Coordinate variables

3.2. Coordinate variables#

NetCDF coordinate variables provide scales for the space and time axes for the multidimensional data arrays, and are included for all dimensions that can be identified as spatio-temporal axes.

3.2.1. Temporal coordinates#

time is the time coordinate of the CCI Sea State datasets. It is computed to the nearest second at least and then coded as continuous UTC time coordinates in seconds from 00:00:00 UTC January 1, 1980 (which is the definition of the CCI Sea State origin time, chosen to precede the start of useful altimeter or SAR data record).

For L2P and L3 datasets, time is defined for each measurement within the data files. For L4 datasets, a single time value is provided, corresponding to the centre time of the averaging or collation window. In both case, the time values are defined along the time axis dimension.

double time(time) ;
    	time:_FillValue = 1.e+20 ;
    	string time:long_name = "reference time of sst file" ;
    	string time:standard_name = "time" ;
    	string time:authority = "CF-1.12, ACDD-1.3" ;
    	string time:axis = "T" ;
    	string time:coverage_content_type = "coordinate" ;
    	string time:units = "seconds since 1981-01-01" ;
    	string time:calendar = "proleptic_gregorian" ;

3.2.2. Spatial coordinates#

lat and lon variables provide respectively the latitude and longitude of each measurement within CCI Sea State dataset files. Latitudes are expressed between -90° and +90° and longitudes between -180° and 180°.

A coordinates variable attribute (coordinates = "lon lat" ;) is provided if the data are on a non-regular lat/lon grid (SAR image, swath or along-track data).

A grid_mapping variable attribute (grid_mapping = "<projection name>" ;) is provided if the data are mapped following a projection. Refer to the CF convention[1] for standard projection names.

3.2.2.1. Along-track latitude/longitude (nadir altimeter)#

For altimeter along-track data (L2P, L3), the latitude and longitude values are defined for each measurement along the time axis dimension.

double lon(time) ;
    	string lon:long_name = "longitude: 1 Hz" ;
    	string lon:units = "degrees_east" ;
    	string lon:standard_name = "longitude" ;
    	string lon:comment = "geographical coordinates, WGS84 projection" ;
    	string lon:authority = "CF-1.11, ACDD-1.3" ;
    	string lon:axis = "X" ;
    	lon:valid_range = -180., 180. ;
    	string lon:coverage_content_type = "coordinate" ;
double lat(time) ;
    	string lat:long_name = "latitude: 1 Hz" ;
    	string lat:units = "degrees_north" ;
    	string lat:standard_name = "latitude" ;
    	string lat:comment = "geographical coordinates, WGS84 projection" ;
    	string lat:authority = "CF-1.11, ACDD-1.3" ;
    	string lat:axis = "Y" ;
    	lat:valid_range = -90., 90. ;
    	string lat:coverage_content_type = "coordinate" ;

3.2.2.2. Non-regular latitude/longitude grids (swath, image)#

For non-regular gridded data following the sensor scanning pattern (such as SAR images), x (columns) and y (lines) grid dimensions are referred either as col (for the across-track dimension) androw (for the along-track dimension).

In this case where data are gridded following the sensor pattern, no projection can be associated and lat/lon data have to be stored in 2-D arrays, as it is the case for along-swath data for low earth orbiting sensors. Therefore it only applies to L2P processing level products.

double lat(row, col) ;
    	lat:_FillValue = 1.e+20 ;
    	lat:standard_name = "latitude" ;
    	lat:long_name = "latitude" ;
    	lat:units = "degrees_north" ;
double lon(row, col) ;
    	lon:_FillValue = 1.e+20 ;
    	lon:standard_name = "longitude" ;
    	lon:long_name = "longitude" ;
    	lon:units = "degrees_east" ;

3.2.2.3. regular latitude/longitude grids#

For orthogonal regular geographic (lat-lon) gridded data (L4), x (columns) and y (lines) grid dimensions are referred either as lat and lon. Coordinate vectors are used. The elements of a coordinate vector array are in monotonically increasing or decreasing order.

double lon(lon) ;
    	string lon:long_name = "longitude" ;
    	string lon:units = "degrees_east" ;
    	string lon:standard_name = "longitude" ;
    	string lon:comment = "geographical coordinates, WGS84 projection" ;
    	string lon:authority = "CF-1.12, ACDD-1.3" ;
    	string lon:axis = "X" ;
    	lon:valid_range = -180., 180. ;
    	string lon:coverage_content_type = "coordinate" ;
double lat(lat) ;
    	string lat:long_name = "latitude" ;
    	string lat:units = "degrees_north" ;
    	string lat:standard_name = "latitude" ;
    	string lat:comment = "geographical coordinates, WGS84 projection" ;
    	string lat:authority = "CF-1.12, ACDD-1.3" ;
    	string lat:axis = "Y" ;
    	lat:valid_range = -90., 90. ;
    	string lat:coverage_content_type = "coordinate" ;
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