I heart APRS
From Adventures with radio. Comments are welcome via email.
According to APRS Direct, my station's coverage is a roughly cardioid shape, 6km across at the longest. It's covering a good patch of the M5, at least. I suppose the reason for the shape is that Richard G7LEE over in Glastonbury has a such a good station providing coverage not only to the Levels but to a good chunk of England and Wales!
(If you are following links, click on the station and then 'Coverage' to see the plots, which may have changed since I posted this).