[postgis-users] A way to split polygons?

[pcreso at pcreso.com]

Hi Peter, 

It sounds like you are wanting to tile your polygons. Not really ideal, as each polygon is rendered via tiles, so you need to render them without borders to hide the tiles, then often plot the border as well, which still has all the vertices. Lots more work.

You might look at simplifying your polygons to reduce the number of vertices in each. If you 
manage your data topologically, this process will work better at 
retaining shared boundaries.

If you can use Google to provide vector zoom layers, then as you zoom in you can get less & less simplified versions... just like pyramid'ed rasters. Zoomed out you can't see so don't provide the unnecessary detail.

I don't use Google myself, the licence terms are too restrictive for much of the data I work with, so can't say how well this works in a Google environment.

Brent Wood


--- On Fri, 3/4/11, Peter N. Schweitzer <pschweitzer at usgs.gov> wrote:

From: Peter N. Schweitzer <pschweitzer at usgs.gov>
Subject: [postgis-users] A way to split polygons?
To: "PostGIS Users Discussion" <postgis-users at postgis.refractions.net>
Date: Friday, March 4, 2011, 9:44 AM

At http://tin.er.usgs.gov/geology/state/ and pages below it, I provide
geologic map units for US states in KML for use within Google Earth.
These are stored in PostgreSQL (9.0.2) with PostGIS (1.5.2).  My problem
is that many of the larger geologic units are represented as polygons that
have a large number of vertices, often with many interior rings.  Google Earth
doesn't want to display polygons that have 100,000 vertices (I think the
actual limit is lower, but I don't know what it is.)  So my geologic maps
have large irregular holes in them when viewed in Google Earth.

Is there a way, using PostGIS functions, to split these polygons so that
the resulting polygons have fewer vertices and thus would appear as expected
in Google Earth?  It seems to me one possibility would be to intersect
these large polygons with Box2D's whose vertical dimensions were, say,
one or two degrees.  But I don't yet see how to carry this operation out.

Can anyone offer suggestions as to how this might be done?

Here are some of the characteristics of the data.  The principal table
containing the polygons is named geol_poly; the field unit_link is a
relational key to other, more interesting data, and the_geom comes through
shp2pgsql.

select unit_link,ST_NPoints(the_geom) npt,ST_NumGeometries(the_geom) ng, ST_NumInteriorRings(the_geom) ni from geol_poly order by npt desc;
    unit_link    |  npt   | ng |  ni
-----------------+--------+----+------
 FLwater;0       | 451759 |  1 | 8945
 PAPAa;6         | 319192 |  1 | 1235
 PAPAcg;6        | 216653 |  1 |  925
 ALwater;0       | 205251 |  1 |  108
 OHPAc;0         | 201310 |  1 |  842
 OHPAm;0         | 200763 |  1 |  895
 OHPAap;0        | 196422 |  1 | 1207
 MNOl;0          | 188753 |  1 |  107
 MNCu;0          | 165061 |  1 |   86
 TNObh;6         | 152421 |  1 |  825
 PAPAp;6         | 144889 |  1 |  324
 MOOjc;0         | 144721 |  1 |  144
 MOMk;0          | 128731 |  1 |  290
 WVPAk;0         | 128402 |  1 |  934
 TXKed;0         | 124666 |  1 |  720
 TNMfp;10        | 122021 |  1 |  117
 OHMlc;0         | 116993 |  1 |  731
 TNMfp;10        | 113034 |  1 |  406
 MOOr;0          | 109195 |  1 |  136
 PAPAm;6         | 105906 |  1 |  395
 PAPAcc;6        | 105798 |  1 |  447
 WVPAm;0         |  99944 |  1 |  474
 CAQ;0           |  97086 |  1 |  455
 WVPAc;0         |  94277 |  1 |  455
 MNOm;0          |  93054 |  1 |   53
 PAPPAw;0        |  89974 |  1 |  218
 TNOca;6         |  89292 |  1 |  564
 WVPAa;0         |  89182 |  1 |  504
 MOOr;0          |  87103 |  1 |  171
 MOOg;0          |  85660 |  1 |  277

Peter
-- Peter N. Schweitzer (MS 954, U.S. Geological Survey, Reston, VA 20192)
(703) 648-6533  FAX: (703) 648-6252  email: pschweitzer at usgs.gov
<http://geology.usgs.gov/peter/>
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