Hi, I’m Sam. I’ve been learning a lot here at Linfiniti (thanks to the brilliant team!) Just like to add a quick note on one of the tasks I learnt this week.

I was working on a shapefile of the suburbs in Cape Town. A client required the suburbs to be grouped by region. After the tedious part of manually grouping the suburbs (using a created field, REGION), the unioning (dissolving) of the suburbs proved to be quick and painless through PostgreSQL, using this SQL command that implements the geomunion function:

create table ct_regions as select geomunion(the_geom), "REGION" \
 from "ct_suburbs" group by "REGION";

“ct_suburbs” is the original shapefile that was loaded into the PostGIS database using the Quantum GIS ‘SPIT’ plugin. “REGION” is the class (attribute) that I wish to union by.  And ct_regions will be the output shapefile. See the result here:

Before dissolving (ct_suburbs shows suburbs)

After dissolving by suburbs (ct_regions shows collections of suburbs that have been merged)

Hopefully this will be of some use when it comes to your own mapping!