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20060313: GB_GUBD

Steve and Scott,

A user was questioning the accuracy of decoded precipitation values
from the NAM104 (grbgrd) files. I tacked down the problem to the
following:

1) The grib tables use "-0.01" as the missing value for P--M (and other
precipitation grids).

2) The grib data uses a bit mask section, so the nagrib.f routine must
call GB_GUBD to unpack the data and replace the missing bits with rmsval
(which is not -9999.00).

3) The GB_GUBD will return "nbits", the number of bits used in the BDS
section possibly modified as follows:
- if 1, increase to 2 bits
- if rmsval is RMISSD, nbits is increased by one, (so that the maxval
will signal the missing value which reading back)
Otherwise, nbits is not changed.

The problem that occurs is:
-0.01 is not in the range specified by the reference value, scale and
precision, and the number of bits is not increased as necessary to
reepresent the new data range without loss of precision in the
subsequent GD_WPGD call. 

Is there an underlying use of -0.01 in contour plots or something else
that would be a problem is 0 were used? I'm assuming that -9999 would be
a bad choice since it would prevent the accumulation calculations
on the intermediate P--M times, so my inclination is to change the the
tables to use 0 as the missing value, rather than change the NAGRIB
or GB code.

Do you have any thoughts about this?

Steve
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Steve Chiswell
Unidata User Support