[wfs-dev] null values

[Andreas v.Dömming]

Hi Simon,

Where are OGC specific URNs defined ? Can you point me to some documentation ?
Thanks in advance,

Regards,
 Andreas

Am Montag, 30. Oktober 2006 13:02 schrieb Simon.Cox at csiro.au:
> Another relative clean option for all GML properties that allow the
> "by-ref" option (xlinks) is to set the value of
> xlink:href="urn:x-ogc:def:nil:OGC:unknown" etc.
>
> The OGC URN schema "def" branch supports all the GML nulls.
> This is *not* the "XML Schema" way (that would use option 3 and as
> Clemens points out is specified in GMl 3.2).
> But it is perhaps more general and less messy in the schema - it applies
> to all props *except* the simpleContent props.
> (which I realise are the most common in App Schemas in the wild).
>
> Simon
>
>
> -----Original Message-----
> From: wfs-dev-bounces+simon.cox=csiro.au at opengeospatial.org
> [mailto:wfs-dev-bounces+simon.cox=csiro.au at opengeospatial.org] On Behalf
> Of Clemens Portele
> Sent: Monday, 30 October 2006 6:28 PM
> To: 'Bart van den Eijnden (OSGIS)'; wfs-dev at opengeospatial.org
> Subject: Re: [wfs-dev] null values
>
> Hi Bart,
>
> As you point out, 1. is an empty string (assuming that the type of
> PRIORITY is string) which is not a NULL. Besides this approach does not
> work at all for numbers or most other data types.
>
> 2. is the representation that has been used in most GML application
> schemas that I have seen and that is the result of the default mapping
> from UML multiplicities for attributes and relationship roles as
> specified in ISO
> 19118 as well as GML.
>
> The Simple Features profile of GML has a discussion about Null values
> and specifies that option 2 is to be used in this profile.
>
> 3. is in general also a possibilty. In ISO/DIS 19136 (GML 3.2.0)
> nillable is explicitly mentioned: "Declaring an element to be nil is an
> implementation of the "Void" datatype of ISO/IEC 11404, i.e. represents
> 'an object whose presence is syntactically or semantically required, but
> carries no information in a given instance' [ISO/IEC 11404]." nillable
> is useful in particular in conjunction with a nilReason attribute (which
> allows to attach reason metadata to a void - nil elements may carry
> attributes but no other
> content) for those application schemas that want to make use of such a
> construct. However, there are issues with the use of nillable. That it
> cannot be used with "ref" has been mentioned by John, i.e. in most cases
> nillable is appropriate only in properties in application schemas that
> are represented by local elements (which is the standard mapping for
> properties from ISO 19109 application schemas in UML). Another is that
> this construct is not an XML mechanism per se, but XML Schema specific.
>
>
> Clemens
>
> > -----Original Message-----
> > From:
> > wfs-dev-bounces+portele=interactive-instruments.de at opengeospatial.org
> > [mailto:wfs-dev-bounces+portele=interactive-instruments.de at ope
> > ngeospatia
> > l.org]On Behalf Of Bart van den Eijnden (OSGIS)
> > Sent: Friday, October 27, 2006 3:06 PM
> > To: wfs-dev at opengeospatial.org
> > Subject: [wfs-dev] null values
> >
> >
> > Hi list,
> >
> > how is a WFS supposed to deal with null values?
> >
> > I can see 3 options (for instance an attribute named PRIORITY):
> >
> >
> > 1) just output an empty element (I guess not the cleanest approach
> > since it cannot be distinguished from an empty string)
> >
> > <MYFIRSTITEM>aaa</MYFIRSTITEM>
> > <PRIORITY></PRIORITY>
> > <MYTHIRDITEM>zzz</MYTHIRDITEM>
> >
> > I think this is how Mapserver WFS operates.
> >
> >
> > 2)
> > Leave out the respective element in the reponse.
> >
> > <MYFIRSTITEM>aaa</MYFIRSTITEM>
> > <MYTHIRDITEM>zzz</MYTHIRDITEM>
> >
> > This is what Ionic WFS does.
> >
> > 3)
> > An option I have not yet seen, output xsi:nil="true" for the
> > respective element?
> >
> > <MYFIRSTITEM>aaa</MYFIRSTITEM>
> > <PRIORITY xsi:nil="true"></PRIORITY>
> > <MYTHIRDITEM>zzz</MYTHIRDITEM>
> >
> > Ofcourse the 1) and 3) behaviour is easier to process (for instance
> > with
> > XSLT) than the second one.
> >
> > Should the WFS spec say something about this? Or is it up to the
> > vendor? It would make life easier if they all did the same thing IMHO.
> > And I guess 3)
> > is the cleanest approach.
> >
> > Thanks in advance for any input on this.
> >
> > Best regards,
> > Bart
> >
> > --
> > Bart van den Eijnden
> > OSGIS, Open Source GIS
> > http://www.osgis.nl
> >
> >
> >
> >
> >
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>
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