Public Contracts Ontology
The ontology developed within the use case covers information related to public contracts that is published by contracting authorities during the procurement process. We built the ontology on the basis of analysis of the existing public procurement portals (especially TED as the European one, but also national ones) and information published by contracting authorities on these portals. We did not consider all information but primarily the information relevant for matching public contracts with potential suppliers. Therefore, for the most part we consider the information that is produced in the tendering phase (description of the public contract, received tenders and the eventually accepted tender). From the evaluation phase we consider the actual ﬁnal price for the contract, as its modeling is identical to that of estimated (in the contract notice) as well as agreed price; no further complexity is thus added to the ontology by including it.
Ontologies Reused by the PCO
Reusing existing, established ontologies when building one's own ontology is crucial for achieving interoperability on the semantic web, since applications capable of working with the original ontologies can then also process the reused elements (and even their derivatives, such as subclasses and subproperties) in the new ontology as well. The PCO reuses the following models:
• GoodRelations Ontology (gr preﬁx) – to model organizations and price speciﬁcations
• VCard Ontology (vcard preﬁx) – to express contact information
• Payments Ontology (payment preﬁx) – to express subsidies
• Dublin Core (dcterms preﬁx) – to express descriptive metadata (e.g., title, description)
• Simple Knowledge Organization System (SKOS) (skos preﬁx) – to express code lists and classiﬁcations
• Friend-of-a-friend Ontology (FOAF) (foaf preﬁx) – to express agents, especially persons and relationships between them
• schema.org (s preﬁx) – to express locations and other generic kinds of entities
• Asset Description Metadata Schema (ADMS) (adms preﬁx) – to express identiﬁers.
Core Concepts of the PCO
Figure 1 depicts the ontology in the form of a UML class diagram. The core concept of the ontology is that of public contract represented by the class pc:Contract. We understand a public contract as a single object that groups pieces of information related to the contract. These pieces of information gradually arise during the public procurement process. They are published by contracting authorities on various public procurement portals in the form of diﬀerent kinds of notiﬁcation documents, e.g., call for tenders (sometimes also called contract notice), contract award notice, contract cancellation notice, or the like. Another important concept of the ontology are business entities, i.e., in this context, contracting authorities and suppliers. Business entities are not represented via a new class in the ontology; we rather reuse the class gr:BusinessEntity from the GoodRelations ontology.
Fig. 1. Public Contracts Ontology – UML class diagram