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Fig. 9 Part of the archaeological time scale as temporal ordinal reference system according to
NBN EN ISO 19108:2005 (NBN 2005 )
are often not precisely known and can only be estimated”. Michalak ( 2005 , p. 868)
passed the same comment and called the choice for the DateTime data type unfor-
tunate. Consequently, both researches adapted the ISO model to be tailored to the
geologic time scale. However, they both opted for a different variant. Michalak
( 2005 ) presents a topological approach, while Cox and Richard ( 2005 ) suggest
a geometric version. In the remainder of this section, we outline the adaptations
made by these two researches and apply the schemes to the archaeological time
Cox and Richard ( 2005 ) introduce a variant on the model described in ISO
19108 in which the boundary between two temporal ordinal (geologic) eras is pre-
sent. This boundary is represented by the class TimeOrdinalEraBoundary which
is associated with TM_Instant (Fig. 10 ). A TimeOrdinalEraBoundary can exist
either with or without a geometric representation. Although the possibility to have
TemporalOrdinalEraBoundaries without known position exists, this model leans
on closely to a geometric variant of the ISO model. Cox and Richard ( 2005 ) refute
the opportunity to express the geological time scale as a topological complex
(Fig. 4 ). They indicate two concerns for this. First, multiple inheritance would be
required in that case, which causes practical problems. Second, some constraints
should be enforced: (i) boundaries and eras should be connected and (ii) an era can
only be divided once (Cox and Richard 2005 , p. 136).
In Fig. 11 , the model presented by Cox and Richard ( 2005 ) is applied to part
of the archaeological time scale. Five instances of the class TM_OrdinalEra are
given with their start and end relationships to five TimeOrdinalEraBoundaries
(Fig. 11 ). For each of these TimeOrdinalEraBoundaries the geometric realization
is performed by defining a value for the temporal position. In this case, there is
simply opted to use the Date data type and specify this until the year level. Other
possible data types for specifying the position of a TimeOrdinalEraBoundary
are Time, DateTime, TM_Coordinate, TM_CalDate, TM_ClockTime or
TM_OrdinalPosition. The first five possibilities are strongly related to the Date
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