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I have a set of postcode boundary data that currently goes down to the most detailed level possible, like this:

AB1 2CD

I would like to 'simplify' the data so that it just shows the first part of the postcode, and thus a simpler, higher-level boundary. Essentially I just want:

AB1

In MapPublisher, I can import the data, apply a Selection Filter that looks for 'AB1' (for example), and then use Illustrator's pathfinder tool to merge this into one polygon consisting of just the outer boundaries of the smaller constituent polygons.

However, the problem I have is that MapPublisher can't export this data out as a Shapefile.

In Quantum GIS, I can select individual sectors by simply searching for 'AB1' in the attribute table. However, I don't know how to go about A)'merging' the individual areas into the one big shape or B)exporting them all to a new Shapefile.

Can anyone help? Is Quantum the best (free) tool for the job?

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up vote 2 down vote accepted

This can be done easily in QGIS.

The operation you are looking for is called dissolving. You can find the dissolve tool from Vector menu -> Geoprosessing tools -> Dissolve. You just have to define the field that contains the first part of the postal code. If this kind of field doesn't exist you can create one using field calulator.

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How can I do this so all the separate dissolved sectors go into one file rather than separate files? Also, is there a way to automate it so that I can run dissolve once and it just dissolves all the sectors? –  ElendilTheTall Dec 3 '12 at 13:02
    
The dissolve tool creates only one shape file that contains these combined/dissolved features. How did you get separate files? Is your source data in many separate files? Then you can use merge tool from Vector menu -> Data Management tools -> Merge shape files to one. Or propably you want to do the merging before and then dissolving to this merged file. –  LauriK Dec 4 '12 at 18:40
    
The files come separately in top level areas, for example AB, CD, EF. Each file is split into many polygons: the lowest postcode level, e.g. AB1 2KL. So I'd like to dissolve these into sectors one level down from the top - AB1, AB2 etc, then merge these back into AB. That way, when I import the file into MapPublisher, I have a file with just sectors rather than all the little postcode polygons. –  ElendilTheTall Dec 4 '12 at 22:01
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