Retrieval of geographic data using ellipsoidal quadtrees

Geographic visualisation systems require methods for efficient data access. Retrieval of geographic data from large databases, of tens to thousands of Gbytes, needs optimisation using spatial indexing mechanisms. This paper describes how the indexing mechanism based on Ellipsoidal Quadtrees, EQT, can be implemented in software, e.g. for real-time or Internet visualisation. EQT fulfils the so-called equal-area criterion, which means that all index cells can be constructed having equal area. For flexible usage, the data structures in the implementation phase are based on how the Lisp programming language handles hierarchical lists. The concerns of handling levels of details, LOD, and optimisation of data flow are presented. The primary objective to handle LOD in the way described in the paper is to manage and structure geographic data for different scales. This structure can reduce the data flow and the amount of data to be rendered, which means that applications of geographic information can be optimised.