[postgis-users] ESRI-style dissolve polygons

[Paul Ramsey]

Tom, thanks for the success story. More and more people are using
PostGIS for data processing, rather than data storage/retrieval. It's
a frightening use case, because it really ups the ante on the
reliability and speed of the spatial functions.  Stop it, you're
scaring me! :)

P.

On Tue, May 27, 2008 at 7:02 AM, Tom Glancy <Tom.Glancy at dnr.state.mn.us> wrote:
> We have a state-wide polygon shapefile (National Wetland Inventory) that originated as a tiled Arc/Info coverage. As a first step to normalizing the data, we wanted to do an ESRI-style dissolve to eliminate the tile boundaries. We were unsuccessful using ArcView, Arc/Info and ArcGIS, but PostGIS worked just fine.
>
> We imported the shapefile data into PostGIS, starting with 1.44M records in 6800 tiles. Here is the process we followed:
> 1 - use ST_Intersects to build a new table of gids of intersecting geometries
> 2 - examine the table of gids and identify new (dissolved) polygons
> 3 - assign all intersecting gids to the new polygons
> 4 - check that the new polygons don't overlap
> 5 - use ST_Union to union the new polygons and add back to the original table
> 6 - delete "dissolved" records from the original table.
>
> We created a function that ran all the steps automatically. The process ran overnight and completed in 8-16 hours. It found 48,000 intersecting polygons that were combined to produce 5200 polygons. The biggest combination was about 80 polygons, the smallest was 2.
>
> We thought you might like to hear another PostGIS success story!
>
> Regards,
> Tom
>
> ---------------------------------------
> Tom Glancy
> Ecological Services Division IT Supervisor
>
> tom.glancy at dnr.state.mn.us
> 651-259-5097
>
> Minnesota Department of Natural Resources
> 500 Lafayette Road - Box 25
> St. Paul, MN 55155-4025
> ---------------------------------------
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