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

Hi Frans, all

[ My 2 cents in line below]

> 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 
> <http://data.ign.fr/def/geometrie#centroid>).
The centroid property was used for the IGN-France use case to publish 
administrative units dataset. Each MULTIPOLYGON representing a 
department for instance, was linked to a POINT representing the centroid 
(pre-computed in the legacy dataset) and then converted into RDF.
>
> 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.
+1 for option B).
>
> 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.opengis.net/def/crs/EPSG/0/28992> ;
> geosparql:asWKT "<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.opengis.net/def/crs/EPSG/0/28992> ;
> geosparql:asWKT "<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.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.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).
I really think this depends on the use case or the intended usage of the 
dataset. A publisher can decide to provide complex geometries in WGS84 
and simple geometry (POINT) in the preferred CRS used in that country. 
This was one of the choices when publishing the French Administrative 
Unit dataset at data.ign.fr.
For example, let's look at the data for AIN [1] department in France, 
according to data.ign.fr endpoint.
You'll find the following in Turtle:

## AIN department according to data.ign.fr
<http://data.ign.fr/id/geofla/departement/01> a 
<http://data.ign.fr/def/geofla#Departement> ;
     rdfs:label "AIN"@fr ;
     <http://data.ign.fr/def/geometrie#geometry> 
<http://data.ign.fr/id/geofla/departement/Multipolygon_1> .

## Geometry for multipolygon here using WGS84
<http://data.ign.fr/id/geofla/departement/Multipolygon_1> a 
<http://data.ign.fr/def/geometrie#MultiPolygon> ;
<http://data.ign.fr/def/geometrie#crs> 
<http://data.ign.fr/id/ignf/crs/WGS84GDD> ;
     <http://data.ign.fr/def/geometrie#centroid> 
<http://data.ign.fr/id/geofla/departement/Point_01> ;
     .

## Geometry for the centroid using RGF93
<http://data.ign.fr/id/geofla/departement/Point_01> a 
<http://data.ign.fr/def/geometrie#Point> ;
     <http://www.opengis.net/ont/geosparql#asWKT> 
"<http://data.ign.fr/id/ignf/crs/RGF93LAMB93> POINT(881400 
6558200)"^^<http://www.opengis.net/ont/geosparql#wktLiteral> ;
     <http://data.ign.fr/def/geometrie#crs> 
<http://data.ign.fr/id/ignf/crs/RGF93LAMB93> .



HTH
Best,
Ghislain

[1] http://dbpedia.org/resource/Ain

-- 
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Ghislain A. Atemezing, Ph.D
R&D Engineer
@ Mondeca, Paris, France
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