Re: Area of spatial objects

On 11/7/18 4:54 AM, Car, Nicholas (L&W, Dutton Park) wrote:
>
> Hi Frans,
>
> What if I wanted to publish Features’ areas without also publishing 
> geometries? What about features with a point geometry and an area, no 
> polygon. Also, I have data where an area is given and also a polygon 
> but I don’t know for sure if the area was calculated from the polygon. 
> In fact I have data with an area and an Albers area and a geometry and 
> don’t really know what happened to make which.
>
> So, I want to be able to represent area as a spatial property of a 
> feature, independently of any geometry. For this I will try using 
> Observations and Measurements-style mechanics for this.
>
> I think I might use subclassing of a sosa:ObservableProperty 
> (https://www.w3.org/TR/vocab-ssn/#SOSAObservableProperty) to express 
> spatial properties such as area and then relate then to a geo:Feature 
> which would also, by SOSA logic, be a sosa:FeatureOfInterest. I may 
> invent modelling to relate that spatial property to a geometry, but 
> this wouldn’t be required, just nice to have if known.
>

Yes, you would subclass sosa:ObservableProperty. Just as a tiny note, 
and sorry for being overly picky, there is a certain risk with 
properties such as area in conjunction with geometries, namely that the 
are not in sync. In fact, we published a paper that shows that this 
happens in many, if not in most, cases. Of course, you could argue that 
the defined area is the 'correct' observation and the area computed from 
a geometry just some approximation but it is worth keeping this in mind 
as we have no uncertainty model linked to sosa.


Cheers,

Jano


> Cheers,
>
> Nick
>
> *From:*Frans Knibbe <frans.knibbe@geodan.nl>
> *Sent:* Monday, 5 November 2018 6:22 PM
> *To:* public-sdwig@w3.org
> *Subject:* Re: Area of spatial objects
>
> Hi Nick,
>
> I wonder: Doesn't the fact that multiple areas for a single spatial 
> thing are published mean that the areas are calculated from different 
> geometric representations of that spatial thing? That would logically 
> make the area a property of a geometry. Besides, the geometry instance 
> could be used to link to the CRS (e.g. Albers), ideally by URI.
>
> CRS data linked to a geometry could also give access to the basic unit 
> of the CRS (e.g. meter) and through that provide information on the 
> units of properties derived from the geometry (like the area), but 
> that would demand a fair amount of reasoning on the part of the data 
> consumer.
>
> Regards,
>
> Frans
>

-- 
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