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3. Conceptual Modeling
A spatiotemporal conceptual model that expresses both spatial and temporal characteristics can meet the main requirements for modeling a cadastral database. Such a model should give an opportunity to model spatiotemporal objects, the attributes, and the relationships between these objects. A number of studies have been carried out on spatiotemporal conceptual modeling.

In this study, the Spatiotemporal Entity-Relationship (STER) model (Tryfona and Jensen 1998a, Tryfona and Jensen 1998b) was used to model the cadastral data. The STER model was chosen because it provides advantages for graphical notation. It provides the representation of both temporal and spatial characteristics of the objects, the attributes, and the relationships between the objects. It also provides a means to represent non-spatial and non-temporal database elements by traditional ER notations. The relationships are represented using the ER notations and the spatial semantics of the relationships are explained by the relationship name.

STER model has two levels of conceptual modeling diagrams (figure 2). The first level, the abstract level, uses components that are easier for the user to follow, free of details, and is not complicated. The second level is a detailed level and allows the designer to explain all the information needed to describe an application. At this level, the new modeling constructs are extended into an ER model. Several EER constructs were added to both STER and ER models to improve the power for representing superclass-subclass and specialization-generalization relationships between the object classes. The EER model includes all the modeling concepts of the ER model. In addition, it includes the specialization-generalization, the superclass-subclass relationship, and the category concepts.


Figure 2: Representation of a spatiotemporal entity set in (a) STER model and (b) ER model.
In STER model, three types of time aspects can be defined: (i) valid time, (ii) transaction time, and (iii) existence time. The valid time of a fact is the time when the fact is true in the modeled reality. In other words, the time interval that a value is valid for a land parcel represents the valid time for the value attribute of the land parcel. The transaction time of a database is the time when the element is the part of the current state of the database. For example, a land parcel may exist between 1951 and 1999 years. However, it may be recorded in a database by 1995. The transaction time of this land parcel is then defined between 1995 and 1999. The transaction time is applied not only to facts but to any element that may be stored in a database. The existence time refers to the time when the object exists. For example, the existence time of above mentioned land parcel is between 1951 and 1999.

In STER model, the entity sets include the (i) temporal, (ii) spatial, and (iii) spatiotemporal aspects of the entities. Temporal entity sets can be assigned either the existence or transaction time or both (then called bitemporal). In figure 3, a part of the proposed abstract conceptual schema of cadastral data is given. The person, parcel, and the building are examples for bitemporal entity types. Spatial entities, such as parcel and building have a position in space that is necessary to capture in the database. The spatial entity sets of SPACE, GEOMETRY, POINT (P), LINE (L), and REGION (R) are defined to represent the space. To capture the temporal aspects of the positions of an object in an entity set, a code of “svt” and “stt” or both are placed in a circle in the lower-right corner of the entity set’s rectangle. The entity sets have two types of attributes: (i) descriptive attributes, such as the “id” of a parcel, and (ii) spatial attributes, such as the “soil type” of a parcel. The values of descriptive attributes for an entity or a relationship often change over time. Therefore, it is necessary to capture this change in the database. Since the spatial attributes may have a temporal dimension they are termed spatiotemporal attributes.

The entity sets have three types of relationships that are (i) the temporal relationship sets, (ii) the spatial relationship sets, and (iii) the spatiotemporal relationship sets. If at least one entity set participating to the relationship has temporal dimension, then the relationship becomes temporal. The relationship between a person and a parcel is temporal. Spatial relationship sets are the associations between the geometries of the spatial entities. The spatiality of a relationship is annotated by using an “s” in a circle (e.g. parcel-building relationship in figure 3). A spatiotemporal relationship is a spatial relationship set with time support. By annotating a spatial relationship set with a temporal aspect, the changes of the spatial relationship over time can be captured.


Figure 3: A part of abstract conceptual schema designed for a cadastral database.
A few EER constructs were added to both STER and ER models to improve the power for representing superclass-subclass and specialization-generalization relationships between the object classes. The EER model includes all the modeling constructs of the ER model. In ER model, all types of superclass-subclass relationships are expressed by an ISA (IS-A) relationship which is denoted by a triangle. EER model provides additional specialization-generalization relationship notations for defining the superclasses and subclasses. Adding EER constructs to STER model has improved its power to express the nature of relationships between object classes in a conceptual model. Due to the orthogonal characteristics of spatial and temporal constructs of STER model, the representation of spatial and temporal characteristics has not been affected. The relationship between real-juridical person and person is an example of denoting specialization with EER and STER model (figure 3).