Slide 18 of 29
What granularity you store timestamps is not just an implementation issue, it is basic to the logical representation.
The table allows 2x4 = 8 different types of t-GIS. Each type comprises several quite different data models, e.g. valid time at the object level (OO GIS) can be done by chaining objects, or by having one object id and a time offset. Object chains can fix ids so that the ‘current’ one always has the same id, or not. For OO GIS, are relationships time-stamped too or are they treated as a kind of attribute value ?
Applying temporality at the attribute level means that the GIS must be extended in that all data types must be temporally enhanced. Applying temporality at the object level can be done additively by adding new datatypes (ADTs) analogous to the spatial datatypes that turn an ordinary database into a spatial database.
Timstamping individual pixels is done in OOGDM (!)
Valid time is not applicable to segments or datasets as they are pure representation: they have no real-world existence.
Transaction time at the segment and valid time at the attribute prevents redundancy and captures appropriate semantics for many purposes.
This viewpoint and table is original to Philip Sargent (May, 1998).
