Finding Ring Smaller than a certain tolerance

Question

In Simplifying a shapefile for world timezones

http://efele.net/maps/tz/world/

all generalize/simplify algorithms fail on 5 countries/states, shown in this shape file

http://enclick.com/tz_world_handpicked.rar

The failure of the simplifying scripts is due to small rings below the size of the tolerance used for simplifying. The entire multi-polygon is deleted for each of these countries

How can I find and delete rings below a certain size with QGIS?

Answer 1

First you have to explode the features in you shapefile, so that you don't have multi-polygones anymore (Multi-to-Single), then you add a column for the area and calculate the size for each polygon. Delete those which are smaller than the required size.