[postgis-users] points to polyline or polygon

[Randy James]

Thanks for all the input looks like i can hack something

Cheers
--- chodgson at refractions.net wrote:
> You can use functions like "pointn()" to get the the nth
> point in the object... 
> but really, the best thing is to parse the WKT outside of
> the database and 
> manipulate it with the language of your choice (real easy
> with perl or php), 
> and then put it back in when you're done.
> 
> The Collect() aggregate function was the first one that I
> ever wrote for 
> postgis, (or for postgres for that matter) and so I am
> somewhat partial to it 
> myself. But unfortunately, the lack of any guarantee of
> order makes it somewhat 
> unreliable and less useful. I believe this is true of all
> aggregate functions - 
> however, if you select them based on a completely indexed
> query ie. SELECT 
> COLLECT(the_geom) FROM table WHERE id1=# and id2=#, and
> there is an index like 
> unique(id1, id2), then you are virtually guaranteed that
> the rows will be 
> retrieved in the order of the index (ie. sorted) and that
> should virtually 
> gaurantee that the collection is in either forward or
> backward, but not mixed 
> up...
> 
> It's still a nice little hack, Tyler!
> 
> Chris
> 
> Quoting Tyler Mitchell <TMitchell at lignum.com>:
> 
> > 
> > 
> > 
> > 
> > 
> > Chris, I'm having fun here and didn't need those
> details ;)
> > Darn!  I was really having fun with this one.  Here's
> what I did, but
> > unfortunately, I used COLLECT() :(
> > 
> > # select 'SRID=-1;LINESTRING' ||
> >
>
substr(astext(collect(the_geom)),11,length(astext(collect(the_geom))))
> as
> > the_geom, line_id from test_pt group by line_id;
> > 
> > I take the WKT collection and replace the geometry type
> with linestring
> > instead.  I know it's hokey, but it worked.  What do
> you mean point order
> > is important for constructing your line?  :)
> > 
> > If some one can tell me a better way using SQL to get
> an array of the
> > points, that'd be helpful.
> > 
> > Does this help at all Randy.
> > 
> > On a similar note can anyone comment or provide
> direction on writing custom
> > functions to help do this kind of thing?  I'd be
> particularly interested in
> > how the proces compares to Oracle's pl/sql.  I guess
> taking a peek at
> > postgis.sql would be a good place for me to start eh? 
> Although I see a lot
> > of your functions are bundled into postgis.dll right?
> > 
> > 
> > Tyler
> > 
> > 
> > 
> >                       chodgson at refractions.net
> >                       Sent by:                         
>         To:      
> > PostGIS Users Discussion
> >                      
> postgis-users-bounces at postgis.refr        
> > <postgis-users at postgis.refractions.net>
> >                       actions.net                      
>         cc:
> >                                                        
>         Fax to:
> >                                                        
>         Subject:  Re:
> > [postgis-users] points to polyline or polygon
> >                       07/15/2003 01:59 PM
> >                       Please respond to PostGIS Users
> >                       Discussion
> > 
> > 
> > 
> > 
> > 
> > You definitely do NOT want to do this - the COLLECT()
> aggregate function
> > doesn't gauarantee any sort of order in the points it
> has collected.
> > Furthermore, a Multipoint object is not necessarily
> ordered (it will always
> > 
> > list the points in the same order, but not by
> necessity, only because it is
> > 
> > simplest).
> > 
> > The best way to solve this problem is to select your
> list of points from
> > the
> > table (they will also need to have an "order" column in
> order to make sure
> > they
> > are made into a line in the correct order) and use PHP
> or your language of
> > choice to build a new WKT string.
> > 
> > HTH.
> > 
> > Chris
> > 
> > > This is an interesting question to me.  If I
> understand correctly, you
> > want
> > > to "cast" a set of points into a linestring.  Getting
> your list of points
> > > should be as easy as:
> > >
> > > SELECT COLLECT(the_geom) FROM mytable GROUP BY
> somefield.
> > >
> > > But this returns the geometry as MULTIPOINT, which
> isn't really want you
> > > want, but if you see the text representation, it does
> look like a
> > > linestring:
> > >
> > > SRID=-1;MULTIPOINT(596681.75 5830977.5,532989.625
> 5869892,547653.0625
> > > 5808612,608081.1875 5799629)
> > >
> > > So can anyone else give a pointer on taking the above
> output and casting
> > it
> > > into a linestring instead?
> > >
> > > Tyler
> > 
> > 
> > 
> > 
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