The OGC STA enables interoperability for two layers: (1) Service interface, and (2) Data model [40]. With regards to the service interface layer, the STA defined a RESTful pattern, based on the OASIS OData standard, that allowed different STA services to exchange and filter entities defined by the STA data model. As for the data model aspect, the STA data model was based on the International Organization for Standardization (ISO) and OGC Observation and Measurement standard model [41]. As a result, the data model can interoperate and is backward compatible with the OGC Sensor Observation Service (SOS) Web service. The following UML diagram describes the entities of the STA data model. In the OGC STA, every Thing can have zero or more locations in space or time ((Figure 2). Furthermore, each Thing can have zero or more “Datastreams”. A Datastream is a collection of “Observation” entities grouped by the same “ObservedProperty”. An Observation is an event performed by a “Sensor”, that is a process producing a result with a value that estimates the ObservedProperty of a “FeatureofInterest”.