[postgis-users] points to polyline or polygon

[Tyler Mitchell]

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                                                                                 
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                                                                Subject:  Re: [postgis-users] points to polyline or polygon    
                      07/15/2003 01:59 PM                                                                                      
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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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