The development of retrieval algorithms for evaluating environmental characteristics on the base of satellite measurements is a key problem in utilizing remote sensing data. Before geographic data can be used in a GIS, the data must be converted into a suitable digital format. The process of converting data from paper maps into computer files is called digitizing. Today many types of geographic data already exist in GIS-compatible formats and it is one more reason to use GIS technologies. Nowadays, all information regarding available geographical data can be found in the Internet.
Some data is readily obtainable on line.
As an example of information on available digital data one can look at the Internet Mapping ESRI Demos Server . At that Server you will find links to demos, price lists and other useful information on available data. Most likely, data types required for a particular GIS project will need to be transformed or manipulated in some way to make them compatible with the system. Data can also be presented in different projections. This could be a temporary transformation for display purposes or a permanent one required for analysis.