Two more thoughts in favour of "reduced precision = interval":
1. Expressiveness - If reduced-precision time strings are taken to mean a
precise instant, then there is no simple way of expressing a standard
interval. Whereas if reduced-precision is interpreted as an interval, it's
still possible to express an instant via a full-precision string.
2. Common Usage: When someone says "2012" they typically mean the entire
year 2012, not the instant at the start of the year. Likewise for other
reduced-precision expressions of time. (I realize that standards often
don't let reality intrude, but it might be taken into consideration in
absence of other clear direction)
On Mon, Jul 16, 2012 at 1:11 PM, Martin Davis <[email protected]> wrote:
> A couple of other data points for this discussion:
>
> 1. The WMS spec for temporal says that it is an extension of ISO 8601.
> References for that spec are:
>
> http://en.wikipedia.org/wiki/ISO_8601
> http://dotat.at/tmp/ISO_8601-2004_E.pdf
>
> ISO 8601 says that it allows for the concept of "reduced accuracy" by
> omitting lower-order time elements. (Sec 3.1 of the standard). The WMS
> spec refers to this as "reduced precision" (Sec D.2.1). To my mind this
> has to be interpreted to mean that a time string with omitted components
> refers to a time *interval* of the appropriate size, rather than a
> high-precision time instant. Otherwise, the term "reduced
> precision/accuracy" has no meaning.
>
>
> 2. This is more of a side comment, but it does have some relevance. While
> the WMS spec says that it "extends" ISO 8601, it appears more that it
> *alters* it. ISO 8601 allows time intervals to be specified using the
> formats "start/end", "start/duration" and "duration/end". WMS appears to
> allow only the syntax "start/end/duration". This forces the user to always
> specify the end time. Since the end time is inclusiv, this is problematic
> for specifying things like "all records from 2012". Using the obvious
> 2012/2013/P1Y or even 2012-01-01T00:00:00Z-2013-01-01T00:00:00Z/P1Y would
> potentially include some records from 2013 as well.
>
> (It seems like the WMS spec should allow for intervals specified as
> start/end as well, but I can't see this in the spec. )
>
> The relevance of this is that if WMS really does only allow
> "start/end/duration", in the absence of the ability to utilize "reduced
> precision" clients are forced to specify a full precision end time, which
> can be problematic to determine (think the classic February in leap years).
>
> The ISO way of allowing a time instance and a duration doesn't have this
> problem, since the duration can be sized appropriately to the time
> specified.
>
> I realize this verges on hair-splitting standards wonkery, but in the
> absence of clearer language and examples there's not much else to go on.
>
>
>
>
--
Martin Davis
OpenGeo - http://opengeo.org
Expert service straight from the developers.
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