# Create polygon from lines in PostGIS

I'm trying to divide a polygon in smaller polygons from a line, but I think I can't use the st_split function. What I need is to create small polygons inside a big one, using linestring grid.

I've tried some ways, but I can't get the result. What I've tried:

Divide one polygon from a LineString using st_split()

From a boundary polygon.

And Linestring table:

I'd need the following polygons:

Problem: I can't Split a polygon from several lines, neither a polygon from a multilinestring.

The other method I'm trying is to create a polygons from the lines with st_polygonize() The SQL I was trying is:

``````SELECT
g.path[1] as gid,
g.geom::geometry(polygon, 22033) as geom
FROM
(SELECT
(ST_Dump(ST_Polygonize(geom))).*
FROM linestable
) as g;
``````

Problem: I can only get one polygon (the boundary).

Can someone tell me which would be the best way to get the polygons from the linestring, or if am I missing something?

Note: Tables are in the same SRID, and geometries are snapped into a grid. In QGIS I can run the polygonize process from lines to polygon perfectly.

As John's demand, here is the linestring table. https://drive.google.com/file/d/0B603y_m735jfS014S0EyVnpMUEU/view?usp=sharing

• Can you post the linestring geometries somewhere? ST_Polygonize along with ST_Dump should work. Commented Dec 9, 2015 at 13:51
• Sure. I've edited the post with the link to the table. Commented Dec 9, 2015 at 14:25
• @JohnPowellakaBarça can you look at this. This question is great. ST_Polygonize on his sample set returns a single polygon. We essentially need to decompose his mess of open LINESTRINGS and find all of them that can make rectangles? Commented May 17, 2017 at 20:35
• @EvanCarroll. Sure. I'm crazily busy atm, so probably not for a few days. Commented May 18, 2017 at 7:09
• @ppardoz. Espero que esto te ayude. I noticed that your table defs were all in Spanish :-) Commented May 25, 2017 at 16:56

I got this working by using ST_Node first, in conjunction with ST_Collect, to convert the lines into a set of noded linestrings within a MultiLinestring.

As it says in the docs for ST_Node:

Fully node a set of linestrings using the least possible number of nodes while preserving all of the input ones.

What this means, is that all of the linestrings are combined in all possible combinations, so as to make up the equivalent to the exterior ring of a polygon. Whereas, if you attempt to ST_Polygonize a set of LineStrings, none of which on it's own describes a polygon, you simply get the LineStrings back. So, this works:

``````WITH multi(geom) AS (
SELECT ST_Node(ST_Collect(geom))
FROM leyenda_digitalizar00
)
SELECT ST_AsText( (ST_Dump(ST_Polygonize(geom))).geom )
FROM multi;
``````

If you just run the first part of this, ie, the CTE multi, the output looks like:

MULTILINESTRING((204.5 69.9000000000004,204.5 69.9000000000004),(204.5 68.9,205.4 68.9),(204.5 68.9,204.5 69,204.5 69.1,204.5 69.2,204.5 69.3,204.5 69.4,204.5 69.5,204.5 69.6,204.5 69.7,204.5 69.8,204.5 69.9,204.5 69.9000000000004),(209.5 68.9,209.5 68.8,209.5 68.7,209.5 68.6,209.5 68.5,209.5 68.4,209.5 68.3,209.5 68.2,209.5 68.1,209.5 68,209.5 67.9,209.5 67.8,209.5 67.7,209.5 67.6,209.5 67.5,209.5 67.4,209.5 .......

Now, when you now feed this MultiLinestring to ST_Polygonize it works as expected, eg,

POLYGON((205.4 68.9,204.5 68.9,204.5 69,204.5 69.1,204.5 69.2,204.5 69.3,204.5 69.4,204.5 69.5,204.5 69.6,204.5 69.7,204.5 69.8,204.5 69.9,204.5 69.9000000000004,205.4 69.9,205.4 69.3,205.4 68.9))

POLYGON((204.5 69.9000000000004,204.5 70,204.5 70.1,204.5 70.2,204.5 70.3,204.5 70.4,206.8 70.4,209.5 70.4,209.5 70.3,209.5 70.2,209.5 70.1,209.5 70,209.5 69.9,205.4 69.9,204.5 69.9000000000004))

POLYGON((206.8 70.4,204.5 70.4,204.5 70.5,204.5 70.6,204.5 70.7,204.5 70.8,204.5 70.9,204.5 71,204.5 71.1,204.5 71.2,204.5 71.3,204.5 71.4,206.8 71.4,206.8 70.4))

Obviously, the ST_AsText is just for illustration, and you will have to tweak, if you want the path ID too.

The key takeaway is that ST_Polygonize expects linestrings that already describe the outline of a polygon, which is what ST_Node(ST_Collect(.... does in the above.

• That's really cool, I can confirm that this solution works! On the flip side, I wonder why `ST_Polygonize()` says it takes line strings, or what the use case is for that without calling `ST_Node()` first Commented May 25, 2017 at 17:42
• @EvanCarroll. I think that the answer is that ST_Polygonize takes Linestrings that are already in a set representing a polygon, whereas the linestrings in this question made up multiple possible polygons. My understanding is that this is what ST_Node does. I will attempt to investigagte further and update the answer. Commented May 25, 2017 at 18:20