DSET
DTYPE
INDEX
TITLE
UNDEF
OPTIONS
XDEF
YDEF
ZDEF
TDEF
VARS
ENDVARS
FILEHEADER
THEADER
XYHEADER
data_filename
is prefixed with a
^
data_filename
must include a full path. For
example, if the data descriptor file is:
/data/wx/grads/sa.ctl
and the binary data file is:
/data/wx/grads/sa.dat
then the data file name in the data descriptor file can be :
DSET ^sa.dat
instead of:
DSET /data/wx/grads/sa.dat
As long as the two files are kept together, they may be moved to any directory without changing the entries in the data descriptor file.
station
or grib
. If the data
type is neither of these, then this entry is omitted completely from
the descriptor file and GrADS will assume the data type is gridded binary.
Here are some examples:
Here is an examples:
The best way to ensure hardware independence for gridded data is to
specify the data's source platform. This allows the data to be used on
any type of hardware without having to worry about byte ordering. The
following
The
The
Here are some examples:
The
The
The Gaussian mapping methods require one additional
argument:
Here are some examples:
The NCEP/NCAR Reanalysis surface variables are on the GAUST62 grid.
The final example shows that there are 20 Y dimension values which
start at Gaussian Latitude 15 (64.10 south) on the Gaussian R40
grid.
The
The
Here are some examples:
where:
If not specified,
where:
Here are some examples:
The format of the
The
Here is an example:
INDEX filename
This entry specifies the name of the grib map file. It is required when using
the
DTYPE grib
entry to read grib formatted data. The file is generated
by running the external utility gribmap
.
This entry gives brief description of the contents of the data set. This will
be included in the output from a
query
command, so it is helpful to put
meaningful information here.
TITLE 5 Days of Sample Model Output
TITLE NCEP Reanalysis Jan 1987 thru Jul 1999
This entry specifies the undefined, or missing, data value. GrADS
operations and graphics routines will ignore data with this value from
this data set.
UNDEF
is a required parameter even if
there are no undefined data.
UNDEF -9.99e33
This entry controls various aspects of the way GrADS interprets the
raw data file. It replaces the old
FORMAT
record. The
keyword
argument may be one or more of the
following:
yrev
Indicates that the Y dimension (latitude) in the data file has
been written in the reverse order from what GrADS assumes. An
important thing to remember is that GrADS still presents the view that
the data goes from south to north. The YDEF statement does not change;
it still describes the transformation from a grid space going from
south to north. The reversal of the Y axis is done as the data is read
from the data file.
zrev
Indicates that the Z dimension (pressure) in the data file has
been written from top to bottom, rather than from bottom to top as
GrADS assumes. The same considerations as noted above for
yrev
also apply.template
Indicates that a template for multiple data files is in use. For
more information, see the section on Using
Templates.
sequential
Indicates that the file was written in sequential unformatted I/O.
This keyword may be used with either station or gridded data. If your
gridded data is written in sequential format, then each record must be
an X-Y varying grid. If you have only one X and one Y dimension in
your file, then each record in the file will be one element long (it
may not be a good idea to write the file this way).
byteswapped
Indicates the binary data file is in reverse byte order from the
normal byte order of your machine. Putting this keyword in the
OPTIONS
record of the descriptor file tells GrADS to swap
the byte order as the data is being read. May be used with gridded or
station data.
OPTION
keywords indicate the actual byte
ordering of the data. If the data are already in the correct order, no
conversion is performed. These options facilitate moving data files
and descriptor files between machines.
big_endian
Indicates the data file contains 32-bit IEEE floats created on a
big_endian platform (e.g., sun, sgi and hp). May be used with gridded
or station data.
little_endian
Indicates the data file contains 32-bit IEEE floats created on a
little_endian platform (e.g., iX86, and dec). May be used with gridded
or station data.
cray_32bit_ieee
Indicates the data file contains 32-bit IEEE floats created on a
cray. May be used with gridded or station data.
365_day_calendar
Indicates the data file was created with perpetual 365-day years,
with no leap years. This is used for some types of model output.
XDEF xnum mapping <additional arguments>
This entry defines the grid point values for the X dimension. The
first argument,
xnum
, specifies the number of
grid points in the X direction. xnum
must be an
integer >= 1. mapping
defines the method by which
longitudes are assigned to X grid points. There are two options for
mapping
:
LINEAR
Linear mapping
LEVELS
Longitudes specified individually
LINEAR
mapping method requires two additional
arguments: start
and
increment
. start
is a floating
point value that indicates the longitude at grid point
X=1. Negative values indicate western longitudes.
increment
is the spacing between grid point values,
given as a positive floating point value.
LEVELS
mapping method requires one additional
argument, value-list
, which explicitly specifies
the longitude value for each grid point.
value-list
should contain
xnum
floating point values. It may continue into
the next record in the descriptor file, but note that records may not
have more than 255 characters. There must be at least 2 levels in
value-list
; otherwise use the LINEAR method.
XDEF 144 LINEAR 0.0 2.5
XDEF 72 LINEAR 0.0 5.0
XDEF 12 LEVELS 0 30 60 90 120 150 180 210 240 270 300 330
XDEF 12 LEVELS 15 45 75 105 135 165 195 225 255 285 315 345
YDEF ynum mapping <additional arguments>
This entry defines the grid point values for the Y dimension. The
first argument,
ynum
, specifies the number of
grid points in the Y direction. ynum
must be an
integer >= 1. mapping
defines the method by which
latitudes are assigned to Y grid points. There are several options for
mapping
:
LINEAR
Linear mapping
LEVELS
Latitudes specified individually
GAUST62
Gaussian T62 latitudes
GAUSR15
Gaussian R15 latitudes
GAUSR20
Gaussian R20 latitudes
GAUSR30
Gaussian R30 latitudes
GAUSR40
Gaussian R40 latitudes
LINEAR
mapping method requires two additional
arguments: start
and
increment
. start
is a floating
point value that indicates the latitude at grid point Y=1. Negative
values indicate southern latitides. increment
is
the spacing between grid point values in the Y direction. It is
assumed that the Y dimension values go from south to north, so
increment
is always positive.
LEVELS
mapping method requires one additional
argument, value-list
, which explicitly specifies
the latitude for each grid point, from south to north.
value-list
should contain
ynum
floating point values. It may continue into
the next record in the descriptor file, but note that records may not
have more than 255 characters. There must be at least 2 levels in
value-list
; otherwise use the LINEAR method.
start
. This argument indicates the first
gaussian grid number. If the data span all latitudes, start
would be 1, indicating the southernmost gaussian grid
latitude.
YDEF 73 LINEAR -90 2.5
YDEF 180 LINEAR -90 1.0
YDEF 18 LEVELS -85 -75 -65 -55 -45 -35 -25 -15 -5 5 15 25 35 45 55 65 75 85
YDEF 94 GAUST62 1
YDEF 20 GAUSR40 15
ZDEF znum mapping <additional arguments>
This entry defines the grid point values for the Z dimension. The
first argument,
znum
, specifies the number of
pressure levels. znum
must be an
integer >= 1. mapping
defines the method by which
longitudes are assigned to Z grid points. There are two options for
mapping
:
LINEAR
Linear mapping
LEVELS
Pressure levels specified individually
LINEAR
mapping method requires two additional
arguments: start
and
increment
. start
is a floating
point value that indicates the longitude at grid point Z=1.
increment
is the spacing between grid point values
in the Z direction, or from lower to
higher. increment
may be a negative value.
LEVELS
mapping method requires one additional
argument, value-list
, which explicitly specifies
the pressure level for each grid point in ascending order.
value-list
should contain
znum
floating point values. It may continue into
the next record in the descriptor file, but note that records may not
have more than 255 characters. There must be at least 2 levels in
value-list
; otherwise use the LINEAR method.
ZDEF 10 LINEAR 1000 -100
ZDEF 7 LEVELS 1000 850 700 500 300 200 100
ZDEF 17 LEVELS 1000 925 850 700 600 500 400 300 250 200 150 100 70 50 30 20 10
TDEF tnum LINEAR start increment
This entry defines the grid point values for the T dimension. The
first argument,
tnum
, specifies the number of time
steps. tnum
must be an integer >= 1. The method by
which times are assigned to T grid points is always
LINEAR
.
start
indicates the initial time value at grid
point T=1. start
must be specified in the GrADS
absolute date/time format:
hh:mmZddmmmyyyy
hh   
= hour (two digit integer)
mm   
= minute (two digit integer)
dd   
= day (one or two digit integer)
mmm  
= 3-character month
yyyy 
= year (may be a two or four digit integer; 2 digits implies a
year between 1950 and 2049).
hh
defaults to 00
,
mm
defaults to 00
, and
dd
defaults to 1
. The month and year
must be specified. No intervening blanks are allowed in the GrADS
absolute date/time format.
increment
is the spacing between grid point values
in the T direction. increment
must be
specified in the GrADS absolute time increment format:
vvkk
vv =
an integer number, 1 or 2 digits
kk = mn
(minute)
hr
(hour)
dy
(day)
mo
(month)
yr
(year)
TDEF 60 LINEAR 00Z31dec1999 1mn
TDEF 73 LINEAR 3jan1989 5dy
TDEF 730 LINEAR 00z1jan1990 12hr
TDEF 12 LINEAR 1jan2000 1mo
TDEF 365 LINEAR 12Z1jan1959 1dy
TDEF 40 LINEAR 1jan1950 1yr
VARS varnum
variable_record_1
variable_record_2
...
variable_record_varnum
ENDVARS
This ensemble of entries describes all the variables contained in the data set.
varnum
indicates the number of variables in the data set and is
therefore also equal to the number of variable records that are listed between
the VARS
and ENDVARS
entries. ENDVARS
must be the final line of the Grads data descriptor file. Any blank lines after
the ENDVARS statement may cause open
to fail!
variable_records
is as follows:
varname levs units description
varname
is a 1-15 character "name" or abbreviation for the
variable. varname
may contain alphabetic and numeric characters
but it must start with an alphabetic character (a-z).
levs
is an integer that specifies the number of
vertical levels the variable contains. levs
may
not exceed znum
as specified in the ZDEF
statement. If levs
is 0, the
variable does not correspond to any vertical level. Surface variables
(e.g. sea level pressure) have a levs
value of
0.
units
component of the
variable_record
is used for data in GRIB and other special formats. If the data your are
describing are not in one of these formats, put a value of
99
here.
description
is text description of the variable, max 40
characters.
VARS 8
PS 0 99 Surface Pressure
U 7 99 U-Winds
V 7 99 V-Winds
Z 7 99 Geopotential Heights
T 7 99 Temperature
Q 5 99 Specific Humidity
TS 0 99 Surface Temperature
P 0 99 Precipitation
ENDVARS
This optional entry tells GrADS that your data file has a header
record of
length
bytes that precedes the data.
GrADS will skip past this header, then treat the remaineder of the
file as though it were a normal GrADS binary file after that
point. This optional descriptor file entry is only valid for GrADS
gridded data sets.
This optional entry tells GrADS that the data file has a header record
of
length
bytes preceding each time block of
binary data. This optional descriptor file entry is only valid for
GrADS gridded data sets. See the section on structure of a
gridded binary data file for more information.
This optional entry tells GrADS that the data file has a header record
of
length
bytes preceding each horizontal grid
(XY block) of binary data. This optional descriptor file entry is only
valid for GrADS gridded data sets. See the section on structure of a
gridded binary data file for more information.