<html>
<head>
<title>NetCDF network Common Data Form</title>
</head>
<body bgcolor="#ffffff">
<h1>NetCDF: Network Common Data Form</h1>
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.<p>
The fill value metadata or missing_value backward compatibililty is preserved
as NODATA value when available.
<p>
NOTE: Implemented as <tt>gdal/frmts/netcdf/netcdfdataset.cpp</tt>.<p>
<h2> Multiple Image Handling (Subdatasets)</h2>
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.
<p>
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. <p>
<p>
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.
<p>
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:
<i>NETCDF:filename:variable_name</i><p>
where <i>filename</i> is the name of the input file, and
<i>variable_name</i> is the dataset selected withing the file.<p>
On the second step you provide this name for <b>gdalinfo</b> to get information
about the dataset or
<b>gdal_translate</b> to read dataset.<p>
For example, we want to read data from a netCDF file:<p>
<pre>
$ 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)
</pre>
This netCDF files contain 4 datasets, lon_bnds, lat_bnds, tim_bnds and tos.
Now select the subdataset, described as:
<tt><p>NETCDF:"sst.nc":tos<p>
[24x17x180] sea_surface_temperature (32-bit floating-point) </tt><p>
and get the information about the number of bands there is inside this
variable.
<pre>
$ 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)
</pre>
.... More metadata
<pre>
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
</pre>
.... More Bands
<pre>
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
</pre>
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 <b>gdal_translate</b> for reading the subdataset.<p>
Note that you should provide exactly the contents of the line marked
<b>SUBDATASET_n_NAME</b> to GDAL, including the <b>NETCDF:</b> prefix.<p>
The <b>NETCDF</b> 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.
<h2>Dimension</h2>
The netCDF driver assume that data follows the CF-1 convention from
<a href="http://www.unidata.ucar.edu/software/netcdf/docs/conventions.html">UNIDATA </A>
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.
<h2>Georeference</h2>
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.<p>
If those 2 mehtods fail, NetCDF driver will try to read the following
metadata directly and set up georefenrencing.
<ul>
<li>spatial_ref (Well Known Text)
</ul>
<ul>
<li>GeoTransform (GeoTransform array)
</ul>
or,
<ul>
<li>Northernmost_Northing
<li>Southernmost_Northing
<li>Easternmost_Easting
<li>Westernmost_Easting
</ul>
<h2>Creation Issues</h2>
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. <p>
Each band will have metadata tied to it giving a short description of
the data it contains.
<h2>GDAL NetCDF Metadata</h2>
All netCDF attributes are transparently translated as GDAL metadata. <p>
The translation follow these directives:<p>
<ul>
<li>Global NetCDF metatadata have a <b>NC_GLOBAL</b> tag prefixed.<p>
<li>Dataset metadata have their <b>variable name</b> prefixed.<p>
<li>Each prefix is followed by a <b>#</b> sign. <p>
<li>The NetCDF attribute follows the form: <b>name=value</b>.<p>
</ul>
Example:<p>
<pre>
$ 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:
</pre>
NetCDF global attributes<p>
<pre>
NC_GLOBAL#title=IPSL model output prepared for IPCC Fourth Assessment SRES A2 experiment
</pre>
Variables attributes for: tos, lon, lat and time
<pre>
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
</pre>
<h2>Driver building</h2>
This driver is compiled with the UNIDATA netCDF library.<p>
You need to download or compile the netCDF library before
configuring GDAL with netCDF support. <p>
Please note that with <b>CygWIN</b> you need to be sure that the DLLs
are executable, or GDAL will not execute.
<pre>
chmod a+rx [NetCDF DLLs]
</pre>
The netCDF DLLs directory <b>must be</b> to be in your <b>PATH</b>.
<h2>See Also:</h2>
<ul>
<li>
<a href="http://www.cgd.ucar.edu/cms/eaton/cf-metadata/index.html">NetCDF CF-1.0 convention </a>
<li><a href="http://www.unidata.ucar.edu/downloads/netcdf/index.jsp">NetCDF compiled libraries</a>
<li><a href="http://www.unidata.ucar.edu/software/netcdf/docs/">NetCDF Documentation</A>
</ul>
<hr>
<a href="formats_list.html">Full list of GDAL Raster Formats</a>
</body>
</html>
