[postgis-users] Create POLYGON from Several LINESTRINGs

[Avery Penniston]

Thanks for the reply.

 

I tried to use the st_buildarea with st_collect as you suggested by
modifying my wedgefunc function that I posted earlier.  However, now I
don't get anything out of the function.  I believe the collect is
returning a MULTILINESTRING with the three elements to my wedge in it,
but for some reason the st_buildarea function is returning nothing.  Any
ideas?

 

Here is my modified wedgefunc function:

 

CREATE OR REPLACE FUNCTION wedgefunc(outerradius float, startangle
float, openingangle float)

  RETURNS text AS

$BODY$

  DECLARE

    startchord geometry;

    midchord geometry;

    endchord geometry;

    revstartchord geometry;

    outerarc text;

    outerarcline geometry;

    polyborder geometry;

    poly geometry;

 

  BEGIN

  -- Rotate a horizontal line to form chords - the 'mid' chord is to
help form the CIRCULARLINESTRING

                startchord :=
st_rotate(st_makeline(st_point(0,0),st_point(outerradius,0)),startangle)
;

                midchord :=
st_rotate(st_makeline(st_point(0,0),st_point(outerradius,0)),startangle
+ (.5 * openingangle));

                endchord :=
st_rotate(st_makeline(st_point(0,0),st_point(outerradius,0)),startangle
+  openingangle);

                

  --Make a reversal of the start chord - not sure if this is needed,
just incase the points need to be in order when putting together the
poly

                revstartchord := st_reverse(startchord); 

                

  --Create an arc line by passing the endpoints of the endchord,
midchord, and startchord

                outerarc :=
'CIRCULARSTRING('||st_x(st_endpoint(endchord))||'
'||st_y(st_endpoint(endchord))||','||st_x(st_endpoint(midchord))||'
'||st_y(st_endpoint(midchord))||','||st_x(st_endpoint(startchord))||'
'||st_y(st_endpoint(startchord))||')';

                outerarcline := st_curvetoline(outerarc);

 

  --TRYING ST_BUILDAREA WITH ST_COLLECT AFTER UNIONING THE 3 LINESTRINGS

                poly := st_buildarea(st_collect(geom))

                FROM (select revstartchord as geom

                                union select endchord as geom

                                union select outerarcline as geom

                ) as u;

                return st_astext(poly);

 

  END;

 

  $BODY$

  LANGUAGE 'plpgsql' IMMUTABLE;

 

From: postgis-users-bounces at postgis.refractions.net
[mailto:postgis-users-bounces at postgis.refractions.net] On Behalf Of
Randall, Eric
Sent: Tuesday, June 22, 2010 2:26 PM
To: PostGIS Users Discussion
Subject: Re: [postgis-users] Create POLYGON from Several LINESTRINGs

 

 

I think perhaps st_buildarea and st_collect might be what you want.
Below is what I used to create a hexagon out of points->lines which was
then translated (using generate_series) across X and Y to create a
hexagonal grid for the County.   There's probably a simpler way to make
a hexagon but it illustrates the st_buildarea.  Maybe that helps.

 

-Eric 

 

 

 

 

--let's make a hexagon with 1000 foot sides and position it with it's 
--center at 1210000, 610000 (the lower left of the pamap tile extent for
Erie County)
--and create a table 

 

drop table if exists hex1000;
create table hex1000 as
select translate(st_buildarea(st_collect(geom)),1210000, 610000) as geom
from 
(select makeline(st_endpoint(makeline(makepoint(0, 0),makepoint(0, -
1000))),st_endpoint(rotate(makeline(makepoint(0, 0),makepoint(0,
-1000)), radians(300)))) as geom 
union select makeline(st_endpoint(rotate(makeline(makepoint(0,
0),makepoint(0, -1000)),
radians(300))),st_endpoint(rotate(makeline(makepoint(0, 0),makepoint(0,
-1000)), radians(240))))  as geom 
union select makeline(st_endpoint(rotate(makeline(makepoint(0,
0),makepoint(0, -1000)),
radians(240))),st_endpoint(rotate(makeline(makepoint(0, 0),makepoint(0,
-1000)), radians(180))))  as geom 
union select makeline(st_endpoint(rotate(makeline(makepoint(0,
0),makepoint(0, -1000)),
radians(180))),st_endpoint(rotate(makeline(makepoint(0, 0),makepoint(0,
-1000)), radians(120))))  as geom 
union select makeline(st_endpoint(rotate(makeline(makepoint(0,
0),makepoint(0, -1000)),
radians(120))),st_endpoint(rotate(makeline(makepoint(0, 0),makepoint(0,
-1000)), radians(60))))  as geom 
union select makeline(st_endpoint(rotate(makeline(makepoint(0,
0),makepoint(0, -1000)),
radians(60))),st_endpoint(rotate(makeline(makepoint(0, 0),makepoint(0,
-1000)), radians(0))))  as geom 
) as t1;

 

 

 

 

 

-----Original Message-----
From: postgis-users-bounces at postgis.refractions.net
[mailto:postgis-users-bounces at postgis.refractions.net]On Behalf Of Avery
Penniston
Sent: Tuesday, June 22, 2010 2:38 PM
To: postgis-users at postgis.refractions.net
Subject: Re: [postgis-users] Create POLYGON from Several LINESTRINGs

	I'm trying to create 'pie wedge' shaped polygons by building
them in a Postgres function.  I am able to create the 2 straight chords
and the arc as separate LINESTRINGs, and I have verified that each
LINESTRING shares its endpoints with the other two LINESTRINGs.
However, I am having trouble putting the individual parts together to
form a POLYGON.

	 

	I tried to ST_UNION the 3 LINESTRINGs together, and the result
is a MULTILINESTRING.  I then call ST_LINEMERGE to convert the
MULTILINESTRING to a single LINESTRING so I can pass it to the
ST_POLYGON function, but the result of the ST_LINEMERGE is a
MULTILINESTRING.  

	 

	I tried using ST_COLLECT instead of ST_UNION, but then I got a
GEOMETRYCOLLECTION containing a MULTILINESTRING and a LINESTRING.

	 

	Does anybody have some advice for creating a POLYGON from 3 or
more separate LINESTRINGs?

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