Geographical data can be organised into two basic structures or models: raster (cellular) or vector. In grid-based GIS, map data entry takes place through digitisation of map features and subsequent vector-to-raster conversion 3 of vector files. In vector-based GIS, scanned maps or digital imagery need to be converted to vector structures.    
In general, efficient spatial data manipulation and analysis require converting structures from vector to raster or from raster to vector. Today, only high-level systems incorporate full raster and vector capabilities.