[postgis-users] A way to split polygons?

[Mike Toews]

Take a look at ST_Simplify
http://postgis.refractions.net/docs/ST_Simplify.html

Or ST_SimplifyPreserveTopology if you want to preserve boundaries:
http://postgis.refractions.net/docs/ST_SimplifyPreserveTopology.html

-Mike

On 4 March 2011 09:44, Peter N. Schweitzer <pschweitzer at usgs.gov> wrote:
>
> 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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