GDM Overview

The conceptual design of the model was patterned after the PODS Pipeline Data Model such that feature tables from PODS might be incorporated into GDM or vice versa.  GDM uses basically the same schema for linear referencing as the PODS Pipeline Data Model, with the exception that GDM supports both stationed and unstationed pipe.

There were several key design considerations for the GDM model, including:

• Support for native spatial data types without relying on external GIS middleware.
• Support for Gas Distribution and Transmission pipelines within the same model – thus providing support for stationing and station equations without requiring it for all pipe.
• Support for linear referencing and spatial data.
• A set of core tables the define the underlying pipeline network.

In PODS, all linear referencing is stored in the core tables and all all feature tables have a foreign key to the Event_Range table to get their linear referenced location. In GDM, the core tables also store the pipeline linear referencing but stations and measures are stored in the feature tables instead of in a central Station_Point table.

Modeling Geometry in the Distribution Model

A relational database approach can be used to define entities and attributes but most gas distribution companies recognize the value of spatial information about the distribution system and, as a result, have or are planning to invest in GIS systems. The GDM does not aim to replace a GIS but to provide a common model that will augment existing systems and improve interoperability between different systems. To provide this interoperability, the committee decided to use RDBMS spatial binary fields to store shapes.

The GDM incorporates the approach of providing a single field that can be used to store geometry. Both SQL Server Spatial and Oracle Spatial state their data types are compliant with the OGC standard. Some issues involving transferring data between different systems remain to be resolved but the single field approach is supported by most systems.

Modeling Distribution Pipe in GDM

Gas distribution system facilities have a hierarchical nature that requires the operator to provide spatial data in a manner compatible with facility parent/child relationships.  Due to the nature of these relationships orphaned data is not permitted or able to be inserted. Therefore, although distribution facilities can be broken down to their least part, they are also a part of the whole system.

View the GDM v1.0 Physical Representation

The tables in the GDM directly related to facilities can be viewed as belonging to separate categories for the purpose of isolating information levels and maintaining reporting relationships. This hierarchy defines the elements that make up the model.