Bryce L Nordgren a écrit :
(For those who are wondering why I care: consider that the "evaluate"
method of a Coverage takes a DirectPosition, which is an array of doubles.
So what is the unit of the time axis? Absolute time? A time interval in
years, months, days, seconds, or timesteps?)
We can get this information from the CoordinateReferenceSystem. The
information is not immediate however; there is a fair amount of
indirection levels. We have to check if the CRS is an instance of
TemporalCRS. If it is not, then we have to check if the CRS is an
instance of CompoundCRS, and then check if one of the CRS elements (as
returned by the getCoordinateReferenceSystems() method, or the
"includesCRS" association in ISO 19111) is an instanceof of TemporalCRS.
Once we have the TemporalCRS, we can get the temporal unit with:
Unit unit = crs.getCoordinateSystem().getAxis(0).getUnit();
and the epoch (for example January 1st, 1970 00:00 UTC on Unix machine)
with:
Date epoch = crs.getDatum().getOrigin();
You now have enough informations for conversions to arbitrary time
units. For example conversions to a java.util.Date object can be
performed with the following code (note: Unit, Converter and SI are part
of the Unit framework, not GeoAPI):
long origin = epoch.getTime();
Converter toMillis = unit.getConverterTo(SI.MILLI(SI.SECOND));
Date time = new Date(Math.round(toMillis.convert(t)) + origin);
Note: Geotools implementation has a convenience method doing exactly
that: DefaultTemporalCRS.toDate(double)
This mean that GeoAPI already has some minimal capability to answer the
question "what is the unit of the time axis". However, it is possible to
go further and to express the time according some calendar (using year
or month units, which are more complex than second units for example).
Maybe this is what ISO 19108 does. I can't tell, since I didn't had a
chance to look at ISO 19108 yet.
Martin.
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