Qualitative thematic maps are not dependent on numbers (data). For example, I can map the hospitals in Manhattan without counting them. I may only be interested in their locations to see where the medical gaps are. Then I could layer the map with population data to determine how many people are served, theoretically, by each one—and then compare that ratio with ones for the other boroughs of New York City.
[Right] Key to James Cowles Pritchard's "Ethnographical Map of Africa, from the Earliest Times, Illustrative of Dr. Pritchard's Natural History of Man and His Researches into the Physical History of Man" (1851).