RE: Area of spatial objects

Rob, Frans

And I think we should extend the question to: “What is the length of the boundary of an area/geometry/polygon etc?”

First answered convincingly by Lewis Fry Richardson: it depends on the length of your ruler. This is an intrinsically different answer to the (2D) areal problem.

I agree they could be good contributions to Frans’ spatial ontology.

Chris

From: Rob Atkinson <rob@metalinkage.com.au>
Sent: 14 November 2018 21:11
To: Krzysztof Janowicz <janowicz@ucsb.edu>
Cc: Car Nicholas <Nicholas.Car@csiro.au>; Frans Knibbe <frans.knibbe@geodan.nl>; public-sdwig@w3.org
Subject: Re: Area of spatial objects

IMHO this is a candidate for discussion in the context of a general spatial ontology.

There is an unhelpful proliferation of solutions to the semantics of the relationships between objects/features and geometries in the Linked Data world.

For example it common practice to have a point representing an address, but that may semantically actually represent many possible things:
* centroid of dwelling
* centroid of land parcel
* centre of road frontage of land parcel/property
* actual entry point to property
* entry point to dwelling
* something unspecified

or for a city:  location of main post office, centroid of designated area, centroid of observed area, etc. etc.  Same goes for an area - what is the area of a city? many possible answers depending on semantic context.

and each geometry has a range of metadata - precision, accuracy, area

is area a property of a specific geometry, or a spatial attribute (again with semantic qualification) of a thing.

On Tue, 13 Nov 2018 at 14:16, Krzysztof Janowicz <janowicz@ucsb.edu<mailto:janowicz@ucsb.edu>> wrote:
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><mailto:frans.knibbe@geodan.nl>
Sent: Monday, 5 November 2018 6:22 PM
To: public-sdwig@w3.org<mailto: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<mailto:jano@geog.ucsb.edu>

Webpage: http://geog.ucsb.edu/~jano/


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