Creating NetCDF CF Files

From Federation of Earth Science Information Partners
Revision as of 16:20, 16 July 2009 by Hoijarvi (Talk | contribs)

Jump to: navigation, search

UNDER CONSTRUCTION


Back to WCS Access to netCDF Files

Back to WCS NetCDF Development

Contents

NetCDF-CF Convention

The reason to use CF convention: Enable plug and play connectivity.

Well done NetCDF files are human readable. After all: what could dimension longitude mean besides longitude. If you get data in NetCDF format, it's usually fairly easy to see what really is there.

It's also easy to write a generic browser, that can display every variable for you.

But since a lot of data in NetCDF files have geographical meaning, a graphical viewer should be able to draw the data ion the map, on it's own. This involves, at minimum:

  • finding the three geographical dimensions
  • Finding time dimension, if any
  • Understanding the geographical projection

From any generic NetCDF, this requires human intelligence. After all, the n-dimensional data variables, dimensional variables and other metadata variables look precisely the same for the program code. There are legion of ways to code projection information, and decoding it reliably is very difficult.

Conventions come to rescue. For example, if a variable has attribute axis='X', there's only one interpretation for the values of this variable: it must have just one dimension, and the values are points on X-axis along that dimension. No more guesswork, and wrong guesses, for the programmers.


The best documentation is at CF Metadata page.

In short, a simple ncdump output for a NetCDF-CF file may look like this:

netcdf TOMS_AI_58 {
dimensions:
	time = 1 ;
	lat = 3 ;
	lon = 4 ;
variables:
	double time(time) ;
		time:standard_name = "time" ;
		time:long_name = "time" ;
		time:units = "days since 1979-01-01" ;
		time:axis = "T" ;
	double lat(lat) ;
		lat:standard_name = "latitude" ;
		lat:long_name = "latitude" ;
		lat:units = "degrees_north" ;
		lat:axis = "Y" ;
	double lon(lon) ;
		lon:standard_name = "longitude" ;
		lon:long_name = "longitude" ;
		lon:units = "degrees_east" ;
		lon:axis = "X" ;
	byte AI(time, lat, lon) ;
		AI:long_name = "Aerosol Index" ;
		AI:units = "fraction" ;
		AI:_FillValue = -1b ;
		AI:missing_value = -1b ;

// global attributes:
		:title = "NASA TOMS Project" ;
		:comment = "NASA Total Ozone Mapping Spectrometer Project" ;
		:Conventions = "CF-1.0" ;
data:

 time = 9952 ;

 lat = 32.5, 33.5, 34.5 ;

 lon = -89.375, -88.125, -86.875, -85.625 ;

 AI =
  0, 2, 1, 2,
  _, 2, 3, 2,
  1, 4, 4, 2 ;
}

Let's go over section by section:

Dimensions

dimensions:
	time = 1 ;
	lat = 3 ;
	lon = 4 ;
}

These names can be anything. The reason is, that sometimes you may want to store two grids, that have different dimensions, into the same file. In that case you could name dimensions lat1, lat2 etc...

Time Dimension Variable

	double time(time) ;
		time:standard_name = "time" ;
		time:long_name = "time" ;
		time:units = "days since 1979-01-01" ;
		time:axis = "T" ;

It's the attribute axis = "T" marks this variable as time dimension.

Latitude and Longitude Dimension Variables

	double lat(lat) ;
		lat:standard_name = "latitude" ;
		lat:long_name = "latitude" ;
		lat:units = "degrees_north" ;
		lat:axis = "Y" ;
	double lon(lon) ;
		lon:standard_name = "longitude" ;
		lon:long_name = "longitude" ;
		lon:units = "degrees_east" ;
		lon:axis = "X" ;

Again, it's the attributes axis = "Y" and axis = "X" that mark grographical dimension variables. Since the linear projection is acknowledged with standard names.

Data Variable

	byte AI(time, lat, lon) ;
		AI:long_name = "Aerosol Index" ;
		AI:units = "fraction" ;
		AI:_FillValue = -1b ;
		AI:missing_value = -1b ;

The dimensions are marked with axis attributes, so this is a data variable.

global attributes

		:title = "NASA TOMS Project" ;
		:comment = "NASA Total Ozone Mapping Spectrometer Project" ;
		:Conventions = "CF-1.0" ;

Only Conventions = CF-1.x is required.

Verifying NetCDF-CF Files

Since CF-1.0 conventions contain a lot of definitions, verifying them by machine is necessary. There is a fairly complete compliance checker online

Creating NetCDF-CF files using NetCDF Markup Language

NCML utlilities here.

Personal tools
Namespaces

Variants
Actions
Navigation
Toolbox