Logical Data Model for the Integrated Hydrologic Forecast System

It is useful to consider the universe of data definitions for Office of Hydrology (OH) information systems in terms of the three-schema approach to information system definition that was first defined by the Standards Planning and Requirements Committee (SPARC) of the American National Standards Institute Information Processing Systems (ANSI/X3) Committee in 1975. The National Institute of Standards and Technology (NIST) Special Publication Number 500-173, Guide to Data Administration discusses this architecture in detail.

The ANSI/SPARC architecture recognizes and defines a separate schema for the three viewpoints from which a data base must be described:

The Conceptual Schema or Logical View describes data definitions for the organization's information as represented in a way that is independent of a particular data structure, representation, or other implementation-specific consideration. The intent of this viewpoint is to both unambiguously represent the enterprise view of the data world, and to support mapping from External to Internal layers.

The External Schema or Application View describes the data corresponding to part of the conceptual schema as seen by one or more users or programs, as cast in a particular data model and as represented in a given programming language. Most applications use a subset of the enterprise data and have particular motives for accessing data in a certain way.

The Internal Schema or Physical View describes how the data is represented and structured on the physical medium. In legacy systems, the internal schema is often represented as hard-coded data definitions within a particular programming language.

ANSI SPARC 3-Schema Data Model