# NetCDF: Network Common Data Form

Driver short name

netCDF

Build dependencies

libnetcdf

This format is supported for read and write access. This page only describes the raster support (you can find documentation for the vector side) NetCDF is an interface for array-oriented data access and is used for representing scientific data.

The fill value metadata or missing_value backward compatibility is preserved as NODATA value when available.

NOTE: Implemented as `gdal/frmts/netcdf/netcdfdataset.cpp`.

## Driver capabilities

Supports CreateCopy()

This driver supports the `GDALDriver::CreateCopy()` operation

Supports Create()

This driver supports the `GDALDriver::Create()` operation

Supports Georeferencing

This driver supports georeferencing

## Multiple Image Handling (Subdatasets)

Network Command Data Form is a container for several different arrays most used for storing scientific dataset. One NetCDF file may contain several datasets. They may differ in size, number of dimensions and may represent data for different regions.

If the file contains only one NetCDF array which appears to be an image, it may be accessed directly, but if the file contains multiple images it may be necessary to import the file via a two step process.

The first step is to get a report of the components images (dataset) in the file using gdalinfo, and then to import the desired images using gdal_translate. The gdalinfo utility lists all multidimensional subdatasets from the input NetCDF file.

The name of individual images are assigned to the SUBDATASET_n_NAME metadata item. The description for each image is found in the SUBDATASET_n_DESC metadata item. For NetCDF images will follow this format: NETCDF:filename:variable_name

where filename is the name of the input file, and variable_name is the dataset selected within the file.

On the second step you provide this name for gdalinfo to get information about the dataset or gdal_translate to read dataset.

For example, we want to read data from a NetCDF file:

    $ gdalinfo sst.nc
    Driver: netCDF/Network Common Data Format
    Size is 512, 512
    Coordinate System is `'
    Metadata:
      NC_GLOBAL#title=IPSL  model output prepared for IPCC Fourth Assessment SRES A2 experiment
      NC_GLOBAL#institution=IPSL (Institut Pierre Simon Laplace, Paris, France)
      NC_GLOBAL#source=IPSL-CM4_v1 (2003) : atmosphere : LMDZ (IPSL-CM4_IPCC, 96x71x19) ; ocean ORCA2 (ipsl_cm4_v1_8, 2x2L31); sea ice LIM (ipsl_cm4_v
      NC_GLOBAL#contact=Sebastien Denvil, sebastien.denvil@ipsl.jussieu.fr
      NC_GLOBAL#project_id=IPCC Fourth Assessment
      NC_GLOBAL#table_id=Table O1 (13 November 2004)
      NC_GLOBAL#experiment_id=SRES A2 experiment
      NC_GLOBAL#realization=1
      NC_GLOBAL#cmor_version=9.600000e-01
      NC_GLOBAL#Conventions=CF-1.0
      NC_GLOBAL#history=YYYY/MM/JJ: data generated; YYYY/MM/JJ+1 data transformed  At 16:37:23 on 01/11/2005, CMOR rewrote data to comply with CF standards and IPCC Fourth Assessment requirements
      NC_GLOBAL#references=Dufresne et al, Journal of Climate, 2015, vol XX, p 136
      NC_GLOBAL#comment=Test drive
    Subdatasets:
      SUBDATASET_1_NAME=NETCDF:"sst.nc":lon_bnds
      SUBDATASET_1_DESC=[180x2] lon_bnds (64-bit floating-point)
      SUBDATASET_2_NAME=NETCDF:"sst.nc":lat_bnds
      SUBDATASET_2_DESC=[170x2] lat_bnds (64-bit floating-point)
      SUBDATASET_3_NAME=NETCDF:"sst.nc":time_bnds
      SUBDATASET_3_DESC=[24x2] time_bnds (64-bit floating-point)
      SUBDATASET_4_NAME=NETCDF:"sst.nc":tos
      SUBDATASET_4_DESC=[24x170x180] sea_surface_temperature (32-bit floating-point)Corner Coordinates:
    Upper Left  (    0.0,    0.0)
    Lower Left  (    0.0,  512.0)
    Upper Right (  512.0,    0.0)
    Lower Right (  512.0,  512.0)
    Center      (  256.0,  256.0)

This NetCDF files contain 4 datasets, lon_bnds, lat_bnds, tim_bnds and tos. Now select the subdataset, described as: `NETCDF:"sst.nc":tos` `[24x17x180] sea_surface_temperature (32-bit floating-point)` and get the information about the number of bands there is inside this variable.

    $ gdalinfo NETCDF:"sst.nc":tos
    Driver: netCDF/Network Common Data Format
    Size is 180, 170
    Coordinate System is `'
    Origin = (1.000000,-79.500000)
    Pixel Size = (1.98888889,0.99411765)
    Metadata:
      NC_GLOBAL#title=IPSL  model output prepared for IPCC Fourth Assessment SRES A2 experiment
      NC_GLOBAL#institution=IPSL (Institut Pierre Simon Laplace, Paris, France)

…. More metadata

      time#standard_name=time
      time#long_name=time
      time#units=days since 2001-1-1
      time#axis=T
      time#calendar=360_day
      time#bounds=time_bnds
      time#original_units=seconds since 2001-1-1
    Corner Coordinates:
    Upper Left  (   1.0000000, -79.5000000)
    Lower Left  (   1.0000000,  89.5000000)
    Upper Right (     359.000,     -79.500)
    Lower Right (     359.000,      89.500)
    Center      ( 180.0000000,   5.0000000)
    Band 1 Block=180x1 Type=Float32, ColorInterp=Undefined
      NoData Value=1e+20
      Metadata:
        NETCDF_VARNAME=tos
        NETCDF_DIMENSION_time=15
        NETCDF_time_units=days since 2001-1-1
    Band 2 Block=180x1 Type=Float32, ColorInterp=Undefined
      NoData Value=1e+20
      Metadata:
        NETCDF_VARNAME=tos
        NETCDF_DIMENSION_time=45
        NETCDF_time_units=days since 2001-1-1

…. More Bands

    Band 22 Block=180x1 Type=Float32, ColorInterp=Undefined
      NoData Value=1e+20
      Metadata:
        NETCDF_VARNAME=tos
        NETCDF_DIMENSION_time=645
        NETCDF_time_units=days since 2001-1-1
    Band 23 Block=180x1 Type=Float32, ColorInterp=Undefined
      NoData Value=1e+20
      Metadata:
        NETCDF_VARNAME=tos
        NETCDF_DIMENSION_time=675
        NETCDF_time_units=days since 2001-1-1
    Band 24 Block=180x1 Type=Float32, ColorInterp=Undefined
      NoData Value=1e+20
      Metadata:
        NETCDF_VARNAME=tos
        NETCDF_DIMENSION_time=705
        NETCDF_time_units=days since 2001-1-1

gdalinfo displays the number of bands into this subdataset. There are metadata attached to each band. In this example, the metadata informs us that each band correspond to an array of monthly sea surface temperature from January 2001. There are 24 months of data in this subdataset. You may also use gdal_translate for reading the subdataset.

Note that you should provide exactly the contents of the line marked SUBDATASET_n_NAME to GDAL, including the NETCDF: prefix.

The NETCDF prefix must be first. It triggers the subdataset NetCDF driver. This driver is intended only for importing remote sensing and geospatial datasets in form of raster images. If you want explore all data contained in NetCDF file you should use another tools.

Starting with GDAL 3.5, the VARIABLES_AS_BANDS=YES open option can be used to indicate to the driver that if the netCDF file only contains 2D variables of the same type and indexed by the same dimensions, then they should be reported as multiple bands of a same dataset.

    $ gdalinfo autotest/gdrivers/data/netcdf/two_vars_scale_offset.nc -oo VARIABLES_AS_BANDS=YES

    Driver: netCDF/Network Common Data Format
    Files: autotest/gdrivers/data/netcdf/two_vars_scale_offset.nc
    Size is 21, 21
    Metadata:
      NC_GLOBAL#Conventions=COARDS/CF-1.0
      x#actual_range={-10,10}
      x#long_name=x
      y#actual_range={-10,10}
      y#long_name=y
      z#add_offset=1.5
      z#long_name=z
      z#scale_factor=0.01
    Corner Coordinates:
    Upper Left  (    0.0,    0.0)
    Lower Left  (    0.0,   21.0)
    Upper Right (   21.0,    0.0)
    Lower Right (   21.0,   21.0)
    Center      (   10.5,   10.5)
    Band 1 Block=21x1 Type=Float32, ColorInterp=Undefined
      NoData Value=9.96920996838686905e+36
      Offset: 1.5,   Scale:0.01
      Metadata:
        add_offset=1.5
        long_name=z
        NETCDF_VARNAME=z
        scale_factor=0.01
    Band 2 Block=21x1 Type=Float32, ColorInterp=Undefined
      NoData Value=9.96920996838686905e+36
      Offset: 2.5,   Scale:0.1
      Metadata:
        add_offset=2.5
        long_name=q
        NETCDF_VARNAME=q
        scale_factor=0.1

## Dimension

The NetCDF driver assume that data follows the CF-1 convention from UNIDATA The dimensions inside the NetCDF file use the following rules: (Z,Y,X). If there are more than 3 dimensions, the driver will merge them into bands. For example if you have an 4 dimension arrays of the type (P, T, Y, X). The driver will multiply the last 2 dimensions (P*T). The driver will display the bands in the following order. It will first increment T and then P. Metadata will be displayed on each band with its corresponding T and P values.

## Georeference

There is no universal way of storing georeferencing in NetCDF files. The driver first tries to follow the CF-1 Convention from UNIDATA looking for the Metadata named “grid_mapping”. If “grid_mapping” is not present, the driver will try to find an lat/lon grid array to set geotransform array. The NetCDF driver verifies that the Lat/Lon array is equally spaced.

New in version 3.4: crs_wkt attribute support

If those 2 methods fail, NetCDF driver will try to read the following metadata directly and set up georeferencing.

- spatial_ref (Well Known Text)

- GeoTransform (GeoTransform array)

or,

- Northernmost_Northing

- Southernmost_Northing

- Easternmost_Easting

- Westernmost_Easting

See also the configuration options GDAL_NETCDF_VERIFY_DIMS and GDAL_NETCDF_IGNORE_XY_AXIS_NAME_CHECKS which control this behavior.

## Open options

The following open options are available:

- HONOUR_VALID_RANGE=YES/NO: (GDAL > 2.2) Whether to set to nodata pixel values outside of the validity range indicated by valid_min, valid_max or valid_range attributes. Default is YES.

- IGNORE_XY_AXIS_NAME_CHECKS=YES/NO: (GDAL >= 3.4.2) Whether X/Y dimensions should be always considered as geospatial axis, even if the lack conventional attributes confirming it. Default is NO.

- VARIABLES_AS_BANDS=YES/NO: (GDAL >= 3.5) If set to YES, and if the netCDF file only contains 2D variables of the same type and indexed by the same dimensions, then they should be reported as multiple bands of a same dataset. Default is NO (that is each variable will be reported as a separate subdataset)

- ASSUME_LONGLAT=[YES/NO] : (GDAL >= 3.7) Whether a Geographic CRS should be assumed and applied when, none has otherwise been found, a meaningful geotransform has been found, and that geotransform is within the bounds -180,360 -90,90, if YES assume OGC:CRS84. Default is NO.

## Creation Issues

This driver supports creation of NetCDF file following the CF-1 convention. You may create set of 2D datasets. Each variable array is named Band1, Band2, … BandN.

Each band will have metadata tied to it giving a short description of the data it contains.

## GDAL NetCDF Metadata

All NetCDF attributes are transparently translated as GDAL metadata.

The translation follow these directives:

- Global NetCDF metadata have a NC_GLOBAL tag prefixed.

- Dataset metadata have their variable name prefixed.

- Each prefix is followed by a # sign.

- The NetCDF attribute follows the form: name=value.

Example:

    $ gdalinfo NETCDF:"sst.nc":tos
    Driver: netCDF/Network Common Data Format
    Size is 180, 170
    Coordinate System is `'
    Origin = (1.000000,-79.500000)
    Pixel Size = (1.98888889,0.99411765)
    Metadata:

NetCDF global attributes

    NC_GLOBAL#title=IPSL  model output prepared for IPCC Fourth Assessment SRES A2 experiment

Variables attributes for: tos, lon, lat and time

    tos#standard_name=sea_surface_temperature
    tos#long_name=Sea Surface Temperature
    tos#units=K
    tos#cell_methods=time: mean (interval: 30 minutes)
    tos#_FillValue=1.000000e+20
    tos#missing_value=1.000000e+20
    tos#original_name=sosstsst
    tos#original_units=degC
    tos#history= At   16:37:23 on 01/11/2005: CMOR altered the data in the following ways: added 2.73150E+02 to yield output units;  Cyclical dimension was output starting at a different lon;
    lon#standard_name=longitude
    lon#long_name=longitude
    lon#units=degrees_east
    lon#axis=X
    lon#bounds=lon_bnds
    lon#original_units=degrees_east
    lat#standard_name=latitude
    lat#long_name=latitude
    lat#units=degrees_north
    lat#axis=Y
    lat#bounds=lat_bnds
    lat#original_units=degrees_north
    time#standard_name=time
    time#long_name=time
    time#units=days since 2001-1-1
    time#axis=T
    time#calendar=360_day
    time#bounds=time_bnds
    time#original_units=seconds since 2001-1-1

On writing, when using the CreateCopy() interface or gdal_translate, dataset level metadata that follows the naming convention NC_GLOBAL#key=value will be used to write the netCDF attributes. Metadata set at the band level using key=value will also be used to write variable attributes.

## Product specific behavior

### Sentinel 5

New in version 3.4.

The most verbose metadata is reported in the `json:ISO_METADATA`, `json:ESA_METADATA`, `json:EOP_METADATA`, `json:QA_STATISTICS`, `json:GRANULE_DESCRIPTION`, `json:ALGORITHM_SETTINGS` and `json:SUPPORT_DATA` metadata domains.

Can be discovered for example with:

    gdalinfo -mdd all -json S5P.nc

## Creation Options

- FORMAT=[NC/NC2/NC4/NC4C]: Set the NetCDF file format to use, NC is the default. NC2 is normally supported by recent NetCDF installations, but NC4 and NC4C are available if NetCDF was compiled with NetCDF-4 (and HDF5) support.

- COMPRESS=[NONE/DEFLATE]: Set the compression to use. DEFLATE is only available if NetCDF has been compiled with NetCDF-4 support. NC4C format is the default if DEFLATE compression is used.

- ZLEVEL=[1-9]: Set the level of compression when using DEFLATE compression. A value of 9 is best, and 1 is least compression. The default is 1, which offers the best time/compression ratio.

- WRITE_BOTTOMUP=[YES/NO]: Set the y-axis order for export, overriding the order detected by the driver. NetCDF files are usually assumed “bottom-up”, contrary to GDAL’s model which is “north up”. This normally does not create a problem in the y-axis order, unless there is no y axis geo-referencing. The default for this setting is YES, so files will be exported in the NetCDF default “bottom-up” order. For import see Configuration Option GDAL_NETCDF_BOTTOMUP below.

- WRITE_GDAL_TAGS=[YES/NO]: Define if GDAL tags used for georeferencing (spatial_ref and GeoTransform) should be exported, in addition to CF tags. Not all information is stored in the CF tags (such as named datums and EPSG codes), therefore the driver exports these variables by default. In import the CF “grid_mapping” variable takes precedence and the GDAL tags are used if they do not conflict with CF metadata. In GDAL 4, spatial_ref will not be exported. The crs_wkt CF metatata attribute will be used instead.

- WRITE_LONLAT=[YES/NO/IF_NEEDED]: Define if CF lon/lat variables are written to file. Default is YES for geographic SRS and NO for projected SRS. This is normally not necessary for projected SRS as GDAL and many applications use the X/Y dimension variables and CF projection information. Use of IF_NEEDED option creates lon/lat variables if the projection is not part of the CF-1.5 standard.

- TYPE_LONLAT=[float/double]: Set the variable type to use for lon/lat variables. Default is double for geographic SRS and float for projected SRS. If lon/lat variables are written for a projected SRS, the file is considerably large (each variable uses X*Y space), therefore TYPE_LONLAT=float and COMPRESS=DEFLATE are advisable in order to save space.

- PIXELTYPE=[DEFAULT/SIGNEDBYTE]: By setting this to SIGNEDBYTE, a new Byte file can be forced to be written as signed byte. Starting with GDAL 3.7, this option is deprecated and Int8 should rather be used.

- WRITE_GDAL_VERSION=[YES/NO]: (GDAL >= 3.5.0) Define if a “GDAL” text global attribute should be added on file creation with the GDAL version. Defaults to YES

- WRITE_GDAL_HISTORY=[YES/NO]: (GDAL >= 3.5.0) Define if the “history” global attribute should be prepended with a date/time and GDAL information. Defaults to YES.

## Creation of multidimensional files with CreateCopy() 2D raster API

Starting with GDAL 3.1, the preferred way of creating > 2D files is to use the the Multidimensional Raster Data Model API. However it is possible to create such files with the 2D raster API using the CreateCopy() method (note that at time of writing, this is not supported using the Create() method).

The `NETCDF_DIM_EXTRA={dim1_name,...dimN_name}` metadata item must be set on the source dataset, where dim1_name is the name of the slowest varying dimension and dimN_name the name of the fastest varying one.

For each extra dimension, the `NETCDF_DIM_{dim_name}_DEF={dimension_size,netcdf_data_type}` metadata item must be set where dimension_size is the size of the dimension (number of samples along that dimension) and netcdf_data_type is the integer value for the netCDF data type of the corresponding indexing variable. Among the most useful data types:

- 4 for Int

- 5 for Float

- 6 for Double

- 10 for Int64

The `NETCDF_DIM_{dim_name}_VALUES={value1,...valueN}` is set to define the values of the indexing variable corresponding to dimension.

`dim_name#attribute` metadata items can also be set to define the attributes of the indexing variable of the dimension.

Example of creation of a Time,Z,Y,X 4D file in Python:

    # Create in-memory file with required metadata to define the extra >2D
    # dimensions
    size_z = 2
    size_time = 3
    src_ds = gdal.GetDriverByName('MEM').Create('', 4, 3, size_z * size_time)
    src_ds.SetMetadataItem('NETCDF_DIM_EXTRA', '{time,Z}')
    # 6 is NC_DOUBLE
    src_ds.SetMetadataItem('NETCDF_DIM_Z_DEF', f"{{{size_z},6}}")
    src_ds.SetMetadataItem('NETCDF_DIM_Z_VALUES', '{1.25,2.50}')
    src_ds.SetMetadataItem('Z#axis', 'Z')
    src_ds.SetMetadataItem('NETCDF_DIM_time_DEF', f"{{{size_time},6}}")
    src_ds.SetMetadataItem('NETCDF_DIM_time_VALUES', '{1,2,3}')
    src_ds.SetMetadataItem('time#axis', 'T')
    src_ds.SetGeoTransform([2,1,0,49,0,-1])

    # Create netCDF file
    gdal.GetDriverByName('netCDF').CreateCopy('out.nc', src_ds)

## Configuration Options

- GDAL_NETCDF_BOTTOMUP=[YES/NO] : Set the y-axis order for import, overriding the order detected by the driver. This option is usually not needed unless a specific dataset is causing problems (which should be reported on GitHub).

- GDAL_NETCDF_VERIFY_DIMS=[YES/STRICT] : Try to guess which dimensions represent the latitude and longitude only by their attributes (STRICT) or also by guessing the name (YES), default is YES.

- GDAL_NETCDF_IGNORE_XY_AXIS_NAME_CHECKS=[YES/NO] : Whether X/Y dimensions should be always considered as geospatial axis, even if the lack conventional attributes confirming it. Default is NO.

- GDAL_NETCDF_ASSUME_LONGLAT=[YES/NO] : (GDAL >= 3.7) Whether a Geographic CRS should be assumed and applied when, none has otherwise been found, a meaningful geotransform has been found, and that geotransform is within the bounds -180,360 -90,90, if YES assume OGC:CRS84. Default is NO.

## VSI Virtual File System API support

Since GDAL 2.4, and with Linux kernel >=4.3 and libnetcdf >=4.5, read operations on /vsi file systems are supported.

## NetCDF-4 groups support on reading (GDAL >= 3.0)

The driver has undergone significant changes in GDAL 3.0 to support NetCDF-4 groups on reading:

- Explore recursively all nested groups to create the subdatasets list

- Subdatasets in nested groups use the /group1/group2/…/groupn/var standard NetCDF-4 convention, except for variables in the root group which do not have a leading slash for backward compatibility with the NetCDF-3 driver

- Global attributes of each nested group are also collected in the GDAL dataset metadata, using the same convention /group1/group2/…/groupn/NC_GLOBAL#attr_name, except for the root group which do not have a leading slash for backward compatibility

- When searching for a variable containing auxiliary information on the selected subdataset, like coordinate variables or grid_mapping, we now also search in parent groups and their children as specified in Support of groups in CF

## Multidimensional API support

New in version 3.1.

The netCDF driver supports the Multidimensional Raster Data Model for reading and creation operations.

The `GDALGroup::GetGroupNames()` method supports the following options:

- GROUP_BY=SAME_DIMENSION. If set, single-dimensional variables will be exposed as a “virtual” subgroup. This enables the user to get a clearer organization of variables, for example in datasets where variables belonging to different trajectories are indexed by different dimensions but mixed in the same netCDF group.

The `GDALGroup::OpenGroup()` method supports the following options:

- GROUP_BY=SAME_DIMENSION. See above description

The `GDALGroup::GetMDArrayNames()` method supports the following options:

- SHOW_ALL=YES/NO. Defaults to NO. If set to YES, all variables will be listed.

- SHOW_ZERO_DIM=YES/NO. Defaults to NO. If set to NO, variables with 0-dimension will not be listed.

- SHOW_COORDINATES=YES/NO. Defaults to YES. If set to NO, variables refererenced in the `coordinates` attribute of another variable will not be listed.

- SHOW_BOUNDS=YES/NO. Defaults to YES. If set to NO, variables refererenced in the `bounds` attribute of another variable will not be listed.

- SHOW_INDEXING=YES/NO. Defaults to YES. If set to NO, single-dimensional variables whose name is equal to the name of their indexing variable will not be listed.

- SHOW_TIME=YES/NO. Defaults to YES. If set to NO, single-dimensional variables whose `standard_name` attribute is “time” will not be listed.

- GROUP_BY=SAME_DIMENSION. If set, single-dimensional variables will not be listed

The `GDALGroup::OpenMDArray()` method supports the following options:

- USE_DEFAULT_FILL_AS_NODATA=YES/NO. (GDAL >= 3.6) Defaults to NO. If set to YES, the default fill value will be used as nodata when there is no _FillValue or missing_value attribute (except on variables of type Byte, UByte, Char)

The `GDALGroup::CreateMDArray()` method supports the following options:

- NC_TYPE=NC_CHAR/NC_BYTE/NC_INT64/NC_UINT64: to overload the netCDF data type normally deduced from the GDAL data type passed to CreateMDArray(). NC_CHAR can only be used for strings of a fixed size.

- BLOCKSIZE=size_dim0,size_dim1,…,size_dimN: to set the netCDF chunk size, as set by nc_def_var_chunking(). There must be exactly as many values as the number of dimensions passed to CreateMDArray()

- COMPRESS=DEFLATE: to ask for deflate compression

- ZLEVEL=number: DEFLATE compression level (1-9)

- CHECKSUM=YES/NO: Whether to turn on Fletcher32 checksums. Checksum generation requires chunking, and if no explicit chunking has been asked with the BLOCKSIZE option, a default one will be used. Defaults to NO.

- FILTER=filterid,param1,…,paramN: Define a filter (typically a compression method) used for writing. This should be a list of numeric values, separated by commas. The first value is the filter id (list of potential values at https://support.hdfgroup.org/services/contributions.html#filters) and following values are per-filter parameters. More details about netCDF-4 filter support at https://www.unidata.ucar.edu/software/netcdf/docs/md__Users_wfisher_Desktop_docs_netcdf-c_docs_filters.html

## Driver building

This driver is compiled with the UNIDATA NetCDF library.

You need to download or compile the NetCDF library before configuring GDAL with NetCDF support.

See NetCDF GDAL wiki for build instructions and information regarding HDF4, NetCDF-4 and HDF5.

## See Also:

- Vector side of the netCDF driver.

- NetCDF CF-1.5 convention

- NetCDF compiled libraries

- NetCDF Documentation