Components of a GrADS Data Descriptor File
DSET data_filename
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This entry specifies the filename of the data file being described. If the data and the descriptor file are not in the same directory, then data_filename must include a full path. If a ^ character is placed in front of data_filename,
then data_filename is assumed to be relative to the path of the descriptor file. If you are using the ^ character in the DSET entry, then the descriptor file and the data file may be moved to a new directory without changing any entries in the data descriptor file, provided their relative paths remain the same. For example:
If the data descriptor file is:
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/data/wx/l
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
If data_filename does not include a full path or a ^, then GrADS will only look for data files in the directory where you are running GrADS.
GrADS allows you u a single DSET entry to aggregate multiple data files and handle them as if they were one individual file. The individual data files must be identical in all dimensions except time, and the time range of each individual file must be indicated it its filename. To accomplish this, the DSET entry has a substitution template instead of a filename. See the ction on Using Templates for a description of all the possible components of the template. Second, the OPTIONS entry must contain the template keyword.
CHSUB t1 t2 string
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(GrADS version 1.9b4) This entry is ud with a new option for templating data files that allows for any ur-specified string substitution, instead of only date string substitution. This is uful when none of the standard template options match the time ranges in the files you wish to aggregate, or if the files are located on different disks. When you put the %ch template in
your DSET entry, then you also need to put additional CHSUB entries in the descriptor file. The string will be substituted for%ch in the data file name for the time steps beginning with t1 and ending with t2.See the ction on Using Templates for examples.
DTYPE keyword
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The DTYPE entry specifies the type of data being described. There are four options: grib, hdfsds, netcdf, or station. If the data type is none of the, then the DTYPE entry is omitted completely from the descriptor file and GrADS will assume the data type is gridded binary.
bufr (GrADS version 1.9) Data file is a BUFR station data file. This data type must be accompanied b
故宫游览攻略y the following special entries: XVAR, YVAR, TVAR, STID. Optional special entries are: ZVAR, TOFFVAR.
grib Data file is an indexed GRIB (version 1) file. This data type requires a condary entry in the descriptor file:INDEX. The INDEX entry provides the filename (including the full path or a ^) for the GRIB index file. The
index file is created by the gribmap utility. You must run gribmap and create the index file before you can
display the GRIB data in GrADS.
grib2 (GrADS version 2.0) Data file is an indexed GRIB2 file. This data type requires a condary entry in the descriptor file: : INDEX. The INDEX entry provides the filename (including the full path or a ^) for the GRIB2
index file. The index file is created by the gribmap utility. You must run grib2map and create the index file
before you can display the GRIB2 data in GrADS.
hdfsds (GrADS version 1.9) Data file is an HDF Scientific Data Set (SDS). Although HDF-SDS files are lf-describing and may be read automatically using the sdfopen/xdfopen commands, this DTYPE gives you
the option of overriding the file's own metadata and creating a descriptor file for some or all of the variables
in the file. This DTYPE may also be ud if the metadata in the HDF-SDS file is insufficient or is not
coards-compliant. This data type requires a special entry in the units field of the variable
音乐之声的插曲declaration. The undef and unpack entries contain special options for this dtype.
hdf5_grid (GrADS version 2.0.a7+) Data file is HDF5 gridded format. The HDF5 format is extremely general and is designed to store a variety of data types. The GrADS interface is only for grids, and requires a complete
descriptor file -- there is no sdfopen/xdfopen interface for HDF5.
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netcdf (GrADS version 1.9) Data file is NetCDF. Although NetCDF files are lf-describing and may b
e read automatically using the sdfopen/xdfopen commands, this DTYPE gives you the option of overriding the file's
own metadata and creating a descriptor file for some or all of the variables in the file. This DTYPE may also
be ud if the metadata in the NetCDF file is insufficient or is not coards-compliant. This data type requires a
special entry in the units field of the variable declaration. The undef and unpack entries contain special
options for this dtype.
station Data file is in GrADS station data format. This data type requires a condary entry in the descriptor file: STNMAP. The STNMAP entry provides the filename (including the full path or a ^) for the station data map
file. The map file is created by the stnmap utility. You must run stnmap and create the map file before you
can display the station data in GrADS.
INDEX filename
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This entry specifies the name of the grib map file. It is required when using the DTYPE grib or grib2 entry to read GRIB formatted data. The file is generated by running the external utility gribmap. or grib2map. Filenaming conventions are the same as tho described for the DSET entry.
STNMAP filename
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This entry specifies the name of the station map file. It is required when using the DTYPE station entry to read
GrADS-formatted station data. The file is generated by running the external utility stnmap. Filenaming conventions are the
same as tho described for the DSET entry.沙字组词
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TITLE string
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This entry gives brief description of the contents of the data t. String will be included in the output from a query command and it will appear in the directory listing if you are rving this data file with the GrADS-DODS Server (GDS), so it is helpful to put meaningful information in the title field. For GDS u, do not u double quotation marks (") in the title.
UNDEF value<undef_attribute_name>
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This entry specifies the undefined or missing data value. UNDEF is a required entry even if there are no undefined data. GrADS operations and graphics routines will ignore data with this value from this data t.
(GrADS version 1.9b4) An optional cond argument has been added for data ts of DTYPE netcdf or hdfsds -- it is the name of the attribute that contains the undefined value. This should be ud when individual variables in the data file have different undefined values. After data I/O, the missing
values in the grid are converted from the variable undef to the file-wide undef (the numerical value in the first argument of the UNDEF record). Then it appears to GrADS that all variables have the same undef value, even if they don't in the original data file. If the data require a transformation using the attributes named in the UNPACK entry, GrADS assumes the variable undef value corresponds to the data values as they appear in the file,
<,before they are transformed using a scale factor and offt. Missing packed data values are thus assigned the file-wide undef value and are never unpacked. Attribute names are ca nsitive, and it is assumed that the name is identical for all variables in the netcdf or hdfsds data file. If the name given does not match any attributes, or if no name is given, the file-wide undef value will be ud.
Example: UNDEF 1e+33 _FillValue
UNPACK scale_factor_attribute_name <add_offt_attribute_name>
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(GrADS version 1.9) This entry is ud with DTYPE netcdf, hdfsds, or hdf5_grid (GrADS version 2.0.a7+) for data variables that are 'packed' -- i.e. non-float data that need to be converted to float by applying the following formula:
y = x * scale_factor + add_offt
If your lf-describing file does not have an offt attribute, the 2nd argument may be omitted, and the offt will be assigned the default value of 0.0. If your lf-describing file has an offt attribute, but not a scale factor, u "NULL" for
the scale_factor_attribute_name. (This "NULL" option is in GrADS version 2.0.0+). Attribute names are ca nsitive, and it is assumed that the names are identical for all variables in the netcdf or hdfsds data file. If the names given do not match any attributes, the scale factor will be assigned a value of 1.0 and the offt will be assigned a value of 0.0. The transformation of packed data is done after the undef test has been applied.
Examples:
UNPACK scale_factor add_offt
UNPACK NULL add_offt
UNPACK Slope Intercept
FILEHEADER length
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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 ts.
THEADER length HEADERBYTES length<
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The two equivalent optional entries tell GrADS that the data file has a header record of length bytes preceding each time block of binary data. U one or the other but not both. The entries are only valid for GrADS gridded data ts. See the ction on structure of a gridded binary data file for more information.
TRAILERBYTES length
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This optional entry tell GrADS that the data file has a trailer record of length bytes following each time block of binary data. This entry is only valid for GrADS gridded data ts. See the ction on structure of a gridded binary data file for more information.
XYHEADER length
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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 entry is only valid for GrADS gridded data ts. See the ction on structure of a gridded binary data
file for more information.
XVAR x,y
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(GrADS version 1.9) This entry provides the x,y pair for the station's longitude. This entry is required for DTYPE bufr.
YVAR x,y
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(GrADS version 1.9) This entry provides the x,y pair for the station's latitude. This entry is required for DTYPE bufr.
ZVAR x,y
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(GrADS version 1.9) This entry provides the x,y pair for the station data's vertical coordinate (e.g., pressure). This is an optional entry for DTYPE bufr.
STID x,y
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(GrADS version 1.9) This entry provides the x,y pair for the station ID. This entry is required for DTYPE bufr.
TVAR yr x,y mo x,y dy x,y hr x,y mn x,y sc x,y
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(GrADS version 1.9) This entry provides the x,y pairs for all the ba time coordinate variables. Each time unit (year=yr, month=mo, day=dy, hour=hr, minute=mn, cond=sc) is prented as a 2-letter abbreviation followed by the x,y pair that goes with that time unit. The time for any individual station report is the ba time plus the offt time (e TOFFVAR). All six ba time units are not required to appear in the TVAR record, only tho that are in the data file. This entry is required
for DTYPE bufr.
TOFFVAR yr x,y mo x,y dy x,y hr x,y mn x,y sc x,y
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(GrADS version 1.9) This entry provides the x,y pairs for all the offt time coordinate variables. The syntax is the same as TVAR. The time for any individual station report is the ba time plus the offt time. All six offt time units are not required to appear in the TOFFVAR record, only tho that are in the data file. This is an optional entry for DTYPE bufr.
CACHESIZE bytes
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(GrADS version 2.0.a8+) This entry overrides the default size of the cache for reading HDF5 or NetCDF4 files. It is not relevant for other data types. It should not be necessary to t the cache size explicitly unless the data file has especially large chunks. Plea e the documentation on compression.
OPTIONS keyword
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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:
pascals (GrADS version 2.0) (For DTYPE grib2 only) Indicates that pressure values that appear in the descriptor file (in the ZDEF entry and in the GRIB2 codes in the variable declarations) are given in
units of Pascals. The gribmap utility requires pressure to be given in Pascals. If this keyword is
prent, the pressure level values will be converted to millibars after the gribmap index is generated
and the descriptor file is opened with GrADS. If this keyword is omitted, pressure levels will remain in
Pascals, and many of the internal functions (which assume a vertical dimension in units of millibars)
will not work properly.
yrev Indicates that the Y dimension (latitude) in the data file has been written in the rever order from what GrADS assumes. An important thing to remember is that GrADS still prents the view that the
简单情话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, ra
ther 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 u. For more information, e the ction on Using Templates.
quential Indicates that the file was written in quential unformatted I/O. This keyword may be ud with either station or gridded data. If your gridded data is written in quential 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).
365_day_calendar Indicates the data file was created with perpetual 365-day years, with no leap years. This is ud for some types of model ouput.
byteswapped Indicates the binary data file is in rever 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
