Re: geosparql:asWkt feedback from NL

Hi, Frans.

My comments inline.

On 27/07/2016 12:47, Frans Knibbe wrote:
> Hi Matt, Josh,
>
> Whether or not it makes sense to have a CRS reference be part of a WKT
> literal is a returning point of debate. It has been discussed at length
> in the Locations and Addresses community group. No clear conclusion was
> found, so I would really welcome further discussion.
>
> I am in favour of removing the CRS reference from the WKT string (or any
> other geometry datatype). Here are some reasons why:
>
>  1. A CRS reference should be a URI, not part of a string literal.
>  2. A geometry is characterised by many attributes (coordinates,
>     geometry type, level of detail, CRS, number of dimensions,..). To
>     conflate all in a single text string would be inflexible.
>  3. In many cases it makes sense to define a CRS at a higher level, for
>     instance for a dataset/graph, or for a class (e.g. class GeomCRS84).

Well, that's a tricky ground - I mean identifying the right level of 
granularity.

Just an example:

In the GeoDCAT-AP WG there was a discussion on whether the CRS should be 
specified at the dataset or distribution level (i.e., you may have 
multiple distributions of the same dataset, each using a different CRS - 
or different sets of CRSs).

Eventually, to be in line with ISO 19115, the CRS was associated with 
the dataset, but without preventing people from specifying CRSs also for 
distributions.

>  4. It seems to me that GML, GeoJSON and KML are more elaborate schemes
>     than WKT, which is a single text string, a data type. So it makes
>     more sense for GML, GeoJSON and KML to include some sort of CRS
>     reference.
>  5. The orginal WKT did not have a CRS part (although CRS can be
>     desribed in WKT, but that is another matter).

True, but, besides GeoSPARQL, we have also other examples of CRS 
included in WKT - as the Extended WKT (EWKT) format used in PostGIS:

http://postgis.net/docs/using_postgis_dbmanagement.html#EWKB_EWKT

Said that, I'm not against having the CRS out of geometry literal. Only, 
I don't think we should say if CRSs should be out or in. We can support 
both the approaches.

We know how to include the CRS in geometry literals. What is missing is 
guidance / BPs on how to specify it outside geometry literals.

> I have to admit it is a not an easy topic. It has to do with the nature
> of geometry: which attributes of a geometry are its intrinsic parts? Can
> a geometry exist as a naked string of coordinates?  I think it can.
>
> I see a parallel with text. Take the word "map". You will probably have
> interpreted the word, and probably the interpretation was wrong. I meant
> the Dutch word "map", which means "folder" in English. I should have
> written "map"^^nl, that would have prevented the misunderstanding...
> This example shows that language is an essential attribute of text
> strings. Omitting it causes errors. But still it works in practice. We
> have the freedom to attach languages to text strings, or to infer the
> language from context. I think it would not hurt to have the same
> freedom with geometric coordinate strings.

I'm not sure, but I guess your point is about Marco's proposal to 
specify the CRS as done with language tags.

If this is the case, I would just like to note that, AFAIK, in RDF a 
literal is either a plain literal (in such a case you can use the lang 
tag) or a typed literal (and then you should specify the datatype).

In GeoSPARQL, :asWKT and :asGML are datatype properties - i.e., take as 
value typed literals. And so is locn:geometry, when used with geometry 
literals.

I'm not aware of examples / proposals to associate both a datatype and a 
language tag with a literal - although I may see the rationale in 
relation to having "localised typed literals".

(Sorry for being pedantic here)

Cheers,

Andrea


> Matt's arguments against are valid, of course. Backward compatibility is
> a problem, but not insolvable. And I can imagine that a solution for
> triple store management can be found too, with some creative thought
> (named graphs for all supported CRSs?). I wonder if modern index
> management in triple stores always assumes that all relevant data is
> contained in a single triple. What if you would want to have an index of
> names of people, or of toponyms? Some form of distinction between types
> of text strings would be needed. What if a recommendation takes the form
> of having CRS-typed predicates, something like (<ex:geom1234>
> <geo:crs84Coordinates> "6 50")?
>
> Greetings,
> Frans
>
>
>
> On 25 July 2016 at 17:03, matthew perry <matthew.perry@oracle.com
> <mailto:matthew.perry@oracle.com>> wrote:
>
>     Hi Josh,
>
>     I would be pretty strongly opposed to removing the encoded CRS
>     reference from wktLiteral.
>
>     If you look at other formats like GML and GeoJSON, those geometry
>     literals include encoded CRS information, and it is implicitly
>     encoded in KML because there is only one possible CRS. WKT is really
>     the only major serialization that lacks CRS information, so to me it
>     seems better to add CRS information to WKT so that it is consistent
>     with the other serializations instead of depending on some other
>     property of the geometry that may or may not exist in a particular
>     dataset.
>
>     From a triplestore implementer's point of view, creating a spatial
>     index for a GeoSPARQL dataset would be an order of magnitude harder
>     if CRS information is not encoded in the geometry literal itself and
>     you have to resort to looking for other triples to determine the
>     CRS, as these triples may be missing and will be updated over time.
>     Plus, such a change for GeoSPARQL 1.1 would not be backwards
>     compatible with GeoSPARQL 1.0, which would cause a lot of headaches.
>
>     Thanks,
>     Matt
>
>
>     On 7/25/2016 10:30 AM, Joshua Lieberman wrote:
>>     Hi,
>>
>>     I am a bit concerned about proliferating versions of geomLiteral
>>     and asWKT properties. It’s a big step already to make a geometry a
>>     first-class object with a global identifier and all the management
>>     issues that raises. Others have also noted that queries get
>>     considerably more complicated if one has to peer into the literals
>>     in order to get the right result. The alternative is to treat the
>>     asWKT and other coordinate properties as the data properties they
>>     are, dependent on the geometry object they are a part of. Then the
>>     coordinate system is defined by the crs property of the geometry.
>>     It may even be best to remove the CRS reference from the
>>     WktLiteral to improve interoperability between that and “regular”
>>     WKT that is returned by database functions.
>>
>>     Another consideration is to make it easier for software systems to
>>     provide the right CRS and translate between CRS’s as needed. One
>>     way might be to negotiate CRS as part of the content format for
>>     the geometry, as we have proposed for encoding format (e.g.
>>     application/ttl; geomLiteral=“WKT”; crs=“CRS84"). This at least
>>     doesn’t proliferate encodings or languages.
>>
>>     It would also, I think, simplify making queries that don’t have to
>>     explicitly select a geometry to test spatial relations or filters
>>     and allow the responding system to select or transform CRS’s as
>>     needed to process the query.
>>
>>     Josh
>>
>>>     On Jul 25, 2016, at 9:34 AM, Brattinga, Marco
>>>     <Marco.Brattinga@ordina.nl <mailto:Marco.Brattinga@ordina.nl>> wrote:
>>>
>>>     Hi Matt,____
>>>     __ __
>>>     Thanks for pointing us to the geof:getSRID, this is indeed what
>>>     we need for that particular requirement.____
>>>     __ __
>>>     As for the encoding of CRS: we propose to encode the CRS into the
>>>     language tag, not the datatype. But you could argue that this
>>>     would proliferate the set of languages…____
>>>     __ __
>>>     Marco____
>>>     __ __
>>>     *Van:* matthew perry [mailto:matthew.perry@oracle.com]
>>>     *Verzonden:* maandag 25 juli 2016 15:26
>>>     *Aan:* Linda van den Brink; public-sdw-comments@w3.org
>>>     <mailto:public-sdw-comments@w3.org>
>>>     *CC:* Brattinga, Marco; Veer, Rein van (Rein.vanVeer@kadaster.nl
>>>     <mailto:Rein.vanVeer@kadaster.nl>); Farla, Joost
>>>     (Joost.Farla@kadaster.nl <mailto:Joost.Farla@kadaster.nl>);
>>>     Maria, Pano (Pano.Maria@kadaster.nl <mailto:Pano.Maria@kadaster.nl>)
>>>     *Onderwerp:* Re: geosparql:asWkt feedback from NL____
>>>     __ __
>>>
>>>     Hi Linda,____
>>>
>>>     Thanks for forwarding the comments.____
>>>
>>>     One of the downsides of encoding the CRS info into the WKT
>>>     literal is that you can't directly process the string with
>>>     existing WKT tools, but it's pretty trivial to read a few bytes
>>>     and strip the CRS URI off. I would be concerned with a
>>>     proliferation of different datatypes if we encoded the CRS into
>>>     the datatype URI. Creating subproperties of ogc:asWKT seems like
>>>     a good, practical approach though.____
>>>
>>>     By the way, GeoSPARQL already defines a function to return the
>>>     CRS of a WKT literal:____
>>>
>>>     *8.7.10 Function: geof:getsrid*____
>>>
>>>     geof:getSRID (geom: ogc:geomLiteral): xsd:anyURI____
>>>
>>>     Returns the spatial reference system URI for geom.____
>>>
>>>     Cheers,
>>>     Matt____
>>>
>>>     __ __
>>>     On 7/25/2016 3:22 AM, Linda van den Brink wrote:____
>>>
>>>         Hi all, ____
>>>          ____
>>>         From the developers at the Dutch Kadaster I got the email
>>>         below, detailing some of the problems they have with CRS
>>>         detection and selection in the current (web) standards. They
>>>         also suggest some interesting solutions. ____
>>>          ____
>>>         (sent to the comments list so they can participate in any
>>>         discussion)____
>>>          ____
>>>         Linda____
>>>          ____
>>>         *Van:* Brattinga, Marco [mailto:Marco.Brattinga@ordina.nl]
>>>         *Verzonden:* zaterdag 23 juli 2016 22:49
>>>         *Aan:* Linda van den Brink; Veer, Rein van
>>>         (Rein.vanVeer@kadaster.nl <mailto:Rein.vanVeer@kadaster.nl>);
>>>         Farla, Joost (Joost.Farla@kadaster.nl
>>>         <mailto:Joost.Farla@kadaster.nl>); Maria, Pano
>>>         (Pano.Maria@kadaster.nl <mailto:Pano.Maria@kadaster.nl>)
>>>         *CC:* Brattinga, Marco (Marco.Brattinga@kadaster.nl
>>>         <mailto:Marco.Brattinga@kadaster.nl>)
>>>         *Onderwerp:* RE: geosparql:asWkt uitdaging icm CRS-en____
>>>          ____
>>>         Linda,____
>>>          ____
>>>         As you know, at the Dutch Land Registry, we are currently
>>>         making all our public data available as Linked Open Data.
>>>         Because most of our data contains a spatial component, we are
>>>         very interested in the work of the spatial on the web
>>>         workgroup.____
>>>          ____
>>>         We would like to raise some questions and have the
>>>         opportunity to share our concerns and experiences.____
>>>          ____
>>>         The situation:____
>>>         -          Our data should not only be available as Linked
>>>         Open Data, but also as JSON-LD and JSON data via REST API’s;____
>>>         -          Currently, we store our spatial information as WKT
>>>         strings;____
>>>         -          Most of the original spatial data is represented
>>>         as RD (the Dutch CRS, EPSG:28992), and some geospatial
>>>         experts would really like to use the data in its original
>>>         CRS;____
>>>         -          But most “regular” webdevelopers would like to use
>>>         the data as CRS84;____
>>>         -          As far as we know, a “regular” WKT string doesn’t
>>>         contain a reference to the CRS, and this should be figured
>>>         out from the context;____
>>>         -          The current geosparql specification specifies that
>>>         the asWKT object is a WKT string, prefixed with a CRS,
>>>         represented with its URI name, or –if absent- CRS84 is
>>>         assumed;____
>>>          ____
>>>         We use the geosparql specification, so a particular resource
>>>         will have a property geosparql:hasGeometry, with a reference
>>>         to a resource of the class geosparql:Geometry, and this
>>>         latter resource has a geosparql:asWKT property, with a WKT
>>>         string as the object.____
>>>          ____
>>>         Our challenges:____
>>>         -          We like the idea of a separate geometry. But we
>>>         would like to include multiple WKT-strings, each with its own
>>>         CRS, just like you would have an rdfs:label with multiple
>>>         languages;____
>>>         -          The current situation means that we would have to
>>>         encode the CRS in the WKT-string and that means that it is
>>>         not really a WKT string any more (which presents problems if
>>>         we want to use it for our REST API, which users don’t
>>>         understand the encoding of the CRS);____
>>>         -          Another problem is, that you’ll get multiple asWKT
>>>         triples, you have to parse the string if you want to select
>>>         just one of the triples. This is not nice (at least we would
>>>         like to have a function available, just like the lang()
>>>         function)____
>>>          ____
>>>         At this moment, we’ve solved the problem by introducing a
>>>         subPropertyOf asWKT, for every CRS:____
>>>          ____
>>>         pdok:asWKT-RD rdfs:subPropertyOf geosparql:asWKT____
>>>          ____
>>>         Every Geometry in our dataset has one geosparql:asWKT with a
>>>         WKT string without a CRS (meaning that it should be CRS84,
>>>         which is fine), and a property pdok:asWKT-RD with the
>>>         semantics that it also shouldn’t contain a CRS, because
>>>         EPSG:28992 is assumed. It works and is compliant to the
>>>         standards, but not very nice.____
>>>          ____
>>>         What we really would like is:____
>>>         -          A more elegant way of encoding the CRS. Maybe you
>>>         could do it just like a language tag, for example: <Geo>
>>>         geosparql:asWKT “POINT(53,2 5,6)”@EPSG:28992;____
>>>         -          A function to check for a particular CRS, similar
>>>         to lang(), for example: crs(?wkt) (which would result a
>>>         literal or maybe a IRI representing the CRS)____
>>>          ____
>>>         Because most spatial encodings can be converted between each
>>>         other, even a better approach might be to have a
>>>         transformation service (toCRS(?wkt,?crs)).____
>>>          ____
>>>         Last, but not least: it would be very much appreciated if a
>>>         user could request for a particular CRS, and the response
>>>         could “tell” what the CRS is. We would like to suggest using
>>>         http-accept-crs and a crs-content-type kind of headers, just
>>>         like a language accept-header or a serialization
>>>         accept-header: having content negotiation available for CRS’s
>>>         as well.____
>>>          ____
>>>         With regards,____
>>>         Marco____
>>>
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>>>
>>>         ____
>>>
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>>>
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>

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Received on Wednesday, 27 July 2016 11:58:18 UTC