[CF-metadata] projections/default cell_methods

From: Jonathan Gregory <jonathan.gregory>
Date: Wed, 05 Jun 2002 09:34:57 +0100

Dear Burkhardt et al.

I would be in favour of including information like this:

> :projection = "<projection name>";
> :projection_params = <projection parameters>;
> (except that I have used float instead of string value for
> projection_params)

I think it might be a little neater to use just one attribute, reminiscent of
the cell_methods and formula_terms attributes, for instance:

  projection="rotated_latitude_longitude north_pole_latitude: 32.5 ",
    "north_pole_longitude: 170."

This would be more work to parse, but a utility could be written to parse it.
The advantages I see are

* More self-describing.

* Less error-prone, because keyword parameters are more likely to be correctly
identified than positional parameters.

* More flexible, because it allows the projection parameters to have any
data type.

What do you think?

For the choice of keyword names to use, we should do some research into what
already exists. When we discussed this before, Brian Eaton identified an
existing software package supporting projections which might suggest some
standard keywords to use.

> However, I would prefer to have at least the opportunity to set "sum"
> optionally also

I think I agree with this too. It should be allowed to set "sum" or "point"
(for extensive and intensive). What do others think?

It would be rather complicated and laborious to specify in the standard name
table whether each quantity for each possible dimension is intensive or
extensive. And anyway, the file might not be using standard names. It is
therefore better, I think, for the data writer to indicate what is intended.

This is not just cosmetic; an application's behaviour is affected by the
choice. For instance,

* If labelling a plot, "point" means the quantity should be labelled "at
such-and-such time", while "sum" means "from earliest_time to latest_time".

* When constructing a time mean of point quantities, the unit does not change.
But for a time mean of accumulated quantities, it does. For instance,
accumulated hourly precipitation has units of kg m-2 or mm. If these values
are meaned, with cell_method of "mean", the units become kg m-2 h-1, and
the standard name has to be changed to indicate a flux or rate.

Cheers

Jonathan
Received on Wed Jun 05 2002 - 02:34:57 BST