Re: geosparql:asWkt feedback from NL

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).
   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).

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.

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> 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>
> 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
> <matthew.perry@oracle.com>]
> *Verzonden:* maandag 25 juli 2016 15:26
> *Aan:* Linda van den Brink; public-sdw-comments@w3.org
> *CC:* Brattinga, Marco; Veer, Rein van (Rein.vanVeer@kadaster.nl); Farla,
> Joost (Joost.Farla@kadaster.nl); Maria, Pano (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
> <Marco.Brattinga@ordina.nl>]
> *Verzonden:* zaterdag 23 juli 2016 22:49
> *Aan:* Linda van den Brink; Veer, Rein van (Rein.vanVeer@kadaster.nl);
> Farla, Joost (Joost.Farla@kadaster.nl); Maria, Pano (
> Pano.Maria@kadaster.nl)
> *CC:* Brattinga, Marco (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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