Geographical Information Systems (GIS) are systems for capturing, storing, checking, manipulating, analysing, and displaying data which are spatially referenced to the Earth. Among the most important applications of GIS are Geographical Information Systems for Transportation (GIS-T).
Two fundamental geographical data models, the raster model and the vector model, are used for representing geographical data: the raster model and the vector model.
In a raster representation, the Earth's surface is divided into an array of cells that are usually square or rectangular. All persistent geographical variation is expressed by assigning attributes to these cells. These attributes can represent the type of the cell, e.g. building or road. When information is represented in raster form all details about variations within the cells are lost and the cell's attributes can only represent a simplification. For a precise representation the cell size has to be small enough in order to minimise the amount of information lost. However, a small cell size dramatically increases the storage memory required for the raster representation. Data encoded using the raster data model are particularly useful as a backdrop map display because they look like conventional maps and can communicate a lot of information quickly to humans. However, the raster representation is not very useful for computerised analysis of the road network.
In the vector model, each object in the real world is classified into a geometric type: point, line, or area. Points are recoded by their coordinates, lines as points defining the vertices of the line, and areas as a series of lines that close to form a polygon. The vector representation of the road network is particularly useful as it is very storage efficient and well suited for various applications. Most transport related applications, e.g. route calculations, are based on a vector data model of the road network, e.g. the Geographic Data File.