search for: baincome

Hi, I am using maptools to plot air quality data on a map. Each measurement point is mapped to a postal code area. This yields pictures with discrete borders, like so: http://dl.dropbox.com/u/27415200/baincome.png The problem is that the size of a postal code area doesn't mean much in this context. Moreover, only a small minority of all the postal code areas has a measurement sation. Are there any ways/tools to "interpolate" the various (strategically chosen) measurement stations? I am look...