NetCDF network Common Data Form

# NetCDF: Network Common Data Form

This format is supported for read and write access. NetCDF is an interface for array-oriented data access and is used for representing scientific data.

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

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

## Multiple Image Handling (Subdatasets)

Nework 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 withing 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 display 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.

## 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 space.

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

- spatial_ref (Well Known Text)

- GeoTransform (GeoTransform array)

or,

- Northernmost_Northing
- Southernmost_Northing
- Easternmost_Easting
- Westernmost_Easting

## 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 metatadata 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

## Driver Improvements (GDAL >= 1.9.0)

The driver has undergone significant changes in GDAL 1.9.0, see NEWS file and NetCDF Improvements.

### Important Changes

- Added support for NC2, NC4 and NC4C file types for reading and writing, and HDF4 for reading. See NetCDF File Format for details.

  - `NC` : NetCDF Classic Format: The Original Binary Format.
  - `NC2` : 64-bit Offset Format: Supporting Larger Variables
  - `NC4` : NetCDF-4 Format: Uses HDF5
  - `NC4C` : NetCDF-4 Classic Model Format: HDF5 with NetCDF Limitations
  - `HDF4` : HDF4 SD Format

- Improved support for CF-1.5 projected and geographic SRS reading and writing

- Improvements to metadata (global and variable) handling

- Added simple progress indicator

- Added support for DEFLATE compression (reading and writing) and szip (reading) - requires NetCDF-4 support

- Added support for valid_range/valid_min/valid_max

- Proper handling of signed/unsigned byte data

- Added support for Create() function - enables to use netcdf directly with gdalwarp

- Added support for CF two-dimensional coordinate variables (see CF Conventions) via GDAL GEOLOCATION arrays (see RFC 4: Geolocation Arrays) (GDAL >= 1.10)

### 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. Files without geo-referencing information will be exported in the netcdf default "bottom-up" order, and the contrary for files with geo-referencing. 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.

- 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.

### 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 in GDAL trac).

## 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:

- NetCDF CF-1.5 convention
- NetCDF compiled libraries
- NetCDF Documentation

Full list of GDAL Raster Formats