Geographic Information Systems (GIS) have been widely used in many applications for exploring large georeferenced data. With the recent advances in Internet technologies, a large volume of GIS data is available on the Web. As a result, an increasing number of emerging applications began to provide tools for accessing these data. However, exploring these large data sets through spatial queries can be very time and resource intensive. The need for interactive spatial queries has arisen in many GIS applications for efficient decision-support. Another issue related to online query processing in...