12 Pages

What is the Proper Unit of Analysis in GIS?—Using Tessellations and Related Database Issues

In the maps we have discussed so far, the units within which data are displayed are defined by city boundaries, U.S. Census Bureau definitions of geography, such as tracts or block groups. There are many good reasons for using such geographic boundaries for geocoding and mapping. First, any address-based system of locating places in geography is based on the definitions each political system uses to identify places. Residences and business in the United States are grouped according to political and administrative units with defined boundaries, such as cities, towns, counties, states, and the national boundary as a whole. Our concepts of space are based on such divisions-when you meet someone, one of the first questions you ask is where are you from? The usual answer involves giving the name of a town, village, or city, and when the question is more specific, such as can I get your address so I can send you a package, the kinds of addresses you have used to geocode locations in this book are in this form. Further, when you started to construct thematic maps, the boundaries used to summarize the data came from the Census Bureau’s definitions of space. All of these boundaries are arbitrary-some time in the past, somebody decided that the boundary of Riverside was going to be in this location, and then the Census Bureau officials used their rules for establishing the boundaries of census tracts and block groups, and so on. These constructions-the nature of which are independent of any consideration of what data you wish to study, understand, and use to inform decision making-were made in some cases many years ago and in a very arbitrary manner. Some of the spatial conflicts engaged in by nations we considered earlier came about because someone far away, and with little understanding of the geography and the social facts on the ground, drew an arbitrary line on a map.