Re: Does 'Feature' = 'Real World Thing'?

> I think you're saying Feature = Real World Thing ... and Feature Description = Information object that describes Real World Thing. Makes sense to me.

This is not how ISO/OGC use the term "feature", or "feature instance" to be more specific (and "feature description" is not a term I have seen used in ISO/OGC standards). 

In the "light house" and "vertical obstruction" example you have two different features - in two datasets and of different feature types, but they describe the same real-world thing. Yet, these are still separate features.

The language is not always clear and "feature" is sometimes also used for the real-world thing and not the information object. I think the reason is that the distinction is blurred in the General Feature Model of ISO/OGC. There will be properties that describe the real-world thing (e.g. height, year of construction, etc.) and those should be consistent between the "light house" and the "vertical obstruction" features. At the same time there will be properties that describe the information object (e.g. provenance information) and these will differ between the datasets. In the General Feature Model these properties are all associated with the feature. So, sometimes when we talk about the feature, we talk about the information object ("last updated today") and sometimes about the real-world thing ("it is 34ft high"). But first of all, the feature is an information object describing a real-world thing.

When representing it in RDF, it may be more natural to distinguish the two subjects. I have some doubts that this would help to make spatial data easier to use for the broader web community, but that will be an interesting point to discuss and learn from existing practice.

Clemens


> On 20 Oct 2015, at 17:57, Jeremy Tandy <jeremy.tandy@gmail.com> wrote:
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> Bill, Andrea, Josh, Simon ... 
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> All great points; thanks!
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> @bill ... understanding the relationship between Thing and Feature is crucial; this echoes the discussions we've had regarding Thing vs. [INSPIRE] Spatial Object within the UK Location Programme. That said, let's bottom out the definitions of both first ...
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> @andrea ... I think that fictional things (like Dicken's London) count as 'real-world Things'. OK; that's counter intuitive :-) ... but I'm implying that we _talk_ about them in the real world; they are part of the "universe of discourse".
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> @josh ... feature _discernment_ vs. feature _representation_; excellent & articulate description. You raise the example of "light house" ... the same physical thing is _discerned_ as, say, "navigation aid" for maritime and "vertical obstruction" for aviation. Clearly, these are both valid 'views' of the light house with respect to their domain of application. Each 'view' of the light house will, necessarily, contain the information which is pertinent for that application ... using your terms, there will be two _representations_ of the lighthouse.
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> However, I think that your conflation of the _discerned_ feature and its representation is problematic. Since working with RDF, I've learned that it's very useful to be specific about the subject of each triple. For example, attributes such as 'owner' could apply to both the light house (physical Thing) _and_ the representation (information resource). It makes sense to identify these separately. This is the advice from W3C URLS in Data (FPWD) <http://www.w3.org/TR/urls-in-data/>.
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> Going back to the point that there are two _discerned_ features (navigation aid & vertical obstruction) it is highly likely that the parties from the responsible communities will mint their own identifiers for the light house. This happens all the time ... the "non-unique-naming" problem. We have to have mechanisms in place to allow those discerned features to be reconciled.
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> That said, I think your point is that (discerned) Feature = Real World Thing. (more or less).
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> @simon ... I think you're saying Feature = Real World Thing ... and Feature Description = Information object that describes Real World Thing. Makes sense to me. I would still say that we need to identify both the Thing and the Information Resource - not conflate them.
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> Jeremy
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> On Tue, 20 Oct 2015 at 04:05 <Simon.Cox@csiro.au> wrote:
> I like to use ‘feature description’ to refer to the digital representation.
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> Elide ‘description’ when it is obvious from context and does no other harm.
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> Or use ‘description’.
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> Ø  Different features, same real world thing...
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> Different descriptions, same feature?
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> Simon
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> From: Linda van den Brink [mailto:l.vandenbrink@geonovum.nl <mailto:l.vandenbrink@geonovum.nl>] 
> Sent: Monday, 19 October 2015 6:24 PM
> To: SDW WG Public List <public-sdw-wg@w3.org <mailto:public-sdw-wg@w3.org>>
> Subject: RE: Does 'Feature' = 'Real World Thing'?
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> Hi all,
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> IMO what we call features in the geospatial domain are usually abstractions of representations on maps of real world things; not abstractions of real world things themselves. I see this a lot in information models in the Netherlands. For example, we have two information models that have features of class Building. They both model exactly the same set of objects, but one model captures the building geometry as seen from above, the other from ground level. Different features, same real world thing...
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> Linda
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> Van: Jeremy Tandy [mailto:jeremy.tandy@gmail.com <mailto:jeremy.tandy@gmail.com>] 
> Verzonden: maandag 19 oktober 2015 8:43
> Aan: SDW WG Public List
> Onderwerp: Does 'Feature' = 'Real World Thing'?
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> Hi-
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> I've been working through the discussion on Linking-Data and this uncovered (or, really, re-found) this issue.
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> By OGC terminology, Feature is "an abstraction of a real world phenomenon". Linked Data folks like to talk about Real World Things (both physical and abstract).
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> There's a disjoint here that we need to resolve.
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> I've captured the question on the wiki [1] and included the content below.
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> Jeremy
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> [1]: https://www.w3.org/2015/spatial/wiki/Linking_Data#Question:_is_a_Feature_the_Real_World_Thing.3F <https://www.w3.org/2015/spatial/wiki/Linking_Data#Question:_is_a_Feature_the_Real_World_Thing.3F> 
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> Question: is a Feature the Real World Thing?
> ISO 19101 -- Geographic information - Reference model states: 
> ·         [4.11] feature: abstraction of real world phenomena
> ·         [4.12] feature attribute: characteristic of a feature ...
> ·         EXAMPLE 2 A feature attribute named ‘length’ may have an attribute value ’82.4’ which belongs to the data type ‘real’.
> The definition of feature attribute is clear- it's a piece of information about the feature.
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> feature is not quite so clear. In this context, what does abstraction mean?
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> Typically, the Linked Data community refer to Real-world ‘Things’ (see Designing URI sets for the UK public sector <https://www.gov.uk/government/publications/designing-uri-sets-for-the-uk-public-sector>); real-world Things (or just Things) are "are the physical and abstract ‘Things’ that may be referred to in statements". Examples include a school, a road, a person (physical); a government sector, an ethnic group, an event (abstract).
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> A commonly used example is Manchester Piccadilly Railway Station. A URI for Manchester Piccadilly Railway Station would refer to the real station, constructed from steel and concrete with thousands of people passing through it each day. Clearly one cannot expect an HTTP request to return the real railway station (!); it returns an information object about the railway station.
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> W3C URLS in Data (FPWD) <http://www.w3.org/TR/urls-in-data/> discusses the need to differentiate between the real Thing and the information resource that describes it. The Publishing Data <http://www.w3.org/TR/urls-in-data/#publishing-data> section provides three strategies for doing so.
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> In the Geographic Community, the Feature is seen as an information resource - which is, in some way, related to the real-world Thing. INSPIRE (Generic Conceptual Model <http://inspire.ec.europa.eu/documents/Data_Specifications/D2.5_v3.4.pdf>) refers to these resources as Spatial Objects: "abstract representation of a real-world phenomenon related to a specific location or geographical area". It notes that the term is "synonymous with "(geographic) feature" as used in the ISO 19100 series" and, later, talks about versioning the Spatial Objects. Clearly, you can only version the record of information held about a real world Thing, not the Thing itself?
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> So the question remains: are we identifying real-world Things (both physical and abstract) or information objects that describe them? Once that's decided, we need to get our terminology clear and stick to it!
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