Sorting Spatial Data

Sorting the underlying spatial data enables the efficient execution of operations on it in geographic information systems (GIS). The chapter differentiates the discussion on the basis of the type of the spatial data which in this case falls into raster and vector. The raster data is sorted via the use of a number of different space-filling curves which are mappings from two dimensions to one dimension. The motivation comes from the fact that the data is extremely voluminous and thus all of it cannot be fit into memory at once. Therefore, external storage must be used. The sorting is applied in order to enable efficient execution of operations. In the case of vector data, two methods are discussed. The first is based on an object hierarchy and distinguishes between occupied and unoccupied space, while the second sorts the data with respect to the space that is occupied and in essence sorts the regions comprising the underlying space so that the number of spatial objects that they contain is within the same range. In both cases of the vector data, the sorting is implicit, whereas in the case of the raster data, the sorting is explicit.