Time-Distance Mapping and Time-Space Convergence: The Southern United States, 1950-1975

Southeastern Geographer Vol. 21, No. 2, November 1981, pp. 67-83 TIME-DISTANCE MAPPING AND TIME-SPACE CONVERGENCE: THE SOUTHERN UNITED STATES, 1950-1975 Laurence W. Carstensen, Jr. Distance is an important aspect of many geographic models, and in much research it is the prime measure of space. However, distance is inclusive of many measures. "How far is the next town?," "How much is a ticket to Pittsburgh?," or "How long until we get there?" are all examples of human perception of distance. (1) The first example is absolute and expresses the idea of physical distance or the spatial interval between two places, A and B. The second relates cost-distance between the origin and destination. The last relates distance to a time interval; it is often more important to know how many days, hours, or minutes a destination is from the starting point than merely how far it is. Warntz has stated that the "tyranny of space," the influence that physical distance places on decision making, is generally reduced through innovation. He also argues that Euclidean geometry may not always be the best system for dealing with space today; other ideas of space and distance perhaps should be used more often in describing human activities. (2) Janelle developed time-space convergence, a concept that treated time and distance as a process which is based upon innovation and distance. The shrinking of time-distance during and immediately following periods of innovation with subsequent reduction of convergence, followed by no change in the time-distance relationship, results in a stair-stepped pattern over time. (3) Cartographers have produced two major types of maps to portray time-space. Isochronal maps hold physical distance constant and apply isarithms which cut the surface at equal travel times from a starting point. This map places a distorted time surface into undeformed cartesian space. Time-distance maps incorporate a constant time scale allowing deformation of cartesian space into time-space to portray the effects of varying time-distance on the physical surface. All sample points on Dr. Carstensen is Assistant Professor of Geography in the Regional Analysis Concentration at the University of Wisconsin-Green Bay in Green Bay, WI 54302. 68Southeastern Geographer such a map are located at their correct azimuths from one central location with their distances from the center set according to the time scale. Excellent examples of these two map types and their differences can be found in Bunge (4) and in Abler, Adams, and Gould. (5) None of the very few examples published covers large areas because the commonly used data collection technique is extremely limiting. Trips must be made from the proposed center point of die map to all the proposed destinations. The traveler marks his location on a cartesian base map after each previously determined time interval. By connecting equal time location marks together, the map becomes isochronal, or, by deforming the surface to produce circular isochrones, a constant time scale is derived creating a time-distance map. This method of data collection severely limits the production of maps of large areas. Neither can maps be made of earlier time periods when travel conditions and modes varied from the present. The major objective of this research is to provide a methodology for producing small-scale, historical time-distance maps by extracting data from existing road maps, and to apply this technique to evaluating the evolving highway network of the South. For automobile travel, road atlases of various years can be used as almost all contain enough information to produce maps of travel time for any desired time period or region. METHODOLOGY. For this study of time-space convergence within the Southern United States focused on Atlanta, Georgia, the following two terms are applied: levels of innovation and time. Levels of innovation refers to road type and quality and includes the number of lanes in either direction, the method of access to the highway, and the speed limit of such highways. Time refers to the travel time accrued from Atlanta to each sample city following the highway network for the year in question. The shortest time found is used in all cases. All aspects of the trips except for the level...