N = parameter
Y = R + i*2**B
R = reference value
i = integer, 0..2**N1
N = binary bit precision, a parameter
B = binary scaling, determined by grib routines
NCEP convention: B = parameter, D = parameter
R = reference value
i = integer, 0..2**N1
N = binary bit precision, determined by grib routines
B = binary scaling, a parameter
D = decimal scaling, a parameter
Note, global model uses a variant: B = 0, D = parameter
Both the ECMWF and NCEP conventions have their advantages and
disadvantages. The ECMWF method is easier to use, you just
set the binary precision to N bits (12? 16?) for all variables
and you are done. With the NCEP convention, you have to
set the scaling for each variable separately. For some variables
such as specific humidity, the scaling should be pressure
dependent. On the other hand, if you are trying to get the smallest
files, the NCEP convention is better. For example, you want to
get the RH to the nearest integer. With the NCEP method, you simply
set D = B = 0. For general use, I suggest that you use the ECMWF
convention because people time is usually more valuable than disk space.
Ok, I value my time more than a few GB. On the othe hand, I've been
involved with more than my share of projects were disk space
has been the critical issue.
By default, wgrib2 will encode using the ECMWF convention using 12 bits.
The number of bits can be changed by the set_bin_prec option.
The set_grib_max_bits option will have to be
used if the binary precision is set to more than 16.
Default Scaling
When you read a field, the scaling of the field (B, D) are saved as
the scaling parameter. However, Some options such as
the rpn option can
change the magnitude of the field and scaling from the
input field may not longer be appropriate. So these
options will revert to the default scaling (ECMWFstyle using
the N bits). However, in some cases such as timeinterpolation
or smoothing, the original scaling is appropriate. In this
case you can set the B and D scaling to text string, same.
Usage
The set_scaling option is used to
change the binary and decimal scaling parameters for the next
write. (The binary and decimal scaling parameters will be
reset by reads and calls to RPN.) This will set wgrib2 to
to encode data using the NCEP convention. If you want to
encode data using the ECMWF convention, you need to use
the set_bin_prec option perhaps with the
set_grib_max_bits option.
set_scaling D B
D = decimal scaling or the text 'same' with no quotes
B = binary scaling or the text 'same' with no quotes
See alse:
set_bin_prec
set_grib_max_bits
scale
scaling
