Derived data layers are completely dependant on the aim of the project. Each data layer would be input individually and topologically integrated to create combined data layers. Based on the data model, e.g. vector or raster, and the topological structure, selected data analysis functions could be undertaken. It is important to note that in vector based GIS software the topological structure defined can only be traversed by means of unique labels to every feature. The proprietary organization of data layers in a horizontal fashion within a GIS is known as spatial indexing .
Spatial indexing is the method utilized by the software to store and retrieve spatial data.
A variety of different strategies exist for speeding up the spatial feature retrieval process within a GIS software product. Most involve the partitioning of the geographic area into manageable subsets or tiles . This is done simply to improve query performance for large data sets that span multiple map sheets, and to ensure data integrity across map sheet boundaries. The method and process of spatial indexing is usually transparent to the user. However, it becomes very important especially when large data sets are utilized.