Re: Area of spatial objects

Frans, Chris,

in addition to the coastline paradox, which I guess is what Chris is referring to, there is the general point that any measurement depends on the geometry characteristics of the space in which the measurement is occurring, in two kinds of ways (that I can think of!)


- the underlying reference system - the shortest distance between two points on a sphere does not look like a straight line when drawn on a flat projection of that sphere. And the world not being a sphere, there are a lot of horrible gotchas in the real world related to this, some of which have serious political or legal consequences. But the basic maths point applies in any non-real "world".


- the length of a line, or the 'area' of an area / polygon - in the real world, it depends on how bumpy it is, as well as any general mathematical curvature. Therein lies much legal & financial debate, for example when it comes to farm payments.


I do support the idea of a general ontology - but it would have to be clearly & explicitly stated what its assumptions are, e.g. for use only in planar spaces, or spherical ones, or whatever.


Note, this is also independent of the semantics of the relationship between the feature/spatial thing & the geometry, as raised by Simon. A "footprint" semantic relationship between a "property" (house, land parcel, flat,....) and a polygon geometry will have a specific semantic - is it the intersection of the property with the actual surface of the earth, or with some "imagined" surface (flattened at some datum). The measured/estimated/calculated area of that polygon is then effected by the characteristics of the spatial system in which it is calculated.


Of course, there are many simple / well defined / closed / well used systems in which this doesn't matter. Or to be more precise, the degree of error caused by ignoring these issues is not relevant to the question being addressed (use case).


But that can't be assumed in any general ontology - the assumptions have to be explicit.


Peter

(OS Principal Data Architect & Open Standards Lead)

________________________________
From: Frans Knibbe <frans.knibbe@geodan.nl>
Sent: 16 November 2018 15:06:00
To: Chris Little
Cc: public-sdwig@w3.org
Subject: Re: Area of spatial objects

Hello Chris,

You are referring to the coastline paradox<https://en.wikipedia.org/wiki/Coastline_paradox>, right? That problem only occurs for real world phenomena, I think a general spatial ontology should not be limited to those.
But I wonder: wouldn't a specification of spatial accuracy (either explicitly or by using significant digits) be sufficient to express the idea of having different sized rulers?

Greetings,
Frans

Op do 15 nov. 2018 om 13:40 schreef Little, Chris <chris.little@metoffice.gov.uk<mailto:chris.little@metoffice.gov.uk>>:

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<mailto:rob@metalinkage.com.au>>
Sent: 14 November 2018 21:11
To: Krzysztof Janowicz <janowicz@ucsb.edu<mailto:janowicz@ucsb.edu>>
Cc: Car Nicholas <Nicholas.Car@csiro.au>; Frans Knibbe <frans.knibbe@geodan.nl<mailto:frans.knibbe@geodan.nl>>; public-sdwig@w3.org<mailto: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>

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