FW: [postgis-users] RE: [postgis-devel] GML and XY,XYM,XYZ,XYZM

[Darko Androsevic]

-----Original Message-----
From: David S. Burggraf [mailto:burggraf at galdosinc.com]
Sent: Wednesday, October 06, 2004 3:19 PM
To: Martin Daly; Darko Androsevic
Cc: Ron Lake (E-mail)
Subject: RE: [postgis-users] RE: [postgis-devel] GML and XY,XYM,XYZ,XYZM


Hi Martin, my previous email bounced from the email reflector as I am not a
member. Can you post my response?

If M is a measure in a Linear reference system, then it is likely a measure
along a curve in some units (percent, meters, ...) and gml:pos (or
gml:posList) can be used to encode XYM, XYZM, ... The srsName needs to point
to a CompoundCRS, which consists of a 2D (or 3D) CRS for the XY (or XYZ) and
a 1D Linear CRS for the M ordinate. Note that if the Linear CRS is defined
in GML, then an application schema will be required as there is no
predefined LinearCRS in GML (the closest thing is an gml:EngineeringCRS that
uses a gml:LinearCS together with a GML Coordinate Operation).

Best regards,

David.

At 12:42 PM 10/6/2004, Martin Daly wrote:


Darko,
 
The M here is nothing to do with SVG.  It is a "measure", e.g. along a
linear geometry or along the perimeter of a polygonal geometry, hence Linear
Referencing System.
 
Regards,
Martin 

-----Original Message----- 

From: Darko Androsevic [ mailto:dandrosevic at galdosinc.com
<mailto:dandrosevic at galdosinc.com> ] 

Sent: Wed 06/10/2004 19:31 

To: 'PostGIS Users Discussion'; 'PostGIS Development Discussion' 

Cc: Ron Lake (E-mail); David Burggraf (E-mail) 

Subject: RE: [postgis-users] RE: [postgis-devel] GML and XY,XYM,XYZ,XYZM


Hello,


GML has no way to emulate M, L or other special characters used in SVG for 

drawing the shape. GML should be used to describe geometries.


GML provides NO support (except deprecated support) for explicit coordinate 

tags and this will not come back in GML. 

We would recommend to use the <gml:pos> element in combination with the CRS 

("srsName" attribute) to desribe the coordinates of the geometries. The 

<gml:coord> element should not be supported in any new softwares.


Thanks, 

Darko




-----Original Message----- 

From: postgis-users-bounces at postgis.refractions.net 

[ mailto:postgis-users-bounces at postgis.refractions.net
<mailto:postgis-users-bounces at postgis.refractions.net> ]On Behalf Of 

strk at refractions.net 

Sent: Wednesday, October 06, 2004 3:17 AM 

To: PostGIS Development Discussion; PostGIS Users Discussion 

Subject: Re: [postgis-users] RE: [postgis-devel] GML and XY,XYM,XYZ,XYZM



On Wed, Oct 06, 2004 at 10:37:28AM +0100, Martin Daly wrote: 

> strk, 

> 

> > I've introduced support for XYM. 

> > How should XYM geometries be rendered in GML ? 

> > And what aboux XYZM ? 

> 

> Excellent questions, with no easy answers. 

> 

> GML 2.x <coord> elements are hard-wired to have elements <X>, <Y> and 

> <Z>, so that is a non-starter.  GML 2.x <coordinates> elements just 

> contains tuples of co-ordinates, so that is a possibility.  I very much 

> doubt that any existing GML 2.x parsers have any knowledge of linear 

> reference systems, and instead assume that the tuples have at most three 

> ordinates, and that they are X, Y and  optionally Z.  Other usage is 

> valid, but is likely to be restricted to a particular vendor, or 

> industry, or whatever, and is not likely to be particularly 

> interoperable. 

> 

> GML 3.x has more support for specifying the CRS of the tuples, including 

> linear reference systems, but using that will reduce the number of 

> available clients for the data, because GML 3 support is not widespread. 

> 

> Not only no easy answers, but no answers at all!  Would it be tolerable 

> to not support AsGML for XYM and XYZM, or to strip the M? 

Sure. 

Current implementation uses: 

        GML:XY  for XY and XYM (strips the M) 

        GML:XYZ for XYZ and XYZM (strips the M) 

thanks for the info. 

--strk;


> 

> Regards, 

> Martin 

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