[postgis-users] polygon/multipolygon limitation?

[Norman Vine]

Hubert Fröhlich writes:
>
> Application: I have ca. 100000 polygons ( a neighborhood of land
> register parcels) and want to get the outer boundary of the area
> enclosing all polygons exactly.
>
> PostGIS could do this by applying several successive operations of
> GeomUnion(geometry, geometry) in a loop (n=100000) . It is clear that I
> will get huge polygons (with several 1000 vertices).
>
> My trials so far have not been very successful: High disk I/O load
> leading even sometimes to postgres errors like
>
> "invalid page header in block ..."
>
> and, therefore, VERY strange results
>
> My question now:
> - Is there a way for PostGIS to process this task?
> - I am looking for a better way than the one I originally proposed.
> - The way I proposed seems to produce too much load. How to reduce it?
> - Is there a limitation on vertices in polygons etc. turning my
> application into crash?

A non-PostGIS method you coul dtry that might work, depending on
your 'neighborhood' is to exploit the fact that all neighboring polygons
will have shared edges and the outer boundary polygons will not.

So it could be that you could make a list of all edges and cull all those
that are referenced twice and the resultant set of edges should be the
boundary you want.  note that most duplicated edges will probably be
in reverse order

HTH

Norman