[tdwg-content] Treatise on Occurrence, tokens, and basisOfRecord

Hilmar Lapp hlapp at nescent.org
Wed Oct 27 04:46:01 CEST 2010

On Oct 25, 2010, at 4:37 AM, Cam Webb wrote:

> But then what exactly are the Occurrences themselves?  From Richard  
> Pyle:
>   ``So, an Occurrence is the intersection of an Individual and an  
> Event.
>   An Event is a Location+Time[+other metadata].  Each Event may have
>   multiple Occurrences (i.e., one for each distinct Individual at  
> the same
>   Location+Time).  Also, an Individual may have multiple Occurrences  
> (one
>   for each Event at which the same Individual was documented).''
> So the Occurrence is the Individual _itself_ bounded by space and  
> time,

While for the purposes of exchanging occurrence data in a commonly  
agreed upon markup, i.e., Darwin Core, this may be perfectly  
acceptable, I think there are some serious issues in the above when we  
try to tighten up the semantics so that machines could do something  
with them, or so they can seamlessly integrate into the semantic web.

First there is an internal inconsistency: on the one hand occurrences  
*are* individuals (albeit only a subset - though see below), and on  
the other hand individuals *have* occurrences.

Second, occurrence is said to be the intersection of an individual and  
an event, or an individual and space and time. In the semantic web,  
OWL models deal with sets of individuals. I would argue that the  
intersection set of an individual organism (or a set of individual  
organisms) and an event (or a set of events) is empty, because there  
are no events that are also individual organisms, and vice versa.

Alternatively, and using "Individuals" as short hand for "instances of  
an organism" we could say that an Occurrence is the intersection of  
all Individuals belonging to a specific taxon, all Individuals at a  
specific location, and all Individuals existing at a specific time.  
Then an instance of an Occurrence would be an Individual in that  
intersection, and taxon, location, and time would be (among) its  

Just some thoughts.

: Hilmar Lapp  -:- Durham, NC -:- informatics.nescent.org :

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