Re: CRS specification (was: Re: ISA Core Location Vocabulary)

Hi Andrea,

I would propose to support only http URIs. The use of literals like EPSG:4326 or URNs like urn:ogc:def:EPSG::4326 has been deprecated by OGC. There is no need to support legacy in the core location vocabulary. (As an aside, the notation EPSG:nnnnn was also meant to be a URI - after registering the URI scheme "EPSG" with IANA, which never happened.)

In addition, and that is my main concern, I am not sure I understand the proposed property. The CRS information is intrinsic to the geometry representation, which is kept open in LOCN, so how exactly would this property be defined or be useful? Of course, every geometry representation should be explicit about its CRS - or specify the standard CRS, if no CRS is or can be specified explicitly in the geometry representation. This is already the case in most of the sample encodings mentioned at http://www.w3.org/ns/locn#locn:geometry, e.g.:

- WKT (GeoSPARQL): CRS info optional, a URI, default is http://www.opengis.net/def/crs/OGC/1.3/CRS84 
- GML: CRS info mandatory, a URI
- RDF+WKT (GeoSPARQL): CRS info optional, a URI, default is http://www.opengis.net/def/crs/OGC/1.3/CRS84 
- RDF+GML (GeoSPARQL): CRS info mandatory, a URI
- KML: fixed to 2D or 3D geodetic WGS84 with long/lat(/alt) axis order, decimal degrees for lat and long, metres for alt
- geo URI: optional, but discouraged (values not specified), default is 2D or 3D geodetic WGS84 with lat/long(/alt) axis order, decimal degrees for lat and long, metres for alt

So each of these geometry representations already includes the information about the CRS. It is not clear to me what the value of an additional, potentially conflicting CRS property would be.

In addition, the CRS is strictly not a property of a geometry, but of a particular representation / serialisation.

In the GeoSPARQL Standards Working Group in OGC we had also discussed having a CRS property. Actually earlier drafts had a separate geo:srid property, but this has been removed in the final version for exactly these reasons (plus this approach results in simpler SPARQL queries).

Best regards,
Clemens


On 30 Dec 2013, at 00:13, Andrea Perego <andrea.perego@jrc.ec.europa.eu> wrote:

> Dears,
> 
> Based on the discussion on the original thread (see mails [1-5]), I would kindly ask your feedback on the following proposal:
> 
> 
> *Proposed resolution*
> 
> Add to the LOCN voc an optional property to specify the CRS of a geometry. The range of such property can be either a literal (e.g., EPSG:4326), a URN (e.g., urn:ogc:def:EPSG::4326) or an HTTP URI reference (e.g., http://www.opengis.net/def/crs/EPSG/0/4326).
> 
> 
> Intentionally, I've not included the following two points, since the position of the group is still unclear on them:
> 
> 1. Should the use of HTTP URIs be recommended? BTW, on this, Krzysztof posted a question that has not been yet answered [1].
> 
> 2. Should we recommend a specific CRS URI register?
> 
> I kindly ask you to use this thread to post any relevant comments.
> 
> Thanks!
> 
> Andrea
> 
> ----
> [1]http://lists.w3.org/Archives/Public/public-locadd/2013Dec/0022.html
> [2]http://lists.w3.org/Archives/Public/public-locadd/2013Dec/0027.html
> [3]http://lists.w3.org/Archives/Public/public-locadd/2013Dec/0028.html
> [4]http://lists.w3.org/Archives/Public/public-locadd/2013Dec/0029.html
> [5]http://lists.w3.org/Archives/Public/public-locadd/2013Dec/0030.html
> 
> 
> 
> On Sun, Dec 22, 2013 at 11:13 PM, Raphaël Troncy <raphael.troncy@eurecom.fr> wrote:
> @Raphaël, could you please provide some more details about the CRS
> conversion service you are about to publish?
> 
> This is a REST service that implements a number of algorithms for doing projections between CRS, similar to what the Circé software is doing, but implemented as a web-based service, see http://fr.wikipedia.org/wiki/Circ%C3%A9_%28logiciel%29 (unfortunately, only in French).
> More details in Jan/Feb when we actually publish the service.
> Best regards.
> 
> 
>   Raphaël
> 
> -- 
> Raphaël Troncy
> EURECOM, Campus SophiaTech
> Multimedia Communications Department
> 450 route des Chappes, 06410 Biot, France.
> e-mail: raphael.troncy@eurecom.fr & raphael.troncy@gmail.com
> Tel: +33 (0)4 - 9300 8242
> Fax: +33 (0)4 - 9000 8200
> Web: http://www.eurecom.fr/~troncy/
> 
> 
> 
> 
> -- 
> Andrea Perego, Ph.D.
> European Commission DG JRC
> Institute for Environment & Sustainability
> Unit H06 - Digital Earth & Reference Data
> Via E. Fermi, 2749 - TP 262
> 21027 Ispra VA, Italy
> 
> DE+RD Unit: http://ies.jrc.ec.europa.eu/DE
> 
> ----
> The views expressed are purely those of the writer and may
> not in any circumstances be regarded as stating an official
> position of the European Commission.