- From: Joshua Lieberman <jlieberman@tumblingwalls.com>
- Date: Mon, 9 May 2016 12:05:34 -0400
- To: Bill Roberts <bill@swirrl.com>
- Cc: Frans Knibbe <frans.knibbe@geodan.nl>, SDW WG Public List <public-sdw-wg@w3.org>
- Message-Id: <BB90CA2D-EC6F-4C86-9D3F-FAEEFA61D265@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> 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 <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 <http://www.opengis.net/def/crs/EPSG/0/28992>> ; >> gsp:asWKT "<http://www.opengis.net/def/crs/EPSG/0/28992 <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 <http://www.opengis.net/def/crs/EPSG/0/28992>> ; >> gsp:asWKT "<http://www.opengis.net/def/crs/EPSG/0/28992 <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 <http://www.opengis.net/def/crs/EPSG/0/28992>> ; >> gsp:asWKT "<http://www.opengis.net/def/crs/EPSG/0/28992 <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 > > >

Received on Monday, 9 May 2016 16:08:26 UTC