[postgis-users] Noob request - using ST_SimplifyPreserveTopology() on Tiger Data

Tue Jun 28 12:48:25 PDT 2011

On Tue, Jun 28, 2011 at 03:00:47PM -0400, Christian Guirreri wrote:
> OK works- thanks.
> 
> So I'm learning now about the difference between what MapShaper does and
> what ST_Stimplify does. Is there any way to make sure that simplified
> boundary lines will match other boundary lines? i.e. even the smallest
> simplifications (like 0.05) produces very visible gaps between adjacent
> districts.

What you could do is convert your layer to a topological representation,
simplify each edge of the topology and then convert back to simple features.

I'm looking for a sponsor to make the first part easier.
Second part is easy already.

If you're unfamiliar with postgis topology,
here's a presentation I gave in Paris last week:
http://strk.keybit.net/projects/postgis/Paris2011_TopologyWithPostGIS_2_0.pdf
And, there's info on the wiki too.

Another approach could be using ST_Snap to snap each simplified geometry
against the collected vertices of each previously simplified one falling 
within a slightly enlarged bounding box of the geometry you're simplifying.

--strk;

> On Tue, Jun 28, 2011 at 1:12 PM, Sandro Santilli <strk at keybit.net> wrote:
> 
> > Christian,
> > I suggest you create a new geometry field w/out enforcing type constraints
> > on it, write the simplified version in there and then see what you get.
> >
> > --strk;
> >
> > On Tue, Jun 28, 2011 at 12:15:13PM -0400, Christian Guirreri wrote:
> > > Same error. Tried both Polygon and Multipolygon. I'm not sure which the
> > > Tiger Data is.
> > >
> > > On Tue, Jun 28, 2011 at 11:41 AM, Ben Madin
> > > <lists at remoteinformation.com.au>wrote:
> > >
> > > > Christian,
> > > >
> > > > My guess here is that because you've told the geometry to be a
> > > > MULTIPOLYGON, but you may have returned a POLYGON, you have indeed
> > violated
> > > > the check constraint on the geometry type.
> > > >
> > > > try using st_multi() to get around it, as in :
> > > >
> > > > update public.tl_2010_us_cd111 set simple_geom =
> > > > ST_Multi(ST_SimplifyPreserveTopology(the_geom, 15));
> > > >
> > > >
> > > > cheers
> > > >
> > > > Ben
> > > >
> > > >
> > > >
> > > > On 28/06/2011, at 11:23 PM, Christian Guirreri wrote:
> > > >
> > > > I get the following error:
> > > > ERROR: new row for relation "tl_2010_us_cd111" violates check
> > constraint
> > > > "enforce_geotype_simple_geom"
> > > > SQL state: 23514
> > > >
> > > > Here's my SQL:
> > > > select addGeometryColumn('public', 'tl_2010_us_cd111', 'simple_geom',
> > 4326,
> > > > 'MULTIPOLYGON', 2);
> > > > update public.tl_2010_us_cd111 set simple_geom =
> > > > ST_SimplifyPreserveTopology(the_geom, 15);
> > > >
> > > >
> > > > On Mon, Jun 27, 2011 at 2:07 PM, Stephen Woodbridge <
> > > > woodbri at swoodbridge.com> wrote:
> > > >
> > > >> On 6/27/2011 1:37 PM, Christian Guirreri wrote:
> > > >>
> > > >>> I'm trying to do something fairly simple. I'd like to take the Tiger
> > > >>> 2010 Data - tl_2010_us_cd111 and tl_2010_us_county10 - and create
> > shape
> > > >>> files that are considerably smaller and more simplified. In the past
> > > >>> I've just used MapShaper and been OK with that, but I was having
> > trouble
> > > >>> uploading tl_2010_us_county10 into it and I've decided its time to
> > learn
> > > >>> some GIS basics anyway.
> > > >>>
> > > >>> I've definitely successfully setup my environment - PostegreSQL 9
> > with
> > > >>> PostGIS. I've used the shp2pgsql wizard included with pgadmin to
> > import
> > > >>> the data sets into their own databases, without the "Load Geography
> > > >>> column" checked. I've been able to export this data back to shp
> > > >>> using pgsql2shp and view the resulting shp in QuantumGIS and ACDSee
> > > >>> Canvas.
> > > >>>
> > > >>> But when it comes to simplification - I'm not sure where to even
> > > >>> start. I'm no database expert beyond some simple mysql so GIS is
> > > >>> especially overwhelming. I only know that I want to
> > > >>> use ST_SimplifyPreserveTopology() as I'm fairly certain a number of
> > > >>> things will disappear if I don't.
> > > >>>
> > > >>> Would love if someone could provide me with the SQL to make this
> > happen!
> > > >>>
> > > >>
> > > >> First you should understand that ST_SimplifyPreserveTopology() works
> > on a
> > > >> single geometry at a time and not on multiple geometries in a
> > coverage. The
> > > >> difference is in a coverage the the polygons that share a common edge
> > that
> > > >> is simplified would continue to have a common simplified edge. When
> > polygons
> > > >> are simplified individually with knowledge of the adjacent polygons it
> > is
> > > >> possible for gaps and overlaps to occur along the simplified edge.
> > > >> ST_SimplifyPreserveTopology() is designed to try and minimize these
> > effects
> > > >> but there are no guarantees.
> > > >>
> > > >> You might try something like:
> > > >>
> > > >> select addGeometryColum('myschema', 'mytable', 'simple_geom', 4326,
> > > >> 'MULTIPOLYGON', 2);
> > > >> update myschema.mytable set simple_geom =
> > ST_SimplifyPreserveTopology(**the_geom,
> > > >> <tolerance>);
> > > >>
> > > >> where <tolerance> is replaces by an appropriate value. This will
> > depend on
> > > >> the some trial and error. The above assume srid=4326 which is probably
> > not a
> > > >> good projection for doing simplification so you might want to project
> > your
> > > >> data into some other UTM or Mercator projection depending on your
> > coverage
> > > >> area and tolerance needs to be set with respect to those units.
> > > >>
> > > >> -Steve W
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