I'm a new user of Quantum GIS (1.7.3) and I need to repeat a task multiple times, so I'm wondering if there is a quicker way to do the following (with three or more polygon layers - each layer has a single polygon):
The effect will be to have three separate single-polygons shapefiles merged into one (larger) single polygon shapefile, with a buffer around it.
Answers in PostGIS would be useful too as my data is stored there. Thanks!
Edit: Adding PostGIS information
The multiple polygons I want to Union-Dissolve-Buffer are the result of a catchment basin point table that I've created in PostGIS, converted (in R) to an alpha-hull (i.e. concave hull) polygon shapefile using the CRAN alphahull package. To create the original catchment table in PostGIS, for example, I do this:
CREATE TABLE catchment_10304 as select id, geom_4269, (select sum(cost) from ( SELECT * FROM shortest_path(' SELECT gid AS id, start_id::int4 AS source, end_id::int4 AS target, newcost::float8 AS cost FROM network', 10304, id, false, false)) as foo ) as cost from node; CREATE TABLE catchment1_10304 AS SELECT * FROM catchment_10304 WHERE cost < 1364;
Then I do it for three different nodes (representing the boundaries of a region). To create the total catchment area for the region, I want to Union-dissolve-buffer with the three or more individual catchment area tables.