[CF-metadata] normalized_radar_backscatter_coefficient

From: Jonathan Gregory <j.m.gregory>
Date: Wed, 30 Jul 2014 14:23:00 +0100

Dear John

> As the radiation wavelength and incidence angle cannot vary over the image
> to obtain reliable measurements, the radiation_wavelength and
> nominal_incidence_angle should be specified as attributes.

That would of course be possible in netCDF, but it is not consistent with the
approach of CF. CF uses attributes to describe the structure of the data
storage, and to point to variables which contain geophysical metadata
(especially coordinates of various kinds), but CF attributes do not contain
geophysical information (see Appendix A). CF always uses coordinates for
geophysical information, even if single-valued. There are various reasons for
that which maybe have already come up in this discussion, including:

* It keeps the convention simple by limiting the number of attributes that are
defined by CF. Names of geophysical quantities (like radiation_wavelength and
nominal_incidence_angle) are entries in a lexicon (the standard name table)
rather than being defined by the convention itself.

* Geophysical quantities (including radiation_wavelength and
nominal_incidence_angle) have units. An attribute can't have an attribute,
and it is inflexible if the units have to be defined, rather than allowing
any physically equivalent unit for a quantity. (We have this inflexibility
in the grid_mapping attributes of Appendix F, unfortunately.)

* In most cases, there may arise a need for them to be multivalued. In some
cases it is obvious. We could use an attribute for the height above the ground,
for example, if it is was single-valued, but it's obvious that we also need
multivalued coordinate variables for this, and it's simpler to use the same
machinery in both cases. I'm sure you're right that in your application you
don't need these quantities to be multivalued, but why rule it out? Other
related applications might want them to be multivalued.

Best wishes

Jonathan
Received on Wed Jul 30 2014 - 07:23:00 BST