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Section 4: Mapping of DLG-E Objects

4. Mapping of Dataset, Theme, Surface, and Feature Object

There are often objects in a data model other than spatial which should be recognized as (SDTS) entities. The DLG-E objects of Dataset, Theme, Surface, and Feature each have attributes and may participate in relationships (i.e. links with other objects in the data model.) To encode attributes and relationships for these objects, there must be a record in the SDTS transfer that represents each instance. This section will address the representation of the objects in the transfer. (See Section 4 and 5 for information on the representation of attributes and Section 7 for the representation of relationships.)

4.1 Dataset

A DLG-E Dataset will be used to create a single transfer, i.e. E Datasets will be neither packed nor split to create transfer filesets. The single record in the Identification Module (IDEN) will represent the DLG-E Dataset. The Identification record contains an attribute record pointer which will be used to reference the datasets' attributes.

4.2 Theme

There is more than one theme in a dataset and each will need its own record. The SDTS composite object can be used to model objects other than spatial aggregates. A Composite module will represent the set of Themes in the Dataset. Each record of this Composite Module will represent a single Theme.

A theme composite record will not have component pointers. (This extension to an SDTS composite is permitted by the TVP.) A DLG-E theme is an association of features, not an aggregation (which implies a mutually exclusive grouping.) The composite has an attribute record pointer which will be used to reference a themes' attributes. The foreign id (i.e. module name and record id pair) of the composite record will be used to reference the theme in the encoding of relationships.

4.3 Surface

There is more than one surface in a dataset and each will need its own record. There will be a Composite module that will represent the set of Surfaces in the Dataset. Each record of this Composite Module will represent a single Surface.

A "surface" composite record will reference its spatial object modules as its component parts. The composite has an attribute record pointer which will be used to reference a surfaces' attributes. The foreign id (i.e. module name and record id pair) of the composite record will be used to reference the surface in the encoding of relationships.

4.4 Feature Objects

A DLG-E feature object will be represented by a SDTS composite object, hence referred to as "feature" composite. The feature composite will reference its composing spatial objects as its component parts. To be more precise, consider each type of DLG-E feature object.

Basic Feature Object
      A "feature" composite that references spatial object 
      records of like dimension.
            0-D BFO composite references a Point-Node record
            1-D BFOcomposite references Line record(s)
            2-D BFOcomposite references Polygon records(s)

Compound Feature Object
      A "feature" composite that references other "BFO" 
      composites and(or) "CFO" composites (other than itself).

Non-spatial Feature Object
      A "feature" composite that references no spatial 
      components. (This extension to a composite is 
      permitted by the TVP.)

The composite has an attribute record pointer which will be used to reference a features' attributes. (In DLG-E there are no attributes on spatial objects. The attributes are carried by the feature objects. This is reflected in the mapping.) The foreign id (i.e. module name and record id pair) of the composite record will be used to reference the feature in the encoding of relationships.

There are many options for partitioning these feature composites into modules. The partitioning chosen reflects the spirit of the DLG-E model. The features are viewed as belonging to the dataset and in that sense are all the same. The fact that a basic feature is spatially represented on a particular surface is not of primary importance. (It does happen that a BFO instance is composed of spatial objects from the same surface.) All BFOs will be in a single composite module and thus not further partitioned. All CFO's will be in a single composite module. All non-spatial will be in a single composite module.

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