Many people are fascinated by maps, and most find them useful, though not in all situations. A lone driver, without a map-reading navigator, will find it difficult to use a map. New in-car navigation systems are designed to help such a driver, or one who is without map-reading skills but is able to follow directions.
The remainder of this subsection uses maps to introduce some important terms and concepts. It also examines a navigation system, used both in cars and in hand-held devices, as an example of the application of computer systems to problem solving.
Maps use latitude and longitude to form a two-dimensional grid that covers the curved surface of the earth.
Altitude or depth (based on a notional sea level) can be superimposed on the latitude and longitude grid using lines connecting adjacent points of the same altitude or depth called contour lines. Contour lines give a map-reader (using a two-dimensional map) an idea of the topography of the area covered by the map.
A map showing only latitude, longitude and contour lines might be of great interest to a geographer. But such a map would be almost useless for, say, a rambler or a driver unless other features, such as roads and villages, were also shown on the map. Examples of two types of map are shown in Figure 4.
My description of a map implies that maps can be made up of separate layers of data. The idea of layering can best be understood if you imagine that each of the following layers is printed on transparent overlays, except the first (the grid) which is printed on opaque paper:
the underlying grid of latitude and longitude;
contour lines showing altitude;
features such as rivers, roads, buildings and boundaries;
the names of towns, roads, hills, rivers and other notable features of the landscape.
Other types of map may have quite different layers. For example, part of a map showing the incidence of cholera is shown in Figure 5. This was produced by Dr John Snow in 1854, and is a classic example of medical cartography. It proved that cholera was a water-borne disease.
Early maps were drawn using counted paces, and local knowledge for place and feature names. However, as the need for greater accuracy grew, geographical data began to be gathered by a painstaking and exacting process of measurement carried out by surveyors. The tools used in the task included chains of an exact length, accurate clocks, sextants and theodolites. Modern mapping still uses some of these, but also relies on aerial photography, on remote sensing from orbiting satellites (described in Example 3) and, increasingly, on the global positioning system (GPS) described below.