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

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>:

> 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> 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.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).
>>
>> 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
>> >:
>>
>>> 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
>>>
>>>
>>
>
>