Clarification: Is sensor a *type* or *instance*?

[matthijskooijman]

Looking through the SensorThings specification, it seems unclear to me what a "Sensor" models exacty. Is that:

1. A type of sensor, or
2. A specific physical sensor?

I looked through the 1.1 and in-progress 2.0 specs, but this seems to not be explicitly defined.

This distinction is not merely a semantic distinction, it is relevant for interpreting the structure of the data. In particular:

• If the same sensor is used by two datastreams belonging to the same thing, then that either means 1) that thing contains the same type of sensor twice, or 2) that sensor instance produces two datastreams (presumably measuring different things).
• If the same sensor is used by two datastreams belonging to different things, then that either means 1) that both things contain the same type of sensor or 2) that a particular sensor is moved from one thing to another.

I suspect that meaning 2 is the intended meaning (each sensor is a specific sensor), but it would be good if the spec could make this more explicit (or if it is intentionally left open, then make that explicit).

The description of sensor does not seem to make this explicit, and the datastream definition contains these two examples which can also still be interpreted both ways (since they lack an example with two datastreams on the same sensor):

A Datastream groups a collection of Observations into a time series measuring the same ObservedProperties by the same Sensor for the same Thing. Examples of Datastreams could be:

• An air quality monitoring station may have multiple Datastreams, each recording a specific pollutant measured by a specific sensor.
• A sensor that measures multiple ObservedProperties can generate a single Datastream with a composite resultType.

This ambiguity was previously mentioned at #161 (comment) (but that main issue is a different one).

I also believe that both interpretations can be useful, and in some cases you might want to express both in the same dataset (e.g. "these obervations are made with this sensor with this serial number and this calibration info, which is an instance of this type of sensor with this model number and these capabilities and characteristics"). Such a typeof relation could now be expressed using a SensorML typeof relation that references one sensor object from another (or references some external URL), but it might also be useful if this is supported more directly in SensorThings. Is this something for the relations that are being added to 2.0?

[KathiSchleidt]

Excellent question, no clear answer! Totally depends on how you're utilizing STA:

1. In simple setups where one has little to no sensor information anyway or when this is not relevant, it could be your version 1, with all physical sensors represented by the same Sensor in STA
2. In more complex systems, where you want to provide specific information, e.g. calibration, on each sensor, you can create a STA Sensor for each physical sensor. This can then also include sensor type information.

As you state, both options are useful in different contexts, so we have no desire to constrain this. When you work with a STA system, it rapidly becomes clear which paradigm is being followed.

Under V2.0 with the OMS extension, it clearly shifts to option 2, as the Deployment links a specific Sensor to a specific Thing.

@hylkevds document this a bit more