The fill value metadata or missing_value backward compatibility is preserved as NODATA value when available.
NOTE: Implemented as gdal/frmts/netcdf/netcdfdataset.cpp.
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, firstname.lastname@example.org 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:
[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 2001-1-1gdalinfo 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.
If those 2 methods fail, NetCDF driver will try to read the following metadata directly and set up georeferencing.
Each band will have metadata tied to it giving a short description of the data it contains.
The translation follow these directives:
$ 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 experimentVariables 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
Added support for NC2, NC4 and NC4C file types for reading and writing, and HDF4 for reading. See NetCDF File Format for details.
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
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.
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.
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 childs as specified in Support of groups in CF
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.