[postgis-users] Invalid Geometries from shapefiles

[Bruce Rindahl]

I have an application that stores floodplains in a PostGIS database for
later display in a web mapping application.  The floodplains are derived
from shapefiles and are converted using shp2pgsql.  The issue is some of the
shapes are invalid when converted to the geometry format in PostGIS.  When
these invalid geometries are manipulated by certain operators, the results
are not as expected (for example intersection returns a geometry collection
instead of a multipolygon).
Are there some guidelines or things to look for in shapefiles to assure the
geometry is valid?  Most of the problems occur when a polygon has an
interior island.  All of the tools we are using accepts these shapefiles as
valid but will cause problems when converted.
Thanks
Bruce Rindahl