[tdwg-content] DwC Occurrence [SEC=UNCLASSIFIED]
pmurray at anbg.gov.au
Tue May 31 09:44:20 CEST 2011
> An Occurrence is a combination of an Individual and an Event.
> An Occurrence is a coupling of an Individual and an Event.
> An Occurrence is a pairing of an Individual and an Event.
An Occurrence is the reification of an individual's involvement in (entaglement with? presence at? relationship to?) an event. It reifies an "Event involvesIndividual Individual" fact.
The need for this construct is that we often need to say a number of additional things about an individual's involvement with (presence at) an event beyond simply assertin that there is some relationship. We need to say what tokens that individual left, what role that individual had (Predator? Prey? Parasite?), perhaps temporal or other limits of that particular individual at the event. Occurrence is the object to which these facts may be attached. An individual might meaningfully have more than one occurrence at an event - particularly in cases where events are part-of larger events, or where an individual somehow has multiple roles (hyenas chased away from their kill by a lion - or is it the other way around?).
To put it another way: "reification" = "tuple" = "association table" = "pulling a property out into an object". More or less.
To put it another another way, an Occurence object stands in relation to an event and an individual much as a TaxonRelationship object stands in relation to the two taxa it mentions. You could simply model taxonomy with a "hasSubtaxon" predicate, but we usually need to say a great deal more about taxonomic relationships than that.
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