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

strk at refractions.net strk at refractions.net
Wed Oct 6 03:16:57 PDT 2004

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?

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.

> Regards,
> Martin
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