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Re: Good practice for publishing geometry of a thing as different geometry types?

From: Andrea Perego <andrea.perego@jrc.ec.europa.eu>
Date: Wed, 18 May 2016 12:28:31 +0200
To: Linda van den Brink <l.vandenbrink@geonovum.nl>
Cc: Rob Atkinson <rob@metalinkage.com.au>, Joshua Lieberman <jlieberman@tumblingwalls.com>, Frans Knibbe <frans.knibbe@geodan.nl>, SDW WG Public List <public-sdw-wg@w3.org>, Andreas Harth <harth@kit.edu>
Message-id: <1306f6b6-a61e-59f4-ab44-8d7e5ef8ff8a@jrc.ec.europa.eu>
Hi, Linda.

Please see my comments inline.

On 18/05/2016 9:50, Linda van den Brink wrote:
> Hi all,
>
> Interesting thread this – we should indeed somehow turn this into
> content for the BP.
>
> I think the consensus is at least that
> -          The geometry of a thing is a model of its real world shape.
> -          Geometries can be expressed in different ways.
>
> And perhaps
> -          Use a separate geometry for each way you want to express it
> (although I don’t know if you all agree)
>
> We should describe the different levels of choice, i.e. as Josh described:
>
> Serialization of the geometry, i.e. WKT or GML (or …)

Just to note again the approach used in GeoDCAT-AP for this:

https://lists.w3.org/Archives/Public/public-sdw-wg/2015Jun/0167.html

Basically, this is specified by using multiple instances of 
locn:geometry, with different literals:

dct:Location locn:geometry
   "POLYGON((-180 90,180 90,180 -90,-180 -90,-180 90))"^^gsp:wktLiteral ,
   "<gml:Envelope 
srsName=\"http://www.opengis.net/def/crs/OGC/1.3/CRS84\"><gml:lowerCorner>-180 
-90</gml:lowerCorner><gml:upperCorner>180 
90</gml:upperCorner></gml:Envelope>"^^gsp:gmlLiteral ,
 
"{\"type\":\"Polygon\",\"crs\":{\"type\":\"name\",\"properties\":{\"name\":\"urn:ogc:def:crs:OGC:1.3:CRS84\"}},\"coordinates\":[[[-180,90],[180,90],[180,-90],[-180,-90],[-180,90]]]}"^^ns0:json 
.

Of course, this doesn't solve the issue of whether these are 
representations of the same geometry or of different geometries, but the 
GeoDCAT-AP WG saw this as a practical solution to provide users 
alternative geometry encodings.

> CRS (I like Rob’s comparison with language choice)
>
> Realization on different scales/generalization levels
>
> Kind of model (boundary, centroid, …)
>
> And possible approaches for each of these. To which extend can content
> negotiation be used?

There's existing work we can refer to for this, especially in relation 
to the use of HTTP URIs for geometries. E.g.:


1. Ian Davis's work (dating back to 2003):

http://vocab.org/placetime/2003/05/geopoint-wgs84-20030516

The service is not maintained, as far as I can see, but a (no longer 
working) example is:

http://placetime.com/geopoint/wgs84/X-126.817Y46.183


2. Ordnance Survey - e.g.:

http://data.ordnancesurvey.co.uk/id/geometry/37256-10

Geometries can be obtained in HTML, RDF/XML, Turtle, JSON. But I'm not 
sure whether HTTP conneg is supported here. In case, John Goodwin should 
be able to provide more details.


3. The work done in the framework of the GADM project, mentioned by 
Andreas (cc'ed) in Amersfoort:

http://gadm.geovocab.org/


4. I have a conflict of interests here, but a few years ago I've also 
set up an experimental API for geometry URIs, able to serve geometries, 
in arbitrary CRSs, in different encodings / representations via conneg. 
Discussing with Andreas, the approach seems to be in line with the one 
used in GADM.

There's not yet an online demo (but should be available soon). In any 
case, source code and documentation is on GH:

https://github.com/andrea-perego/geoiri/wiki

The supported formats are HTML, RDF (different serialisations), WKT, 
GML, KML, GeoJSON.

An example of the RDF/XML output is here:

https://github.com/andrea-perego/geoiri/blob/master/examples/point(0_0).rdf

As you will be able to see, the different encodings / representations 
are modelled by linking them from the geometry with dct:hasFormat, 
whereas alternative RDF geometry representations are related with 
owl:sameAs:

<http://some.site/geoiri/id/geometry/4326/point(0_0)>
   foaf:primaryTopicOf 
<http://some.site/geoiri/doc/geometry/4326/point(0_0)> ;
   rdfs:label "Geometry (WKT): POINT(0 0) - EPSG:4326 (WGS 84)"@en ;
   owl:sameAs 
<http://some.site/geoiri/id/geometry/4326/point(0_0)#geosparql> ;
   owl:sameAs <http://some.site/geoiri/id/geometry/4326/point(0_0)#wgs84> ;
   owl:sameAs 
<http://some.site/geoiri/id/geometry/4326/point(0_0)#schema.org> ;
   owl:sameAs <http://placetime.com/geopoint/wgs84/X0Y0> ;
   owl:sameAs <http://geohash.org/s0000000000000000000> ;
   owl:sameAs <geo:0,0;u=0;crs=wgs84> .


Cheers,

Andrea


>
>
>
> Linda
>
>
>
> *Van:*Rob Atkinson [mailto:rob@metalinkage.com.au]
> *Verzonden:* vrijdag 13 mei 2016 02:44
> *Aan:* Joshua Lieberman; Frans Knibbe
> *CC:* SDW WG Public List
> *Onderwerp:* Re: Good practice for publishing geometry of a thing as
> different geometry types?
>
>
>
> Lots of overlapping concerns here - but one unified way of looking at
> this is to look at the OpenWorld approach - if we assume an object can
> have properties we dont know about (yet) then we have a basic model for
> an object
>
> - object has an identifier and is a member of a set (a registration
> viewpoint)
>
> We can then provide an addition view of that object, with a "named
> collection of properties" using the Linked Data API semantics - but
> basically what we do every time we define a FeatureType. so for
> "spatial" features we have
>
> - object isa FeatureType has p1,p2,p3 etc
>
>
>
> This view can then be accessed in multiple serialisations (Content-type
> negotiation) and potentially also language negotiation.
>
>
>
> So in principle one identifier maps to multiple resources. An we accept
> that if we work on the web.
>
>
>
> but there is no reason to limit the number of views to 1 - and if we
> accept the Web architecture that would be an inconsistent thing to do
> anyway.
>
>
>
> So - you can have
>
>
>
> FeatureTypes with multiple geometry properties
>
>
>
> AND/OR
>
>
>
> Multiple FeatureTypes (e.g. simple features) bound to different geometry
> models
>
>
>
> AND/OR
>
>
>
> content negotiation to access different encodings of a feature ( perhaps
> CRS choice is analagous to language choice for text?)
>
>
>
> but you do probably end up with an architectural requirement to
> advertise what views are available... :-)
>
>
>
> In summary, I recommend publishing multiple encodings using different
> information models, but consistent under an OpenWorld interpretation -
> i.e. you should not have different models using the same property
> predicate with different values unless it is truly a multi-valued
> property and these are separate valid values.  My suspicion is that
> owl:sameAs ought to apply across instances using different views
> (multiple "information resources" c.f. http-range-14) - as in you can
> safely mix the assertions about the object from multiple views. This
> last point needs some serious thinking, but I believe the fundamental
> pattern of supporting multiple views is an inevitable consequence of the
> Web architecture and the potential for multiple clients.
>
>
>
> Cheers
>
> Rob Atkinson
>
>
>
>
>
>
>
>
>
>
>
>
>
>
>
>
>
> On Wed, 11 May 2016 at 00:44 Joshua Lieberman
> <jlieberman@tumblingwalls.com <mailto:jlieberman@tumblingwalls.com>> wrote:
>
>     Franz,
>
>
>
>     There are certainly many shades of difference that can distinguish
>     how models are constructed and expressed. Serialization may be the
>     equivalent (in a literal) of the difference between RDF/XML and
>     Turtle. It’s complicated for CRS since WKT requires any CRS
>     different from EPSG4326 to be explicitly included in the text
>     string, although in principle it’s the same geometry. The next step
>     up is probably realization of a geometry for different scales /
>     levels of generalization. These are all the same conceptual geometry
>     but with a different realization in coordinate positions. If we go
>     to the difference between a feature boundary and a feature centroid,
>     that’s more clearly a different geometric model of the feature. It’s
>     less clear that there is a difference in geometry type between a
>     road centerline and a road marginal line, although it’s clearly a
>     different property of the feature.
>
>
>
>     From a practical viewpoint of geometries living independently on the
>     Web that can be both discovered and linked to features, it makes
>     sense for these properties (CRS, geometric object type, generality)
>     to be top level functional geometry properties, and there should be
>     a set of feature geometry properties (envelope, centroid, boundary,
>     hull, buffer are in 19107) so that the appropriate property can be
>     chosen for a given application and people don’t all end up in Kansas
>     looking for something in the USA because the centroid property is
>     not identified as such.
>
>
>
>     Whether alternate serializations should be allowed in one geometry
>     object is less clear. It is ambiguous in 19107, not allowed in GML,
>     generally not in Simple Features, and not forbidden in GeoSPARQL. My
>     sense is that serializations themselves are easily transformable and
>     multiple coordinate strings are too useful for composite geometries
>     and other purposes to risk confusion. GeoJSON is an issue here
>     because it requires one serialization and one CRS to be present
>     whether or not another one is desirable. There doesn’t seem to be an
>     prohibition against multiple geometry elements per feature, though,
>     so it makes sense to put different serializations into different
>     GeoJSON geometry objects as an extension to support JSON-LD / RDF
>     (not recognized by the GeoJSON specification, of course).
>
>
>
>     Josh
>
>
>
>
>
>         On May 10, 2016, at 6:09 AM, Frans Knibbe
>         <frans.knibbe@geodan.nl <mailto:frans.knibbe@geodan.nl>> wrote:
>
>
>
>         Hello Josh, all,
>
>
>
>         Sorry for misunderstanding what you meant. But it is good to
>         share thoughts about when and how to split geometry data in
>         different objects. Probably things like this deserve a place in
>         the BP document.
>
>
>
>         You mentioned different serializations. That makes me wonder
>         what a serialization is. If I change the CRS or the type of a
>         geometry, I get a new geometry. That more or less follows from
>         the definition that a geometry is a model of a shape. If I use a
>         different CRS or different geometry type, I use a different
>         model. In other words, if the actual sequence of numbers in the
>         coordinates changes, we have a new geometry. So is that sequence
>         of numbers the serialization? I could use the same numbers in a
>         WKT string or in a GML object. Isn't that also serialization?
>         Probably we need two terms, one for different sequences of
>         coordinates, and one for packaging the same sequence of
>         coordinates in different ways. In notice that the GeoSPARQL
>         standard talks of WKT and GML as different serializations.
>
>
>
>         To me it would make sense to let the same coordinate sequence in
>         different data types be part of the same geometry - they are
>         different expressions of the same model. For example:
>
>
>
>         ex:geom6789
>
>           a geom:Geometry, geom:Point ;
>
>           geom:crs <http://www.opengis.net/def/crs/EPSG/0/28992> ;
>
>           geosparql:asWKT "<http://www.opengis.net/def/crs/EPSG/0/28992>
>         POINT(131216.968 461378.333)"^^geosparql:wktLiteral ;
>
>           geosparql:asGML "<gml:Point
>         srsName="EPSG:28992"><gml:coordinates>131216.968,461378.333</gml:coordinates></gml:Point>"^^geosparql:gmlLiteral
>         .
>
>
>
>         Regards,
>
>         Frans
>
>
>
>
>
>
>
>
>
>         2016-05-09 18:05 GMT+02:00 Joshua Lieberman
>         <jlieberman@tumblingwalls.com
>         <mailto:jlieberman@tumblingwalls.com>>:
>
>         Yes, of course a geometry is a model. For that matter, a feature
>         is a model, but one that directly references the “real world".
>         The question was really whether multiple serializations can be
>         included in a geometry, e.g. for different scales or CRS’s. It
>         makes sense to me, but upon investigation, it is just not
>         something that is practiced, although there is no present axiom
>         in GeoSPARQL that prevents it. It doesn’t seem to be an explicit
>         part of either 19107 or GML, and the general semantic is that
>         multiple positions represent a composite serialization of a
>         geometry, not alternate ones.
>
>
>
>         So we should probably stick with each unique combination of role
>         (centroid, etc. are actually defined terms in 19107), scale,
>         CRS, interpolation method, etc. as a distinct geometry.
>
>
>
>         Backlinks from geometry to feature are problematic. If
>         geometries are to be stored separately, it’s probably to share them.
>
>
>
>         Josh
>
>
>
>
>
>             On May 9, 2016, at 9:50 AM, Bill Roberts <bill@swirrl.com
>             <mailto:bill@swirrl.com>> wrote:
>
>
>
>                 But I suspect at the heart of your comments is the
>                 question what a geometry really is. There are at least
>                 two possibledefinitions:
>                 A) The geometry of a thing is its real world shape.
>                 B) The geometry of a thing is a model of its real world
>                 shape.
>
>
>
>             I agree: in practice, (B) is always the case.  No
>             representation of geometry will be completely accurate, and
>             different levels of approximation (different models) are
>             appropriate in different contexts.
>
>
>
>
>
>
>
>             On 9 May 2016 at 15:35, Frans Knibbe <frans.knibbe@geodan.nl
>             <mailto:frans.knibbe@geodan.nl>> wrote:
>
>             Hello Josh,
>
>
>
>             It could be possible to add more context to the geometries,
>             to express that they are a footprint or a centroid for
>             instance. But I think that extra context will not be crucial
>             for many use cases. Especially since there is no standard
>             vocabulary for that extra meaning yet  (although the
>             vocabulary I try to use does have a centroid
>             property: http://data.ign.fr/def/geometrie#centroid).
>
>
>
>             But I suspect at the heart of your comments is the question
>             what a geometry really is. There are at least two possible
>             definitions:
>
>             A) The geometry of a thing is its real world shape.
>
>             B) The geometry of a thing is a model of its real world shape.
>
>
>
>             I think I silently use definition B. But if others assume
>             definition A that could lead to problems. I am ashamed to
>             have to admit that I don't know the official OGC party line
>             in this case. But it would be great if an updated GeoSPARQL
>             standard could have a direct link to a core definition of
>             geometry.
>
>
>
>             As for your last example (two coordinate strings that differ
>             in their CRS) in my line of thinking (adherent of definition
>             B) that would be modelled as separate geometries. An
>             extended example:
>
>
>
>             ex:location1234
>
>                a dcterms:Location ;
>
>                locn:geometry ex:geom1234_1, ex:geom1234_2,
>             ex:geom1234_3, ex_geom1234_4 ;
>
>
>
>             ex:geom1234_1
>
>                a geom:Geometry, locn:Geometry, geom:Point ;
>
>                locn:location ex:location123 ;
>
>                geom:crs <http://www
>             <http://www/>.opengis.net/def/crs/EPSG/0/28992> ;
>
>                geosparql:asWKT "<http://www
>             <http://www/>.opengis.net/def/crs/EPSG/0/28992>
>             POINT(...)"^^geosparql:wktLiteral .
>
>
>
>             ex:geom1234_2
>
>                a geom:Geometry, locn:Geometry, geom:Polygon ;
>
>                locn:location ex:location123 ;
>
>                geom:crs <http://www
>             <http://www/>.opengis.net/def/crs/EPSG/0/28992> ;
>
>                geosparql:asWKT "<http://www
>             <http://www/>.opengis.net/def/crs/EPSG/0/28992>
>             POLYGON(...)"^^geosparql:wktLiteral .
>
>
>
>             ex:geom1234_3
>
>                a geom:Geometry, locn:Geometry, geom:Point ;
>
>                locn:location ex:location123 ;
>
>                geom:crs <http://www
>             <http://www/>.opengis.net/def/crs/OGC/1.3/CRS84> ;
>
>                geosparql:asWKT "POINT(...)"^^geosparql:wktLiteral .
>
>
>
>             ex:geom1234_4
>
>                a geom:Geometry, locn:Geometry, geom:Polygon ;
>
>                locn:location ex:location123 ;
>
>                geom:crs <http://www
>             <http://www/>.opengis.net/def/crs/OGC/1.3/CRS84> ;
>
>                geosparql:asWKT "POLYGON(...)"^^geosparql:wktLiteral .
>
>
>
>
>
>             Note that I also included a backlink from geometry to
>             location (locn:location).
>
>
>
>             The question still is: can this be considered a good
>             practice, given currently available standards/vocabularies?
>
>
>
>             Regards,
>
>             Frans
>
>
>
>
>
>
>
>
>
>
>
>
>
>             2016-05-04 19:10 GMT+02:00 Joshua Lieberman
>             <jlieberman@tumblingwalls.com
>             <mailto:jlieberman@tumblingwalls.com>>:
>
>             Do you mean:
>
>
>
>                 ex:location1234
>
>                    a dcterms:Location, ex:feature ;
>
>                    ex:centroid ex:geom1234 ;
>
>                    ex:footprint ex:geom6789 .
>
>
>
>                 ex:geom1234
>
>                    a geom:Geometry, gsp:Point ;
>
>                    geom:crs <http://www.opengis.net/def/crs/EPSG/0/28992> ;
>
>                    gsp:asWKT
>                 "<http://www.opengis.net/def/crs/EPSG/0/28992>
>                 POINT(...)"^^geosparql:wktLiteral .
>
>
>
>                 ex:geom6789
>
>                    a geom:Geometry, gsp:Polygon ;
>
>                    geom:crs <http://www.opengis.net/def/crs/EPSG/0/28992> ;
>
>                    gsp:asWKT
>                 "<http://www.opengis.net/def/crs/EPSG/0/28992>
>                 POLYGON(...)"^^geosparql:wktLiteral .
>
>
>
>             In that case, the range of gsp:asWKT is not a geometry, but
>             a set of coordinate positions locating the geometry, so
>             “POLYGON” is the format of the coordinate string, not the
>             geometry class per se.
>
>
>
>
>
>             The coordinate information is more problematic, since one
>             could easily want to have
>
>
>
>                 ex:geom6789
>
>                    a geom:Geometry, gsp:Polygon ;
>
>                    geom:crs <http://www.opengis.net/def/crs/EPSG/0/28992> ;
>
>                    gsp:asWKT
>                 "<http://www.opengis.net/def/crs/EPSG/0/28992>
>                 POLYGON(...)"^^geosparql:wktLiteral .
>
>                    gsp:asWKT "POLYGON(...)"^^geosparql:wktLiteral .
>
>                 gap:asGML “…”
>
>
>
>             I consider asWKT to be problematic for this reason, and one
>             ground for updating the GeoSPARQL standard.
>
>
>
>
>
>              Josh
>
>
>
>
>
>
>
>
>
>
>
>
>

-- 
Andrea Perego, Ph.D.
Scientific / Technical Project Officer
European Commission DG JRC
Institute for Environment & Sustainability
Unit H06 - Digital Earth & Reference Data
Via E. Fermi, 2749 - TP 262
21027 Ispra VA, Italy

https://ec.europa.eu/jrc/
Received on Wednesday, 18 May 2016 10:29:15 UTC

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