13

My goal is take a single table and st_union all polygons that are either touching or close to each other into single polygons

I'm a C# developer who is starting to learn about PostGIS. Using the code below, I was able to accomplish this, but it seems inefficient, and there's much to PostGIS that is new to me.

From my initial attempt (still in comments), I was able to reduce the iterations by using array_agg with ST_UNION instead of unioning just polys at a time.

I wind up with 133 polys from my orig 173.

sql = "DROP TABLE IF Exists tmpTable; create table tmpTable ( ID varchar(50), Geom  geometry(Geometry,4326), Touchin varchar(50) ); create index idx_tmp on tmpTable using GIST(Geom); ";
                CommandText = sql;
                ExecuteNonQuery();

                sql = "";
                for (int i = 0; i < infos.Count(); i++)
                {
                    sql += "INSERT INTO tmpTable SELECT '" + infos[i].ID + "', ST_GeomFromText('" + infos[i].wkt + "', 4326), '0';";
                }
                CommandText = sql;
                ExecuteNonQuery();

                CommandText = "update tmpTable set touchin = (select id from tmpTable as t where st_intersects(st_buffer(geom, 0.0001), (select geom from tmpTable as t2 where t2.ID = tmpTable.ID ) ) and t.ID <> tmpTable.ID limit 1)";
                ExecuteNonQuery();

                CommandText = "select count(*) from tmpTable where touchin is not null";
                long touching = (long)ExecuteScalar();
                string thisId = "";
                // string otherId = "";
                while (touching > 0)
                {
                    CommandText = "select touchin, count(*)  from tmpTable where touchin is not null group by touchin order by 2 desc limit 1";
                    //CommandText = "select id, touchin from tmpTable where touchin is not null";
                    using (var prdr = ExecuteReader())
                    {
                        CommandText = "";
                        if (prdr.Read())
                        {
                            thisId = prdr.GetString(0);
                             // otherID = prdr.GetString(1);
                            CommandText = @"update tmpTable set geom = st_union(unioned) 
                                from (select array_agg(geom) as unioned from tmpTable where touchin = '" + thisId + "' or id = '" + thisId + @"') as data
                                where id = '" + thisId + "'";
                             // CommandText = "update tmpTable set geom = st_union(geom, (select geom from tmpTable where ID = '" + otherId + "')) where id = '" + thisId + "'";
                        }
                    }

                    if (!string.IsNullOrEmpty(CommandText))
                    {
                        ExecuteNonQuery();
                        //CommandText = "update tmpTable set geom = null, touchin = null where ID = '" + otherId + "'";
                        CommandText = "update tmpTable set geom = null, touchin = null where touchin = '" + thisId + "'";
                        ExecuteNonQuery();                            
                    }

                    CommandText = "update tmpTable set touchin = (select id from tmpTable as t where st_intersects(st_buffer(geom, 0.0001), (select geom from tmpTable as t2 where t2.ID = tmpTable.ID ) ) and t.ID <> tmpTable.ID limit 1)";
                    ExecuteNonQuery();

                    CommandText = "select count(*) from tmpTable where touchin is not null";
                    touching = (long)ExecuteScalar();
                }

Here's sample of the dataset I am using:

INSERT INTO tmpTable SELECT '872538', ST_GeomFromText('POLYGON((-101.455035985 26.8835084441,-101.455035985 26.8924915559,-101.444964015 26.8924915559,-101.444964015 26.8835084441,-101.455035985 26.8835084441))', 4326), '0';
INSERT INTO tmpTable SELECT '872550', ST_GeomFromText('POLYGON((-93.9484752173 46.0755084441,-93.9484752173 46.0844915559,-93.9355247827 46.0844915559,-93.9355247827 46.0755084441,-93.9484752173 46.0755084441))', 4326), '0';
INSERT INTO tmpTable SELECT '872552', ST_GeomFromText('POLYGON((-116.060688575 47.8105084441,-116.060688575 47.8194915559,-116.047311425 47.8194915559,-116.047311425 47.8105084441,-116.060688575 47.8105084441))', 4326), '0';
INSERT INTO tmpTable SELECT '872553', ST_GeomFromText('POLYGON((-116.043688832 47.8125084441,-116.043688832 47.8214915559,-116.030311168 47.8214915559,-116.030311168 47.8125084441,-116.043688832 47.8125084441))', 4326), '0';
INSERT INTO tmpTable SELECT '872557', ST_GeomFromText('POLYGON((-80.6380222359 26.5725084441,-80.6380222359 26.5814915559,-80.6279777641 26.5814915559,-80.6279777641 26.5725084441,-80.6380222359 26.5725084441))', 4326), '0';
INSERT INTO tmpTable SELECT '872558', ST_GeomFromText('POLYGON((-80.6520223675 26.5755084441,-80.6520223675 26.5844915559,-80.6419776325 26.5844915559,-80.6419776325 26.5755084441,-80.6520223675 26.5755084441))', 4326), '0';
INSERT INTO tmpTable SELECT '872559', ST_GeomFromText('POLYGON((-80.6400224991 26.5785084441,-80.6400224991 26.5874915559,-80.6299775009 26.5874915559,-80.6299775009 26.5785084441,-80.6400224991 26.5785084441))', 4326), '0';
INSERT INTO tmpTable SELECT '872560', ST_GeomFromText('POLYGON((-80.6530226307 26.5815084441,-80.6530226307 26.5904915559,-80.6429773693 26.5904915559,-80.6429773693 26.5815084441,-80.6530226307 26.5815084441))', 4326), '0';
INSERT INTO tmpTable SELECT '872568', ST_GeomFromText('POLYGON((-90.7892258584 30.7365084441,-90.7892258584 30.7454915559,-90.7787741416 30.7454915559,-90.7787741416 30.7365084441,-90.7892258584 30.7365084441))', 4326), '0';
INSERT INTO tmpTable SELECT '872569', ST_GeomFromText('POLYGON((-90.7832259127 30.7375084441,-90.7832259127 30.7464915559,-90.7727740873 30.7464915559,-90.7727740873 30.7375084441,-90.7832259127 30.7375084441))', 4326), '0';
  • Is the actual data itself needed in your question? – Paul Oct 2 '15 at 18:15
  • @Paul - wasn't sure if it would be helpful or not. – Carol AndorMarten Liebster Oct 2 '15 at 18:37
20

The simplest way would be to ST_Union the entire table:

SELECT ST_Union(geom) FROM tmpTable;

This will give you one huge MultiPolygon, which probably isn't what you want. You can get the individual dissolved components with ST_Dump. So:

SELECT (ST_Dump(geom)).geom FROM (SELECT ST_Union(geom) AS geom FROM tmpTable) sq;

This gives you a separate polygon for each set of touching inputs, but groups of inputs that were separated by a short distance will remain as separate geometries. If you have access to PostGIS 2.2.0rc1, you can merge geometries that are close together into a single GeometryCollection using ST_ClusterWithin:

SELECT unnest(ST_ClusterWithin(geom, 0.0001)) AS grp FROM tmpTable;

If you want Polygons within the GeometryCollection to be dissolved, you can run ST_UnaryUnion on each GeometryCollection in the result, like:

SELECT ST_UnaryUnion(grp) FROM
(SELECT unnest(ST_ClusterWithin(geom, 0.0001)) AS grp FROM tmpTable) sq;
  • That's definitely much faster, but 2 things: (1) Can I preserve the ID field in the results? It's not important which one, but I need to take the result and get other data from it. (2) Is there a way to add the ST_Buffer back in? – Carol AndorMarten Liebster Oct 2 '15 at 18:42
  • 1
    (1) Not easily, but a simple way to get attributes back is to spatially join your result polygons against an interior point of your input polygons. (2) Added some explanation for handling geometries that are close but don't touch. – dbaston Oct 2 '15 at 19:03
  • Thanks for the help - I currently don't have 2.2, so I'll have to revisit this when I upgrade to that. For now the exclusion of the buffer is not a deal breaker. – Carol AndorMarten Liebster Oct 2 '15 at 21:13
  • I agree this is the simplest. I do wonder if there would be a way to do a recursive query that finds touching geoms, but I gave up on it- postgresql.org/docs/current/static/queries-with.html – chrismarx Apr 20 '18 at 10:23
  • 1
    @chrismarx, check out gis.stackexchange.com/a/94228/18189 for some ideas on the recursive solution. – dbaston Apr 20 '18 at 13:03

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