Cartographers and computers can also use parallax, or the difference in angle between two images of the same subject, to measure altitudes. The process is similar to the way your eyes perceive depth. It allows cartographers to use remote sensing imagery to create physical and topographical maps.
For thematic maps, the shape of the world is just the beginning. When making a thematic map, cartographers have to find accurate, up-to-date sources of information for a range of social and environmental phenomena. "We use a variety of sources to best generalize the feature we want to display," says Turner. "For example, for a population density map, every 10 years in the U.S. there is a census…the new census data will be made available to the public, and we'll be able to take that information and make new maps from that."
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Cartographers must also determine which source of information is the most current, accurate and complete. Turner explains, "If we're doing a state map of Virginia, we might receive information from the state at one period, that was developed at one time. We might receive information from a city or a county that was developed at another time, and part of the fun of my job is interpreting [which source] is correct."
Most thematic maps contain a citation explaining where the information came from. A few common sources are:
Along with data about the size and shape of the planet, much of this thematic information is stored in databases. The cartographer's job is to combine the information from the various databases and existing maps to create a new, understandable map. We'll look at how this happens in the next section.