Primary data sources. Those that generate data that has to be adapted or converted are called secondary

independent from the data itself. That allows us to perform opera- tions separately on the metadata, which opens many possibilities and gives them a greater value.

Two of the main functions of metadata are ensuring the correct use of data and facilitating its manage- ment, discovery and exploitation.

Geographical data, as with many other types of data, are usually created for a given purpose, and that purpose does not have to be contained in the data themselves or be easy to infer from them. When data are then used for a dif- ferent purpose, problems might arise, since the data might have some deficiencies when used in this new context. With the help of metadata, this can be solved. Metadata may help, for instance,