[georss] elevation and Simple<->GeoRSS GML interop questions

Tue May 6 21:43:28 EDT 2008

On May 6, 2008, at 7:09 PM, Chris Beaudette wrote:

>
> Thanks Josh!
>
> Pardon my XML (and CRS) ignorance, but from your responses it seems  
> like the answers to my questions are:
>
> 1.        If one wants to define elevation, they should use the  
> ‘dimension’ attribute w/ a 3D CRS.  If you can avoid it, don’t use  
> <georss:elev> with <gml:…> directives.
> - (Potentially) silly question:  how does one know if a CRS is 3D?   
> For example, is UTM a 3D CRS?

It is generally necessary to look at the definition of the CRS. One  
can look at the EPSG code definitions here, for example: http://www.epsg-registry.org/ 
  and find that the 3D equivalent of EPSG:4326 is EPSG: 4979. There  
only appear to be CompoundCRS versions of projected 3D CRS's, e.g.  
EPSG:7416 is a combination of  ETRS89 / UTM zone 32N + DVR90 height.

>
> - is the ‘dimension’ attr/3D CRS combo the only way (outside of  
> using <georss:elev>) to define an elevation in GeoRSS GML?

It is the only way in GML, hence in GeoRSS GML.

>
>
> 2.       <georss:where> is not required when using gmlgeorssXX.xsd  
> and when not using Atom/<entry>, but:
> - if one does so, they are operating outside the application  
> profile, and…
> - they must define what the individual GML tags mean relative to  
> other info in the schema, since they will have no “context”.  In  
> other words, unless one has good reason to do so, use <georss:where>  
> with the GeoRSS GML app profile to avoid extra work.

georss:where is the geometry property defined for GeoRSS GML. If you  
don't use it, you have to define your own means of relating geometry  
to something else.

>
>
> 3.       Since georssXX.xsd and gmlgeorssXX.xsd are distinct from  
> ea. other, one would  not use them together.  What’s throwing me is  
> the use of the <georss:where> tag, which I can’t find defined in  
> gmlgeorssXX.xsd.

gmlgeorss12.xsd profiles (is a proper subset of) GML. georss12.xsd  
defines other elements specifically for GeoRSS and brings in  
gmlgeorss12.xsd so that GML elements can be included in georss:where.  
If you use GeoRSS GML, both schema documents apply. If georss:where  
were put into gmlgeorss12.xsd, it would no longer be a profile.

If you just use GML, you could just use gmlgeorss12.xsd. If you just  
use GeoRSS Simple, you probably don't have any interest in XML schema  
at all and can dump both.

>
> Thanks for bearing with me.
>
> Cheers,
>
> Chris
>
> _____________________
> Chris Beaudette
> Windsor Solutions, Inc.
> www.windsorsolutions.com
> 503.675.7833  ext. 232
>
> From: Joshua Lieberman [mailto:josh at oklieb.net]
> Sent: Tuesday, May 06, 2008 3:06 PM
> To: Chris Beaudette
> Cc: georss at lists.eogeo.org
> Subject: Re: [georss] elevation and Simple<->GeoRSS GML interop  
> questions
>
> Chris,
>
> The gmlgeorss12.xsd file is for a profile of GML 3.1.1, where the  
> coordinate dimensions are (more) closely  tied to the coordinate  
> reference system, so one needs generally to declare both a 3D  
> coordinate system (compound or not) and dimensions defined by that  
> system.
>
> The schema allows a georss:where and georss:elev to be used together  
> (yes, they are properties at the same level). I think we've  
> discussed discouraging it, but not sure it is actually banned  
> outright. It's a conceivable use case, I suppose, of needing  
> specific 2D information and only general elevation information, but  
> awkward at best.
>
> The two schemas georss.xsd and gmlgeorss12.xsd is distinct. The  
> latter is a proper profile of GML 3.1.1, meaning that any XML which  
> validates under it will also validate under the full GML 3.1.1  
> schema. You are free to use those elements just as you are free to  
> use GML. The former schema defines the elements which are specific  
> to GeoRSS, which import and use GML elements and are "not"  
> themselves part of GML.
>
> GeoRSS as a specification only has something to say about what one  
> communicates with XML which uses GML within the GeoRSS elements, so  
> if you just use elements alone from gmlgeorss12.xsd, you are on your  
> own in defining what your application schema or XML elements mean  
> relative to other information.
>
> There has been an interest expressed in creating GML application  
> schemas (having a feature element derived from gml:AbstractFeature)  
> where the geometry properties are constrained to those used in  
> GeoRSS (In other words, valid according to gmlgeorss12.xsd). A paper  
> describing that will hopefully be released by OGC in June or so.
>
> Cheers,
>
> Josh
>
> On May 6, 2008, at 4:45 PM, Chris Beaudette wrote:
>
>
>
> 1. I don't see an elevation spec for GeoRSS GML in gmlgeorssXX.xsd,  
> but there's one for Simple in georssXX.xsd (<georss:elev>).  Looking  
> through the user forums at http://lists.eogeo.org/pipermail/georss/2007-January/000936.html 
>  I see that it is necessary to use the "dimension" attribute for the  
> <gml:pos[List]> tag, e.g.
>
> <gml:LineString>
>    <gml:posList dimension="3">0.0 0.0 1000.0 ...</gml:posList> </ 
> gml:LineString>
>
> Where the '1000.0' is the elevation.
>
> - Is this correct for gmlgeorss12.xsd?  I don't see the 'dimension'  
> attribute in there. I see only 'srsDimension'.
> - If this is the correct use of 'dimension', what units are they  
> in?  Does it depend on the 'srsName' used?  Does 'dimension' only  
> work when using a 'compound CRS'? Or are a compound CRS and  
> 'dimension' not used together?
> - Would we ever still have occasion to use both GeoRSS GML and  
> Simple together, like this (fromhttp://lists.eogeo.org/pipermail/georss/2008-April/001848.html) 
> ...
>   <georss:where>
>     <gml:point>
>       <gml:pos>blah blah</gml:pos>
>     </gml:point>
>   </georss:where>
>   <georss:elev>313</georss:elev>
>
> ... assuming the <georss:elev> tag is at the same level as the  
> <georss:where> tag, or this if it is not:
>   <georss:where>
>     <gml:point>
>       <gml:pos>blah blah</gml:pos>
>     </gml:point>
>     <georss:elev>313</georss:elev>
>   </georss:where>
>
>
> 2. For an application to use the XML components only of the GeoRSS  
> GML spec w/o Atom (and the requisite <entry> tag), would it be  
> possible to use the <gml:...> elements without the <georss:where>  
> tag?  Or is <georss:where> required even when not using Atom/<entry>?
>
>
> 3. As a follow-up to #s 1 & 2, I see the whereType defined in  
> georssXX.xsd, but not in gmlgeorssXX.xsd. Is it necessary to use  
> both schemas when using GeoRSS GML structures only (and no Simple  
> features)?  Or is that only necessary when using the <georss:elev>  
> tag w/ GeoRSS GML?
>
> _____________________
> Chris Beaudette
> Windsor Solutions, Inc.
> www.windsorsolutions.com
> 503.675.7833  ext. 232
>
> _______________________________________________
> georss mailing list
> georss at lists.eogeo.org
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>

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