There are three classes of maps: 1) general reference maps which show the locations of a variety of features; 2) thematic maps which show the distribution of a single attribute (characteristic) or the relationship between several attributes, and; 3) charts which are used primarily for navigational purposes.
Thematic maps can cover a variety of characteristics from soil types to population density. It is the cartographers responsibility to make sure that the map shows the correct distribution or the relationship between the various attributes.
Thematic maps can show not only the distribution of a single attribute, such as the result of a presidential election by state, but they can also show the relationship between several different attributes. For instance, a thematic map could show the results of that same presidential election with the number of votes divided according to the gender or age group of the voters.
Some maps that deal with a single attribute such as population, may not necessarily be thematic maps. If the map shows the actual location where the people live, it would be a general reference map. A map showing the distribution of that same population would be a thematic map.
Until fairly recently, thematic maps were usually made with a small scale, because the data was rather coarse, and it was more important to show the basic distribution pattern than the map location for the data. In recent years, however, better data has become available and thematic maps are being made with a larger scale to show more accurate spatial information.
When designing a thematic map, a cartographer must be careful to portray the data on the map so that it will be easy for the audience to use and understand. This is accomplished mainly with the marks and symbols that the cartographer uses to represent the data. The designer should also give an adequate locational base for the map.