Augmenting SAND with a Spherical Data Model ∗

SAND is a spatial database and information browser system developed at University of Maryland which supports 2D and 3D data models. The experience in extending SAND to include a spherical data model thereby enabling SAND to perform spatial queries on spherical data and to browse their results in an incremental manner is described. The focus is on the issues that arise in adding a spherical data model to a database that has been built on the basis of the planar data model. Special emphasis is placed on how the data structure was chosen for the task. The various geometrical primitives that were needed for modeling spherical data as well as the algorithms that were required in the implementation are also described. As SAND is based on the quadtree representation, a natural method for adding spherical data to SAND was to find a spherical adaptation of the quadtree. The sphere was mapped into a quadtree representation by projecting the sphere onto a cube, and then constructing the quadtree on the faces of cube. Alternatively, it is also possible to map the sphere into a plane, and then simply use a planar quadtree. Some of the differences between these approaches are discussed. In addition, the display models and visual query mechanisms used in SAND with our new spherical data model to ease the task of navigating through the data are described. By adding a spherical sector primitive, it is possible for users to locate data on the sphere which are located in a spherical lune anchored at two antipodal spherical points. ∗The support of the National Science Foundation under Grants EIA-99-00268 and IRI-97-12715 is gratefully acknowledged.