Create, merge, and write NetCDF files using high-level functions
netcdf package low-level functions.
Network Common Data Form (NetCDF) is a set of software libraries and machine-independent data formats that support the creation, access, and sharing of array-oriented scientific data. NetCDF is used by a wide range of engineering and scientific fields that want a standard way to store data so that it can be shared.
MATLAB® high-level functions make it easy to export data to a netCDF file. MATLAB low-level functions provide access to the routines in the NetCDF C library. To use the low-level functions effectively, you should be familiar with the NetCDF C Interface. The NetCDF documentation is available at the Unidata website.
For information about exporting to Common Data Format (CDF) files, which have a separate and incompatible format, see Export to CDF Files.
This example shows how to create a new NetCDF file that contains the variable, dimension, and group definitions of an existing file, but uses a different format.
Create a file containing one variable, using the
Write data to the file.
A = 99; ncwrite('myfile.nc','myvar',A)
Read the variable, dimension, and group definitions from the file using
ncinfo. This information defines the file's schema.
S = ncinfo('myfile.nc');
Get the format of the file.
file_fmt = S.Format
file_fmt = 'netcdf4_classic'
Change the value of the
Format field in the structure,
S, to another supported NetCDF format.
S.Format = 'netcdf4';
Create a new version of the file that uses the new format, using the
ncwriteschema function. A schema defines the structure of the file but does not contain any of the data that was in the original file.
ncwriteschema('newfile.nc',S) S = ncinfo('newfile.nc');
Note: When you convert a file's format using
ncwriteschema, you might get a warning message if the original file format includes fields that are not supported by the new format. For example, the netcdf4 format supports fill values but the NetCDF classic format does not. In these cases,
ncwriteschema still creates the file, but omits the field that is undefined in the new format.
View the format of the new file.
new_fmt = S.Format
new_fmt = 'netcdf4'
The new file,
newfile.nc, contains the variable and dimension definitions of
myfile.nc, but does not contain the data.
Write data to the new file.
This example shows how to merge two NetCDF files using high-level functions. The combined file contains the variable and dimension definitions of the files that are combined, but does not contain the data in these original files.
Create a NetCDF file named
define a variable named
myvar. Then, write data
to the variable and display the file contents.
nccreate('ex1.nc','myvar'); ncwrite('ex1.nc','myvar',55) ncdisp('ex1.nc')
Source: pwd\ex1.nc Format: netcdf4_classic Variables: myvar Size: 1x1 Dimensions: Datatype: double
Create a second file and define a variable named
Then, write data to the variable and display the file contents.
nccreate('ex2.nc','myvar2'); ncwrite('ex2.nc','myvar2',99) ncdisp('ex2.nc')
Source: pwd\ex2.nc Format: netcdf4_classic Variables: myvar2 Size: 1x1 Dimensions: Datatype: double
Get the schema of each of the files, using the
info1 = ncinfo('ex1.nc')
info1 = Filename: 'pwd\ex1.nc' Name: '/' Dimensions:  Variables: [1x1 struct] Attributes:  Groups:  Format: 'netcdf4_classic'
info2 = ncinfo('ex2.nc')
info2 = Filename: 'pwd\ex2.nc' Name: '/' Dimensions:  Variables: [1x1 struct] Attributes:  Groups:  Format: 'netcdf4_classic'
Create a new NetCDF file that uses the schema of the first
example file, using the
Then, display the file contents.
Source: pwd\combined.nc Format: netcdf4_classic Variables: myvar Size: 1x1 Dimensions: Datatype: double Attributes: _FillValue = 9.969209968386869e+36
Add the schema from
View the contents of the combined file.
Source: pwd\combined.nc Format: netcdf4_classic Variables: myvar Size: 1x1 Dimensions: Datatype: double Attributes: _FillValue = 9.969209968386869e+36 myvar2 Size: 1x1 Dimensions: Datatype: double Attributes: _FillValue = 9.969209968386869e+36
The file contains the
myvar variable defined
in the first example file and the
defined in the second file.
This example shows how to use low-level functions to write data to a NetCDF file. The MATLAB® low-level functions provide access to the routines in the NetCDF C library. MATLAB groups the functions into a package, called
netcdf. To call one of the functions in the package, you must prefix the function name with the package name.
To use the MATLAB NetCDF functions effectively, you should be familiar with the information about the NetCDF C Interface.
To run this example, you must have write permission in your current folder.
Create a 1-by-50 variable of numeric values named
my_data in the MATLAB workspace. The vector is of class
my_data = linspace(0,49,50);
Create a NetCDF file named
my_file.nc, using the
netcdf.create function. The
NOCLOBBER parameter is a NetCDF file access constant that indicates that you do not want to overwrite an existing file with the same name.
ncid = netcdf.create('my_file.nc','NOCLOBBER');
netcdf.create returns a file identifier,
ncid. When you create a NetCDF file, the file opens in define mode. You must be in define mode to define dimensions and variables.
Define a dimension in the file, using the
netcdf.defDim function. This function corresponds to the
nc_def_dim function in the NetCDF library C API. You must define dimensions in the file before you can define variables and write data to the file. In this case, define a dimension named
my_dim with length 50.
dimid = netcdf.defDim(ncid,'my_dim',50)
dimid = 0
netcdf.defDim returns a dimension identifier that corresponds to the new dimension. Identifiers are zero-based indexes.
Define a variable named
my_var on the dimension, using the
netcdf.defVar function. This function corresponds to the
nc_def_var function in the NetCDF library C API. Specify the NetCDF data type of the variable, in this case,
varid = netcdf.defVar(ncid,'my_var','NC_BYTE',dimid)
varid = 0
netcdf.defVar returns a variable identifier that corresponds to
Take the NetCDF file out of define mode. To write data to a file, you must be in data mode.
Write the data from the MATLAB workspace into the variable in the NetCDF file, using the
netcdf.putVar function. The data in the workspace is of class
double but the variable in the NetCDF file is of type
NC_BYTE. The MATLAB NetCDF functions automatically do the conversion.
Close the file, using the
Verify that the data was written to the file by opening the file and reading the data from the variable into a new variable in the MATLAB workspace.
ncid2 = netcdf.open('my_file.nc','NC_NOWRITE'); x = netcdf.getVar(ncid2,0);
View the data type of
Name Size Bytes Class Attributes x 50x1 50 int8
MATLAB stores data in column-major order while the NetCDF C API uses row-major order.
x represents the data stored in the NetCDF file and is therefore 50-by-1 even though the original vector in the MATLAB workspace,
my_data, is 1-by-50. Because you stored the data in the NetCDF file as
NC_BYTE, MATLAB reads the data from the variable into the workspace as class
Close the file.