4. The method of claim 1 , wherein the geographical viewport region comprises a circular geographical region that encompasses a rectangular geographical region displayed on the client system.
4. The method of claim 1 , wherein the geographical viewport region comprises a circular geographical region that encompasses a rectangular geographical region displayed on the client system. 5. The method of claim 4 , where the geographical viewport region has a size corresponding to a size of the rectangular geographical region displayed on the client system.