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

[Christian Guirreri]

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
>> ______________________________**_________________
>> postgis-users mailing list
>> postgis-users at postgis.**refractions.net<postgis-users at postgis.refractions.net>
>> http://postgis.refractions.**net/mailman/listinfo/postgis-**users<http://postgis.refractions.net/mailman/listinfo/postgis-users>
>>
>
> _______________________________________________
> postgis-users mailing list
> postgis-users at postgis.refractions.net
> http://postgis.refractions.net/mailman/listinfo/postgis-users
>
>
>
> _______________________________________________
> postgis-users mailing list
> postgis-users at postgis.refractions.net
> http://postgis.refractions.net/mailman/listinfo/postgis-users
>
>
-------------- next part --------------
An HTML attachment was scrubbed...
URL: <http://lists.osgeo.org/pipermail/postgis-users/attachments/20110628/f285f254/attachment.html>