I. Introduction
Spatial data is everywhere. Geospatial Web services such as Google Maps, in-vehicle GPS navigation systems, GPS-enabled mobile phones, and a host of accompanying location-based services have become part of our daily experience. Enormous quantities of spatial data is constantly being generated from various sources such as satellites, sensors and mobile devices. NASA's Earth Observing System (EOS), for instance, generates 1 terabyte of data every day [1]. A decade ago, it was estimated that 80% of all business data stored in existing databases had spatial attributes [2]. The percentage today is probably even higher, as the ability to track customers and inventory has become cheaper and easier.