At the center of all spatial analysis is the concept of place. The Earth’s surface comprises some 500,000,000 sq km, so there would be room to pack half a billion industrial sites of 1 sq km each (assuming that nothing else required space, and that the two-thirds of the Earth’s surface that is covered by water was as acceptable as the one-third that is land); and 500 trillion sites of 1 sq m each (roughly the space occupied by a sleeping human). People identify with places of various sizes and shapes, from the room to the parcel of land, to the neighborhood, the city, the county, the state or province, or the nation-state. Places may overlap, as when a watershed spans the boundary of two counties, and places may be nested hierarchically, as when counties combine to form a state or province.
Places often have names, and people use these to talk about and distinguish between places. Some names are official, having been recognized by national or state agencies charged with bringing order to geographic names. In the U.S., for example, the Board on Geographic Names exists to ensure that all agencies of the federal government use the same name in referring to a place, and to ensure as far as possible that duplicate names are removed from the landscape. A list of officially sanctioned names is termed a gazetteer, though that word has come to be used for any list of geographic names.
Places change continually, as people move, climate changes, cities expand, and a myriad of social and physical processes affect virtually every spot on the Earth’s surface. For some purposes it is sufficient to treat places as if they were static, especially if the processes that affect them are comparatively slow to operate. It is difficult, for example, to come up with instances of the need to modify maps as continents move and mountains grow or shrink in response to earthquakes and erosion. On the other hand it would be foolish to ignore the rapid changes that occur in the social and economic makeup of cities, or the constant movement that characterizes modern life. Throughout this Guide, it will be important to distinguish between these two cases, and to judge whether time is or is not important.
People associate a vast amount of information with places. Three Mile Island, Sellafield, and Chernobyl are associated with nuclear reactors and accidents, while Tahiti and Waikiki conjure images of (perhaps somewhat faded) tropical paradise. One of the roles of places and their names is to link together what is known in useful ways. So for example the statements “I am going to London next week” and “There’s always something going on in London” imply that I will be having an exciting time next week. But while “London” plays a useful role, it is nevertheless vague, since it might refer to the area administered by the Greater London Authority, the area inside the M25 motorway, or something even less precise and determined by the context in which the name is used. Science clearly needs something better if information is to be linked exactly to places, and if places are to be matched, measured, and subjected to the rigors of spatial analysis.
The basis of rigorous and precise definition of place is a coordinate system, a set of measurements that allows place to be specified unambiguously and in a way that is meaningful to everyone. The Meridian Convention of 1884 established the Greenwich Observatory in London as the basis of longitude, replacing a confusing multitude of earlier systems. Today, the World Geodetic System (WGS84) of 1984 and subsequent adjustments provide a highly accurate pair of coordinates for every location on the Earth’s surface (and incidentally place the line of zero longitude about 100m east of the Greenwich Observatory). Elevation continues to be problematic, however, since countries and even agencies within countries insist on their own definitions of what marks zero elevation, or exactly how to define “sea level”. Many other coordinate systems are in use, but most are easily converted to and from latitude/longitude. Today it is possible to measure location directly, using the Global Positioning System (GPS) or its Russian counterpart GLONASS (and in future its European counterpart Galileo). Spatial analysis is most often applied in a two-dimensional space. But applications that extend above or below the surface of the Earth must often be handled as three-dimensional. Time sometimes adds a fourth dimension, particularly in studies that examine the dynamic nature of phenomena.