SEACOOS CDL v2.0 December 9, 2004 10 heights, but the constant z value is also provided with the specific dependent variable  (e.g. wspd: z=5;).    6. Required Variables As described in the previous section on Format Categories, SEACOOS CDL files must always contain the four independent variables  (time, longitude, latitude and vertical) that describe the spatial and temporal setting of the data.  Each independent variable must be described in netCDF at the beginning of the variable section. Each independent variable must always explicitly include the units and standard_name attributes.    6.1 Time Variable For real-time data, only the last 48 ho urs of observations should be retained in the netCDF file and should be updated as new data are available. Only one time frame for SEACOOS CDL is allowed and that is the number of seconds since January 1, 1970 at 00Z.  All time data must be converted to this time frame.  The DMCC may consider allowing more flexible time values in future versions of this standard. long time(time); time: standard_name = "time"; time: units = "sec since 1970-1-1 00:00:00"; time: axis = "T"; 6.2 Longitude  Variable Longitude may be represented as a coordinate variable, (e.g. lon(lon), or a function of time, e.g. lon(time). float lon(lon); //or lon(time) lon: standard_name = "longitude"; lon: units = "degrees_east" ; lon: reference = "geographical coordinates" ; lon: axis = "X"; 6.3 Latitude  Variable Latitude may be represented as a coordinate variable (e.g. lat(lat))or a function of time (e.g. lat(time)). float lat(lat); //or lat(time) lat: standard_name = "latitude"; lat: units = "degrees_north"; lat: reference = "geographical coordinates"; lat: axis = "Y";