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Re: ACTION-251: (ISSUE-88) write up how an ssn:platform and a sosa:platform are essentially the same, with an example (Spatial Data on the Web Working Group)

From: Krzysztof Janowicz <janowicz@ucsb.edu>
Date: Mon, 16 Jan 2017 19:34:21 -0800
To: Spatial Data on the Web Working Group <public-sdw-wg@maia.w3.org>, SDW WG Public List <public-sdw-wg@w3.org>, Armin Haller <armin.haller@anu.edu.au>, "Cox, Simon (CESRE, Kensington)" <Simon.Cox@csiro.au>, Kerry Taylor <kerry.taylor@anu.edu.au>
Message-ID: <6b1d6e43-62a0-e6fc-344d-ad2dd05f14fd@ucsb.edu>
Hi,

This message discusses the relation between ssn:platform and 
sosa:platform. An argument was made that both are 'completely different' 
and cannot be aligned. This message will show that this is not the case 
and that both concepts are indeed conceptually very similar. In fact, 
the ssn:platform is problematic and ill-defined. In the following SSN 
will refer to the 'old' SSN, not necessarily the currently revised version.

Let us start by looking at the textual definitions/descriptions.

According to SOSA, a platform is defined as "[a] device, (computational) 
system, or agent (including humans). A platform carries at least one 
sensor, actuator, or sampling device to produce observations, 
actuations, or samples, by following a procedure. In case of virtual 
sensors, a platform can be a computing system which hosts software 
implementations, e.g., simulations."

According to SSN, a platform is  "An entity to which other entities can 
be attached - particuarly [sic] sensors and other platforms. For 
example, a post might act as a platform, a buoy might act as a platform, 
or a fish might act as a platform for an attached sensor."

The key characteristic of a platform in both SOSA and SSN is that it 
carries something, e.g., a sensor, actuator, or another platform. The 
SOSA definitions states more explicitly what is carried while the old 
SSN definition is broader in the sense that any 'entity' to which other 
entities can be attached constitutes a platform (e.g., a wall hook is a 
platform as one can attach a picture to it). From a formal perspective, 
however, there is no difference here, e.g., an SSN platform can carry an 
actuator and a SOSA platform can carry a platform. We can make this more 
explicit in the SOSA platform definition if this would help to remove 
confusion.

Both SOSA and SSN explicitly include agents such as fish or humans as 
platforms for sensors such as their eyes. This is another part where the 
definitions are well aligned. The SOSA definitions explicitly mentions 
devices and so forth, as it has fewer axioms and thus relies more on 
textual descriptions to convey meaning. Also, SSN does not deal with 
sampling, so understandably the SOSA definitions mentions sampling 
devices while the SSN definition does not.

Most importantly, SOSA explicitly lists virtual sensors in the platform 
definition while SSN does not. In both cases, the developers of SOSA and 
SSN have explicitly stated that they support virtual sensors. SOSA is 
simply more consistent in doing so as SSN has been repeatedly criticized 
for an uneven handling of virtual sensors; more details below.


More formally speaking:

SSN platform is defined as a subclass of DUL:PhysicalObject. Thus, 
virtual sensors cannot be mounted on platforms. If I remember correctly, 
it was Claus who provided the nice example of the simulation of 
self-driving cars in which the positioning of the virtual sensors is 
key. SOSA would support such scenario, while the old SSN would not. 
Clearly, this had to be changed.

The new SSN does not have a DUL alignment anymore and thus the only 
axioms left are two forall quantifications on the fillers of 
attachedSystem and inDeployment. These axioms, however, do not carry any 
meaning as far as platforms are concerned. This is a similar situation 
to the recent subsystems discussion. I explained in said discussion why 
removing the existential restriction is problematic and the same problem 
appears for SSN platform. Simply put platform(x) AND inDeployment(x,y) 
--> deployment(y). In other terms, the two axioms tell us something 
about systems and deployments but not about platforms. Long story short, 
there is no real formal definition left (after removing DUL) and thus 
really anything can be an ssn:platform. Consequently, all SOSA platforms 
can be SSN platforms. By design (roughly (!)) the same can be said about 
platform in SOSA. Thus, the claim that the two definitions would be in 
any way incompatible or not align-able is wrong.


Recommendation: I like to think of SOSA as being to the new SSN what SSO 
was to the old SSN and not as some sort of entirely separate entity. I 
do not see any need for a ssn:platform in the new SSN and would propose 
using the platform class from SOSA instead. Alternatively, one could 
define ssn:platform as a subclass or sosa:platform if there would be any 
specific reason to distinguish both.


Best,
Krzysztof


On 01/11/2017 04:01 AM, Spatial Data on the Web Working Group Issue 
Tracker wrote:
> ACTION-251: write up how an ssn:platform and a sosa:platform are essentially the same, with an example  (Spatial Data on the Web Working Group)
>
> http://www.w3.org/2015/spatial/track/actions/251
>
> On: Krzysztof Janowicz
> Due: 2017-01-18
> Issue: ISSUE-88 (Why is a sosa-core platofrm completely different to an ssn:platform?)Product: Semantic Sensor Network Ontology
>
> If you do not want to be notified on new action items for this group, please update your settings at:
> http://www.w3.org/2015/spatial/track/users/43518#settings
>


-- 
Krzysztof Janowicz

Geography Department, University of California, Santa Barbara
4830 Ellison Hall, Santa Barbara, CA 93106-4060

Email: jano@geog.ucsb.edu
Webpage: http://geog.ucsb.edu/~jano/
Semantic Web Journal: http://www.semantic-web-journal.net
Received on Tuesday, 17 January 2017 03:34:57 UTC

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