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Comparison between SSN and WoT TD

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This page contains a comparison of some of the conceptual models taken into account in the Spatial Data on the Web (SDW) WG and in the Web of Things (WoT) IG.

In particular, it compares the Semantic Sensor Network (SSN) ontology and the Thing Description (TD) model (http://w3c.github.io/wot/current-practices/wot-practices.html#thing-description).

Both models are currently (February 2017) under development and are expected to change in the future before they are standardised. Therefore, the goal of the comparison is not to perform a detailed comparison of these models but a light one in order to identify general similarities and differences.


The main concept in the Thing Description model is a Thing.

WoT Things can be either physical (real-world artifact, such as a device) or virtual (aggregation of things that represents physicality, such as a room or group of devices).

SSN Systems are WoT Things (either physical or virtual) and SSN Devices correspond to WoT Physical Things.

Sensors and actuators

The WoT Thing Description model does not include concepts to represent Sensors and Actuators, while the SSN model does.

WoT Things conflate the sensing/actuating capabilities with the Thing, while in SSN the capabilities are separated: i.e., SSN allows describing separately Devices and Sensors/Actuators, and in order to represent those Devices that have sensing capabilities there exists a SensingDevice concept.

Note that if a SSN Core part does not include a Device concept, aligning such Core with the WoT TD model will not be straightforward.

Features of Interest and properties

In the WoT Thing Description model, Features of Interest and Properties are not explicitly described. It includes a concept named Property but it refers to one of the interaction patterns that can be used to deal with SSN Properties (see next section).

SSN allows representing both Features of Interest and their Properties.

Observations and actuations

The SSN model includes classes to represent Observations and Actuations (i.e., the outcomes of the respective processes). The WoT Thing Description model does not cover the representation of these concepts.

The WoT Thing Description model focuses on describing the interaction patterns that can be used for a concrete Thing (i.e., which operations can be performed to interact with the Thing). Right now, three interaction patterns are defined: Property, Action and Event.

These interaction patters describe the format of inputs and outputs, to allow interacting correctly with a Thing, but the model does not cover representing the concrete data used as input/output.

In SSN, Observations and Actuations follow some Process for which inputs and outputs are defined. But SSN allows representing the concrete data used as input/output, not the data format as in the WoT model.