Help:Introduction

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Welcome to Gridpedia! This page explains the basic idea behind Gridpedia and introduces the core concepts that are used. If you are already familiar with RDF or ontologies, you may want to have a look at the other Help pages, in order to get familiar with our Gridpedia interface.

If you have not heard of Semantic Web Technologies - that's perfectly fine! You will not need detailed knowledge on the Semantic Web to make contributions to this project!


Purpose of Gridpedia

The purpose of Gridpedia is to collect and structure information regarding the Smart Grid. The information is entered via simple forms, which are easy to understand. Please also note that you cannot 'break' anything - the history of all pages is stored automatically (which can be restored easily). See also the Gridpedia:About page for further information on the contents of Gridpedia.

Gridpedia Data Model

Information is described via 'resources' - a resource can be anything that one wants to associate information with. There are three kinds of resources:

  • Classes
  • Instances
  • Properties

If you would like to read more about these concepts and the underlying description language, RDF, the RDF Primer is a good starting point.

Classes

Classes describe a group of similar instances. Further, classes have a hierarchical structure, putting each other into context. This hierarchy is described via the 'subclassof' property. That is, if 'class A' is a subclass of 'class B', all instances in class A are also in class B.

More formally, we use the concept of a class, defined as follows[1]:

Resources may be divided into groups called classes. The members of a class are known as instances of the class. Classes are themselves resources. They are often identified by RDF URI References and may be described using RDF properties. The rdf:type property may be used to state that a resource is an instance of a class.

Example

Assume that class B is the class 'Power Plant', class A may then be the class 'Nuclear Power Plant'. These classes have a hierarchical relationship. Each Nuclear Power Plant is always also a Power Plant, but there are many Power Plants which are not Nuclear Power Plants - Coal, Gas, Wind and Hydro Power Plants, for example. A more formal introduction on classes can be found on the W3C website.

Instances

Instances can be thought of as class members, i.e., each instance is assigned to one (or more) class(es) and represents a concrete instantiation of that particular class (or classes).

Properties

Properties describe relationships between resources. That is, properties connect two resources in the form of a 'subject' - 'property' - 'object' triple. A property can therefore be considered a 'link' between two other resources.

Formal definition[2]:

[...] An RDF property as a relation between subject resources and object resources. The rdfs:subPropertyOf property may be used to state that one property is a subproperty of another. If a property P is a subproperty of property P', then all pairs of resources which are related by P are also related by P'. [...]

Note, the property itself is a resource, so one may define it: Most importantly, one may specify the domain and range for a given property. The domain of property specifies the type of the subject that the property connects. The range, on the other hand, specifies the object type. Regarding the range, there are two different types of properties: 'object-properties' and 'data-properties'. Object-properties have a resource as object, while data-properties assign a data-value as object.

Further information: W3C website.

References

  1. http://www.w3.org/TR/rdf-schema/#ch_classes
  2. http://www.w3.org/TR/rdf-schema/#ch_properties
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