[postgis-users] A way to split polygons?

[Peter N. Schweitzer]

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/>