I would disagree with it being impossible, especially in the drone delivery business. Definitely hard but not impossible. The drone leave a known location to go to another location using a direct path. It is a point A to point B algorithm. Amazon could build or use a database of information regarding obstacles and traffic, to program and test well ahead of time destination between A (the warehouse) to B (i.e. every doorstep in a 30 minutes radius) and back to A. GPS is often accurate within 1 meter which should be enough for door delivery (http://www.gps.gov/systems/gps/performance/accuracy/histogra...). I agree that humans will be involved in monitoring the drones 24/7.