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The Make Up of SEDRIS
SEDRIS is fundamentally about: (a) representation of environmental data, and (b) the interchange of environmental data sets.
To achieve its representation and interchange objectives, SEDRIS relies on its five core technology components. These are the SEDRIS Data Representation Model (DRM), the Environmental Data Coding Specification (EDCS), the Spatial Reference Model (SRM), the SEDRIS interface specification (API), and the SEDRIS Transmittal Format (STF).
Three of these (DRM, EDCS, and SRM) are used to achieve the unambiguous representation of environmental data. The combination of these three core components provides the mechanism for description of environmental data. In some respect, this capability within SEDRIS can be viewed as analogous to a language for describing data about the environment. The DRM, the EDCS, and the SRM enable us to capture and communicate meaning and semantics about environmental data. The SEDRIS API and the STF allow the efficient sharing and interchange of the environmental data represented by the other three components. In the following, each of these five components is briefly described.
The SEDRIS Data Representation Model
The Data Representation Model (DRM) is based on an object-oriented data representation model that provides not only a clear description of data from all environmental domains (space, atmosphere, terrain and ocean) through a single schema, but also includes the logical relationships between the data elements. This allows for polymorphic representation of the same data, which means the same environmental "thing" can be represented through various means. In addition, the DRM permits the "association" of these various representations, which indicate the connectivity of the alternate representation of the same "thing". The DRM also ensures the syntax and the structural semantics of the data are fully expressed and correctly understood by users.
The DRM is at the heart of the SEDRIS technologies, and is developed and maintained based on object-oriented techniques using the Unified Modeling Language (UML) software. It consists of more than 300 object-oriented classes that allow the description of any environmental data, regardless of resolution, domain or density.
The combination of these classes and their relationships provides a rich, powerful, and expressive schema that can be thought of as the grammar of a language for describing environmental data.
The Environmental Data Coding Specification
The EDCS provides the means for classification (naming, labeling, identification) of environmental objects, as well as articulating their attributes (characteristics) based on a known and agreed upon convention. The foundation of this convention started from the Feature Attribute Coding Catalog (FACC) standard (used mainly in the cartographic and geographic communities), and although it has since changed substantially, many of the FACC definitions and codes are still utilized, and a mapping between EDCS and FACC is maintained. Since the nature and the types of data that SEDRIS needs to represent are broader than the cartographic domain, the FACC entries constitute only a sub-portion of the EDCS. Today, the EDCS schema goes far beyond its FACC start. It has been expanded both in breadth and depth to allow for additional (new) entries, as well as inclusion of the ocean, atmosphere and space domains by incorporating classification and attribute items from standards in those domains.
The EDCS unifies the characterization of environmental "things", regardless of the means by which such "things" are represented (e.g. as surfaces, features, point samples or others), or whether they are cast as individual primitives or structured collections.
This allows a clear separation between the DRM and EDCS, where the EDCS can act as the dictionary to the language and complement the grammar (the DRM).
Fundamentally, EDCS provides answers to three types of questions. What something is; what are its characteristics; and what units are used to measure those characteristics. These questions are independent of any data model and, as a result, EDCS is designed as a standalone technology that can be utilized independent of the rest of SEDRIS technologies any time the semantics of identification and characteristics of environmental data are called for.
The Spatial Reference Model
The most basic representation of anything environmental is its location. Without the ability to clearly specify the position of an object in reference to a designated origin and in reference to other objects, very little else can be said about the whole environment that can be shared with others.
To represent a location in space, an infinite number of coordinate systems and reference frames can be described, and each of these would be a valid representation. In practice, spatial reference frames are designed to meet specific goals or needs. Some are better tailored for use in specific applications, or have properties that are of value to the users. There is no rationale that dictates everyone must use one or a limited set of spatial reference frames. Different communities and sub-communities use a variety of systems in order to maximize efficiency, ease of use, mathematical properties, reduction in cost, or any number of domain-specific objectives. The key is to provide a model that can unify these different location representations, and allow for clear mapping and transformation of one to others.
The SRM captures and unifies the spatial models used by SEDRIS, plus the many others that are not currently used but can be easily added. These models include inertial, quasi-inertial, geo-based, and non-geo-based (purely arbitrary Cartesian) systems. Through its coordinate conversion library (a component of the SEDRIS API libraries), the associated SRM software provides a fast, accurate, and efficient means to transform coordinates from one frame of reference to another.
The SRM provides a unifying mechanism for specification and inclusion of any spatial reference frame and coordinate system. Its algorithms are designed to retain a high degree of accuracy during transformation and conversion operations (1mm accuracy).
Coordinate conversions and transformations must not only be accurate, but also fast. When dealing with millions of objects within even small environmental data sets, it is critical that operations during data extraction or insertion be very efficient without introducing any errors or loss in the data stream. For these reasons, the implementation of the SRM is highly optimized, and achieves very high performance measures without compromising accuracy in its algorithms.
Currently the SRM supports some 151 spatial reference frames, in addition to a large set of object reference models (Earth reference models, such as ellipsoids).
Similar to the EDCS, the SRM is recognized and designed as a standalone technology. SEDRIS depends on the SRM, but the SRM can be used in a variety of other domains and applications.
The SEDRIS Interface Specification and the SEDRIS Transmittal Format
The interchange of data must take into account platform independence, practical efficiency (both in storage and processing), and ease in software development (lowering the barrier to entry).
The SEDRIS interface specification (commonly known as API, or the Application Program Interface) and the STF (the SEDRIS Transmittal Format) are designed to achieve these goals. The API and STF, along with the associated tools and utilities, play the primary role in the data interchange, while being semantically coupled to the data representation model.
The API is the encapsulation of the functionality needed to produce and consume SEDRIS transmittals. It does this by de-coupling the user's application from the transmittal data structures, and by providing a consistent interface between a user's application and the SEDRIS transmittals. Its interface is bound by using C and C++, and is implemented in C++. The SEDRIS-provided reference implementation of the interface specification is designed to be portable and runs on multiple computing platforms. It is currently supported on a variety of platform and operating system combinations including Unix (SUN, SGI, IBM), Win9x, NT, XP (Intel-based machines), and Linux.
The STF is a platform-independent format designed to support the full capabilities of the DRM. Similar to other SEDRIS technology components, the STF is designed to de-couple the media-based format requirements from the API or the DRM. The supporting STF software (part of the SEDRIS API) adapts to the platform's word order (little or big endian), and frees the users from worries regarding platform dependencies. The STF is a file-based storage system, and is designed to be as space efficient as possible to reduce storage needs.
All five SEDRIS technology components comprise the total system, and are currently employed in a number of applications dealing with data conversion, data analysis, visualization and a variety of others, and operate on very large data sets used in oceanographic, atmospheric, space and terrain domains.