Maps are rich historical sources. Like narrative documents, both the form and substance of historical maps tell a story. The form of an historical mapits artwork, its style and presentationin itself provides an insight into past eras and cultures. The substance of a map (what it shows, literally) provides a record of past landscapes and features that may no longer exist. It also reflects the priorities, sensibilities, fears, and the state of knowledge of the mapmaker and his or her cultural context.
A map offers a reader a new dimension of analysis, a visual dimension. One of the particular advantages of a map is that it conveys nonlinear and simultaneous knowledge. In a single glance at a map, a reader can tell whats going on over the whole map at a single moment in time.
In a single frame, a map also offers rich contextualization that might otherwise take pages and pages of text to convey. Because of its visual properties, almost by definition a map represents its subject in a broader contextthat is, in drawing any single place, the mapmaker also situates that place in relation to other nearby places. Sometimes relational geography is the primary purpose of a map (to show how close A is to B), and sometimes this contextualizing effect is just a fortuitous byproduct of the nature of graphic representation.
On the other hand, a single map is not very effective at showing process. A narrative can move a reader through time very quickly; a map tends to be static and to show a single place at a single moment. A map then, in distinction from written texts, can be understood as privileging place over process, contextuality over linearity.
Thematic mapping is a particularly powerful geographic tool. A thematic map shows the distribution of non-geographic features and phenomena (social, cultural, political, or economic features) in their geographic context: for example, a map of poverty rates across a city or country or a map of the distribution of deaths from cholera are thematic maps. In showing not only what is happening but where, thematic mapping makes patterns visiblesimilarities and differences, continuities and contrasts across spacethat would be extremely difficult to discern in a narrative treatment.