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Hi Simon,
You asked:
>
> Has anybody else been thinking about metadata for compound types?
> How is metadata expected to be represented for individual fields?
and Ed Hartnett wrote:
> We have also considered that one could create an attribute of compound
> type, in which each element was a unit for an element of the data
> compound type.
Just to expand on this a bit, I think this is an adequate way to
provide attributes to members of a compound type, because attributes
in netCDF-4 can be of user-defined types, not just of primitive types.
As an example, consider a user-defined compound type "obs_type" and a
1D variable "obs_var" of that type:
types:
compound obs_type { // a user-defined compound type
int station_id;
double time;
float temperature;
float pressure;
};
variables:
obs_type obs_var(n); // a 1D variable of type obs_type
To assign units to all the members of the obs_var variable (and any
other variables of that type), just define a related type named
"obs_atts", for example, and declare a variable attribute "units"
of that type for the obs_var variable:
types:
...
compound obs_atts { // a user-defined compound type
string station_id;
string time;
string temperature;
string pressure;
}
variables:
...
obs_atts obs_var:units = { // a units attribute of type obs_atts
time = "hours since 2000-01-01 00:00";
temperature = "degreesC";
pressure = "hPa";
};
Notice that since the station_id member of the obs_type has no units,
we do not include a station_id member in the associated obs_var:units
attribute. Strictly speaking, there's no need to even use the same
order for the members in the obs_atts type, since an association can
be made by using the same member names.
Also if you wanted to associate a "standard_name" attribute string
with each member of the obs_type, you would need to define another
variable attribute named "standard_name" for the obs_var variable, but
you wouldn't need another type:
variables:
...
obs_atts obs_var:standard_name = { // another attribute of type obs_atts
temperature = "air_temperature";
pressure = "air_pressure";
};
There is a convention implicit in the above, that identity of member
names between attribute compound types and variable compound types can
be used to associate attributes with members of compound types, but I
think this convention is natural and easy to understand.
--Russ
_____________________________________________________________________
Russ Rew UCAR Unidata Program
russ@xxxxxxxxxxxxxxxx http://www.unidata.ucar.edu
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