Re: CRS specification

Dear Kostis,

>     True, if you only have the literal, e.g. "POLYGON((97372
>     487152,97372 580407,149636 580407,149636 487152,97372 487152))", you
>     can not get the CRS. But is that a problem? If you have the triple
>     you can get the CRS (provided one is specified).
>
> You cannot rely on two triples to interpret a single RDF term. See my
> comment on the formal definition of a datatype. On the other hand, since
> we adopt the open world assumption we certainly cannot assume that only
> a single pair of coordinates and CRS will exist in a knowledge base.

This really comes back to the class vs datatype representation issue 
that has already been pointed out by Frans.

In your view, if I understand well, you want the geometry value to be of 
a (complex) datatype, therefore, putting the CRS information *and* the 
coordinates into a literal that needs to be (correctly) parsed to be 
used. This has some limitations as we already shown in other threads, 
e.g. 1/ use of regular expression for some SPARQL queries, 2/ hard-coded 
interpretation of the value if no CRS is provided, i.e. your software 
relies on a default CRS, but this default is not made explicit in the 
vocabulary definition, etc.

In Frans view (and also mine), a geometry can be a class, that has 
explicit attributes such as a list of coordinates and a CRS. The CRS and 
the set of coordinates are still tied together in the definition of this 
class. Do you see the difference?

> Well, I do not agree with you in this. We will definitely have multiple
> CRS and there is a good reason for this. Each CRS consider a different
> approximation of the earth. For example, WGS84 consider the earth to be
> spheroidal, which is perfect in some cases and absolute disaster for
> others. It may sound a bit funny at first, but there is also the need in
> some cases to model extraterrestrial coordinates :)

+1 for this! We definitively need multiple CRS depending on the use 
case. WGS84 is fine for many use cases, but a disaster when you need 
precision.
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/

Received on Tuesday, 7 January 2014 11:31:11 UTC