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

[Christian Guirreri]

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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