[CF-metadata] size-one axes

From: Jonathan Gregory <j.m.gregory>
Date: Fri, 4 Jul 2003 17:36:19 +0100

Dear All

I have a proposal regarding axes (dimensions) of size one. We propose the use
of such axes for attaching metadata that can be regarded as a single-level
coordinate. Temperature at 1.5 m has such a vertical coordinate, for instance.
Other examples could be: radiation wavelength band, analysis time for a set of
forecasts (discussed in the other thread), reference pressure (for potential
temperature, exner function, etc.). It is convenient to use coordinate vars
for such things because they may need units, standard names, bounds, etc., which
can be stored on coordinate variables.

My proposal is that, for coordinate variables having a dimension of one,
except for the basic xyzt (the ones which might have an axis attribute), it
should be permissible to name the coordinate variable in the coordinates
attribute *without* the dimension being a dimension of the data variable.
That would allow many of such size-one coordinate variables to be associated
with a data variable without insisting that it has many dimensions of size
one. Since such dimensions do not affect the ordering of the elements of the
array, they are not really needed as dimensions.

The reason for excluding xyzt is that COARDS applications would expect the
data variable to have these dimensions, even if they do have size one.

So, for example, instead of

dimensions:
  lon=96;
  lat=73;
  forecast_time=3;
  analysis_time=1;
  wavelength=1;
variables:
  float LWflux(wavelength,analysis_time,forecast_time,lat,lon);
  float lon(lon);
  float lat(lat);
  double forecast_time(forecast_time);
  double analysis_time(analysis_time);
  float wavelength(wavelength);

where the LWflux array is dimensioned (1,1,3,73,96), we would have

  float LWflux(forecast_time,lat,lon);
    LWflux:coordinates="wavelength analysis_time";

where it is dimensioned (3,73,96). All the other variables remain the same.

Any views?

Best wishes

Jonathan
Received on Fri Jul 04 2003 - 10:36:19 BST