Could not retrieve add offset

[Willis, Paul ENV:EX]

Trying to concantenate two netCDF files:  each, a year's worth of daily
slp means.  data can be plotted but is clearly wrong.  It appears to be an
offsest problem.  See below.

Do not understand the warning about the missing value as well.

Running GrADS 1.9b4 on linux.


Thanks in advance for any guidance.

-paul
*********************


ga-> q dim

Default file number is: 1

X is varying   Lon = 0 to 360   X = 1 to 145

Y is varying   Lat = -90 to 90   Y = 0.999999 to 73

Z is fixed     Lev = 1  Z = 1

T is fixed     Time = 00Z01JAN1999  T = 1

ga-> d slp
"  -- using -1e+15 insteade data type for "missing_value
" not handled
-- using -1e+15 instead

Warning: Could not retrieve add offset data type -- setting add offset to
0.0

Contouring: -20000 to -16500
 interval 500

*********************
here is my descriptor file
*********************
dset /home/pfwillis/2006/Grads/circulation/Climate/data/slp_daily.%y4.nc
dtype netcdf
options yrev template
undef -1e15 missing_value
unpack scale_factor add_offset
title NCEP Reanalysis Daily Averages
xdef 144 linear 0 2.5
ydef 73 linear -90 2.5
zdef   1 linear 1 1
tdef 731 linear 00Z01jan1999 24hr
vars 1
slp 0 t,y,x mean Daily Sea Level Pressure [Pa]
endvars


*********************
should an ncdump provide useful information on this, see below.
*********************


netcdf slp_daily.1999 {
dimensions:
 lon = 144 ;
 lat = 73 ;
 time = UNLIMITED ;
 // (365 currently)
variables:
 float lat(lat) ;
  lat:units = "degrees_north" ;

  lat:actual_range = 90.f, -90.f ;
  lat:long_name = "Latitude" ;

 float lon(lon) ;
  lon:units = "degrees_east" ;

  lon:long_name = "Longitude" ;

  lon:actual_range = 0.f, 357.5f ;
 double time(time) ;

  time:units = "hours since 1-1-1 00:00:0.0" ;

  time:long_name = "Time" ;
  time:actual_range = 17514144., 17522880. ;

  time:delta_t = "0000-00-01 00:00:00" ;

time:avg_period = "0000-00-01 00:00:00" ;
 short slp(time, lat, lon) ;

slp:long_name = "mean Daily Sea Level Pressure" ;

  slp:valid_range = 87000.f, 115000.f ;

  slp:actual_range = 92970.f, 110552.f ;

  slp:units = "Pascals" ;

  slp:add_offset = 119765.f ;

  slp:scale_factor = 1.f ;

  slp:missing_value = 32766s ;

  slp:precision = 0s ;

  slp:least_significant_digit = -1s ;

 slp:GRIB_id = 2s ;

  slp:GRIB_name = "PRMSL" ;

  slp:var_desc = "Sea Level Pressure\n",
   "P" ;

  slp:dataset = "NCEP Reanalysis Daily Averages\n",
   "AJ" ;

  slp:level_desc = "Sea Level\n",
   "I" ;

  slp:statistic = "Mean\n",
   "M" ;

  slp:parent_stat = "Individual Obs\n",
   "I" ;


// global attributes:
  :Conventions = "COARDS" ;

  :title = "mean daily NMC reanalysis (1999)" ;

  :base_date = 1999s, 1s, 1s ;

  :history = "created 99/01/04 by Hoop (netCDF2.3)" ;

  :description = "Data is from NMC initialized reanalysis\n",

   "(4x/day).  It consists of most variables
interpolated to\n",

   "pressure surfaces from model (sigma) surfaces." ;

  :platform = "Model" ;


data:

 lat = .......