[postgis-users] line from points (& feature manipulation/editing)

[strk at refractions.net]

On Tue, Oct 05, 2004 at 12:02:54AM -0700, Wood Brent wrote:
> 
> --- strk at refractions.net wrote:
> 
> > > 
> > > how I can to create a line or polyline  from a collection of points with
> > > coordinates, inside of postgis of course
> > 
> > This is an unavailable feature, being discussed.
> > How would you specify the ordering of the composing points ?
> 
> Summat I'm also interested in, improved feature editing & manipulation in
> PostGIS, building lines from points, assembling polygons from lines, merging
> polygons by removing shared borders, splitting polygons by inserting a new
> line, etc....

Isn't "merging polygons" already implemented by GeomUnion ?

> 
> At present, I believe GRASS 5.7 is perhaps the best tool for this? JUMP/JTS is
> also there, but doesn't do what I need. QGIS may get there, it is certainly
> adding functionality quite rapidly. 
> 
> Is there a way to have PostGIS use something like the JTS natively, much as it
> uses GEOS now, to implement such capabilities?

mmm. what do you mean by natively ?
GEOS is a JTS port. Everything available in JTS can be made available
in PostGIS.

> 
> But to specify the line vertex order from point features, you would need some
> attribute(s) to provide an explicit order. In many cases I work with, this
> could be a timestamp, to turn a set of stations into a track.
> 
> ie: something like 
> 
> select linefrompoint(the_geom) where <...> order by <ordering_attr(s)>
> 
> to create a line from a set of points selected & ordered from a table of
> points.
> (unless I'm missing something...) 

Yes. You're missing that ORDER BY acts on the result set, which in this
case is a single record.
If you want to use it to order aggregate input you should do:

SELECT linefrompoint(
	(SELECT the_geom FROM <table> ORDER by <ordering_attr(s)>)
);

Which is use a subselect to feed ordered values to the aggregate.

--strk;

> 
> 
> 
> & I second the thanks for both PostGIS itself & the help from this forum!!
> 
> 
> Cheers,
> 
>   Brent Wood
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