# CDL File Questions

I'm confused by what exactly a CDL file is and what its purpose is. Unidata specifies the syntax of such a CDL file in its netCDF documentation.

Questions:
Are CDL files designed to be opened by a simple text editor like Notepad or gedit? Or should I use some other tool to open it?

What is its purpose and how is it used?

A CDL file is basically a text output from a netcdf file. If you want to know the contents of a netcdf file but don't have the time (or ability) to use programs built to read/write netcdf, you can use the simple text output of "ncdump" and then read/write it with a basic text editor. You can also use "ncgen" to regenerate a netcdf file based on the new CDL file. I've used this process to make simple changes to netcdf files, and it works very quickly compared to generating a netcdf file using programming methods.

See http://www.atmos.washington.edu/ive/ive_help/writing_netCDF_files.html , quoted below:

A CDL (network Common data form Description Language) file is an ASCII descripton of the binary data in a netCDF file that is designed to be easily read by humans. CDL files can be generated from netCDF files via the ncdump', command. For example,

ncdump -c sample.nc generates the file sample.cdl' that contains the file name, the dimensions, the specification of the variables, any attributes and the data for any "coordinate variables." A CDL file of this type is shown below. Note that the double slash indicates a comment in the CDL file.

netcdf implicit_grid{

dimensions: lon = 101; lat = 101; level = 5; time = UNLIMITED ; //(7 currently) variables:

float A(time,level,lat,lon); A:units = "meters/second"; float level(level);

level:units = "millibars"; float time(time);

time:units = "hours"; //global attributes:

:x_min = -180.f; :x_max = 180.f; :x_units = "degrees_east"; :x_label = "longitude"; :y_min = -90.f; :y_max = 90.f; :y_units = "degrees_north"; :y_label = "latitude";

:z_label = "level"; :t_label = "time";

data: level = 1000, 850, 700, 500, 300 ; time = 0, 2, 4, 6, 8, 10, 12 ;

The command ncgen' is the inverse ofncdump'; it converts an ASCII CDL file to a binary netCDF file. For example

ncgen -o sample.nc sample.cdl converts the CDL file sample.cdl' to the netCDF filesample.nc'. The easiest way to create a netCDF file is to (1) write all the header data (the name, dimensions, variable and attribute specifications, and the values of any coordinate variables) to a CDL file, (2) convert the CDL file to a netCDF file using ncgen, and (3) continue writing the main data arrays to this netCDF file.