Hi Pat, (01)
In a 4D Ontology, what you refer to as a point in time is actually an extent
with one dimension (time) tending towards zero and the other three tending
towards infinity. More usually, one is interested in a "point in time" over
a finite 3D extent though (ignoring relativistic considerations, which in
most earth-bound situations we can). We can talk about 4:00 UST all over the
world. That is not a point, it's an extent in four dimensions. (02)
I'm sure Chris Partridge or Matthew West can explain this more precisely
than me, so apologies if my terminology is not quite correct. (03)
>From a purely pragmatic point of view (I'm what Chris describes as a
"hairy-arsed engineer"), 4D works and works very well - we've been working
with for years in ISO15926, and we chose the same approach for the IDEAS
Ontology (www.ideasgroup.org) 'cos it works. I've seen far too many bodged
and inconsistent attempts to manage time in data models and ontologies over
the years. The 4D approach gives you a pattern for representing change over
time that works in all cases and works consistently. (04)
Cheers
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But those start and end times ARE points. Really,
you can't get away from it: if you have
intervals, you have the points at their ends.
They might not be points in the mathematical real
line, but they are points in the following senses: (06)
1. they are indivisible (unless you change your interval topology)
2. they are the places where intervals meet one another
3. they are uniquely determined by your intervals
4. they determine the time ordering on the intervals (07)
And this is independent of whether or not you have continuous or dense time. (08)
Pat (09)
>All this for me is independent of whether time is ultimately granular
>or continuous. Ultimately this only means at what point we can not longer
>tell whether one event happened before another.
>
>
>Regards
>
>Matthew West
>Reference Data Architecture and Standards Manager
>Shell International Petroleum Company Limited
>Registered in England and Wales
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>
>Tel: +44 20 7934 4490 Mobile: +44 7796 336538
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>
>
>> -----Original Message-----
>> From: ontolog-forum-bounces@xxxxxxxxxxxxxxxx
>> [mailto:ontolog-forum-bounces@xxxxxxxxxxxxxxxx]On Behalf Of
>> John F. Sowa
>> Sent: 21 January 2008 17:48
>> To: [ontolog-forum]
>> Subject: Re: [ontolog-forum] Time representation
>>
>>
>> Pat,
>>
>> The position I most strongly advocate is not a specific ontology,
>> but a framework of conventions for organizing a multiplicity
>> of special cases (not necessarily consistent with one another),
>> making the implicit relationships explicit, and providing tools
>> and guidelines for mixing and matching. The lattice of theories
>> is an example. Robert Kent's IFF is a much more ambitious example.
>>
>> I would recommend a fairly simple framework for starters, since
>> there's a danger of freezing half-baked ideas before they're fully
>> baked. (RDF, for example, was hardly out of the oven before
>> Tim Bray tried, unsuccessfully, to pull it back in.)
>>
>> > Do you have any granularity axioms? That is one of the hardest
>> > ontological problems, in my experience.
>>
>> There are so many hard problems, it's hard to say which are harder.
>> But the idea of taking the least significant digit as the criterion
>> for implicit granularity is fairly common for experimental data
>> (unless some explicit margin of error is stated).
>>
>> Re PTim: I realize that calling an interval a point is problematical.
>> But in anything that has to do with the physical world, there is no
>> way to specify a true point. Perhaps a better term would be "grain
>> in time", abbreviated "Grit".
>>
>> John
>>
>> PS re HTML email formats: Your note of 11:18 was in a readable font
>> for Thunderbird, but your note of 11:37 appeared in a tiny, tiny font.
> > I had to increase the font size by two steps to make it the same as
>> the previous note. But then the fonts for all other notes were too
>> big, and I had to decrease the default by two steps.
>>
>> At least each of your notes was entirely in one font size. I've
>> received some email in which each paragraph was in a progressively
>> smaller font. That's why I hate HTML email.
>>
>>
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