Combine Geometry

Doing the union operation on two geometries is one of the most expensive operations you can possibly do; while it is reasonable for small numbers (say 5-10) when you start to get up to hundreds of geometries the cost can be measured in minutes.

• Using GeometryCollection union()

  In JTS 1.9 there is a new union() method that will “do the right thing”:

  static Geometry combineIntoOneGeometry( Collection<Geometry> geometryCollection ){
       GeometryFactory factory = FactoryFinder.getGeometryFactory( null );
  
       // note the following geometry collection may be invalid (say with overlapping polygons)
       GeometryCollection geometryCollection =
            (GeometryCollection) factory.buildGeometry( geometryCollection );
  
       return geometryCollection.union();
   }
  

• Using buffer( 0 )

  You can get the same effect in JTS 1.8 using buffer(0):

  GeometryFactory factory = FactoryFinder.getGeometryFactory( null );
  
  // note the following geometry collection may be invalid (say with overlapping polygons)
  GeometryCollection geometryCollection =
           (GeometryCollection) factory.buildGeometry( geometryCollection );
  
  Geometry union = geometryCollection.buffer(0);
  

• Using union( geometry )

  Using JTS versions prior to 1.9 you will need to combine the geometries one by one using geometry.union( geometry ):

      static Geometry combineIntoOneGeometry( Collection<Geometry> geometryCollection ){
          Geometry all = null;
          for( Iterator<Geometry> i = geometryCollection.iterator(); i.hasNext(); ){
          Geometry geometry = i.next();
          if( geometry == null ) continue;
          if( all == null ){
              all = geometry;
          }
          else {
              all = all.union( geometry );
          }
      }
      return all;
  }
  

  The above code is pretty much too simple to live; the correct way to do things is to break up your data into regions, union all the geometries in one region together; and then combine these at the end into one big geometry (this is the approach used above by the union() method).