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2.1 Conceptual Model

The SDTS conceptual model has three parts: a model of spatial phenomena, a model of the spatial objects used to represent phenomena, and a model of spatial features that explains how spatial objects and spatial phenomena are related (also see Annex B.)

The following terms define the parts of the model.

a) Phenomenon. A fact, occurrence, or circumstance. Route 10, George Washington National Forest, and Chesterfield County are all phenomena.

b) Classification. The assignment of similar phenomena to a common class. An individual phenomenon is an instance of its class. Route 10 is an instance of the class road.

c) Generalization. A process in which classes are assigned to other classes. The general class includes all the instances of the constituent classes. Sewers are included in the more general class of utilities.

d) Aggregation. The operation of constructing more complex phenomena out of component phenomena. A lock is an aggregation of walls, gates, and a reservoir.

e) Association. The assignment of phenomena to sets, using criteria different from those used for classification. Concrete roads may be associated with concrete sewers, concrete locks, and other phenomena constructed of concrete.

2.1.1 Model of Spatial Phenomena

SDTS transfers information about phenomena that are defined in space and time and are described by using a fixed location-- spatial phenomena. All phenomena are defined as belonging to a class of phenomena. (Smith's Farm belongs to Farm.) A characteristic of such a class is called an attribute. (Acreage is an attribute for Farm.) An attribute value is a specific quantity or quality of the attribute assigned to a phenomenon in that class. (Smith's Farm has an Acreage of 160 acres.)

Whether a given phenomenon belongs to a class is determined by the definition of the class. The definition consists of a statement about characteristics that all members of the class have in common. It also includes characteristics that distinguish the class from other classes. These definitional characteristics are necessary and sufficient conditions for classifying some phenomena into the class and excluding others. The data collector may define which classes of phenomena are of interest. Those classes of phenomena are called entity types, and the individual phenomena are called entity instances.

Certain attributes are identified with each class. The attributes of a class include key attributes. The combination of values of the key attributes forms a unique identifier for each entity instance.

Entity instances may be aggregated into instances of a different type of entity.

Entity types can be generalized into themes based on definitional characteristics shared by more than one class. A theme can also have its own attributes, including name.

Associations of entity instances are defined in terms of characteristics other than those used to define an entity type. A common association is the spatial domain, which groups all entity instances having coordinates within a specified range. Another useful association is temporal domain. Values of a temporal attribute such as Age or a user-defined temporal dimension as in x,y,t may be assigned to entity instances and used to associate them into sets with a common extent in time.

A relationship is a special case of an association. A relationship exists between entity types. A relationship instance is an association between entity instances with a unique relationship value.

2.1.2 Model of Spatial Objects

Entity instances have a digital representation. That digital representation consists of one or more spatial objects. A spatial object may be an aggregation of other spatial objects, not all of which necessarily represent an entity instance. A spatial object that represents all of a single entity instance is an entity object. It may be classified into an entity object class. Entity objects have generalizations and associations as well: the representation of an entity theme is an object theme, and an entity spatial domain, an object spatial domain. In general, the correspondence between entity instance and entity object is paralleled by all characteristics of entities and objects.

Entity objects have locational attributes (spatial address), nonlocational attributes, and relationships (topology). The attributes and relationships of entity objects need not be as extensive as those of their corresponding entity instances. The key attributes used to distinguish a particular entity instance may not be present in the actual transfer; instead, the entity object record identifier may be the only way to distinguish between instances.

Spatial objects may have attributes independently of whether they are entity objects or not. All objects may be classified, aggregated, and associated in the same general manner that entity instances may be.

This standard defines a set of simple spatial objects. These simple spatial objects are either primitive objects (not aggregated from any other objects), or aggregated only from spatial objects belonging to different classes (polygons, for example, are not aggregated from polygons, only from rings, chains or strings). The only exception is the composite object. Composite spatial objects may be aggregated from simple objects or from other composites.

Spatial objects are classified into module types, one of the basic building blocks of the standard. Once defined, modules may be associated into sets by spatial domain, temporal domain, data quality, security requirements, topological relationships, or any other criteria.

2.1.3 Model of Spatial Features

The terminology in geographic data handling has traditionally not distinguished between the phenomenon and its representation, referring to both as features. For the sake of clarity, a feature is defined as the combination of the phenomenon and its representation. A feature instance consists of an entity instance and the entity object that represents it, and belongs to a feature type (see Table 1).

Table 1 - Feature Instance

An instance of a defined entity and its object representation
Feature Instance
is
Entity Instance: a spatial phenomenon of a defined type.
and
Entity object: a digital representation of an entity instance.