One of the most powerful ways to use GIS techniques to show and understand how our world changes is to factor in the passage of time in our maps. As you probably recall, Hurricane Katrina had a devastating effect on the physical landscape of New Orleans, coastal Mississippi, and parts of Alabama in September 2005, but it was perhaps in the city of New Orleans itself where the devastation became best known to Americans elsewhere in the country and to the entire world. One of the tragedies of this hurricane and its aftermath was the way in which poverty, especially among African-American residents in New Orleans, left many people vulnerable and stranded. One question to ask about this is how did certain parts of the city end up being populated by poor people with few resources to deal with this disaster? Is this something that was always the case, and no one realized the problematic nature of the situation because New Orleans was very lucky during the past 40 years and had experienced no direct hits by such a power storm as Katrina? Did this situation develop recently, and therefore local residents and officials alike could not have anticipated the problems that might result? Issues like these are of dire importance to examine when planning for disaster relief. One way to examine this question is to map the rates of poverty among African-American residents in New Orleans over time and see if any patterns emerge that might illuminate the tragic situation and help New Orleans, other cities, and the Federal government plan more effectively for future storms hitting major urban centers.
Example: Rates of Poverty over Time in New Orleans
Using a specially prepared database, the Neighborhood Change Database, 19702000 (Geolytics, 2003), we can make valid over time comparisons possible because the census tracts have been standardized across the four census points (1970, 1980, 1990, and 2000). What was the extent of poverty among African-American residents of New Orleans in 1970?
The map in Figure 2.43 is focused on what we have all come to learn was the critical area of the city, between Lake Pontchartrain at the top of the map (the empty white space) and the Mississippi River curving through the city at the lower portion of the map (white curving lines); it was in the area where the river makes an ox-bow that many of the residents were stranded with no power, food, or means to evacuate after the storm hit and passed through the city. You can see that poverty was well established among this population in 1970. Was the situation constant over the next thirty years?
FIGURE 2.43 African-American poverty rates, New Orleans, 1970
As you can see comparing Figure 2.44 to Figure 2.43, conditions in many census tracts of this area of New Orleans improved during the decade of the 1970s. Many more areas show light gray (yellow), the color used here to show no poverty among this population in the tract, and some significant changes towards less poverty occur inside the ox-bow of the river.
FIGURE 2.44 African-American poverty rates, New Orleans, 1980
However, one section to the right where the river turns upward again seems to have become an area of concentrated poverty with several contiguous tracts with rates of 75 percent or more of the residents below the 1980 poverty line in terms of family income.
FIGURE 2.45 African-American poverty rates, New Orleans, 1990
By the 1990 Census, the trend towards less poverty in many of these tracts had substantially reversed. Fewer tracts show light gray (yellow), and poverty is beginning to hit areas between the lake and the river on the right side of the map that had been consistently better off in 1970 and 1980. The area of concentrated poverty described in Figure 2.44 has improved somewhat, but now instead of 75 percent or more residents being poor in many of those tracts, now 50 percent to 75 percent are poor—an improvement, but not a great deal of improvement.
FIGURE 2.46 African-American poverty rates, New Orleans, 2000
Between 1990 and 2000 there seems to have been little change in the pattern of overall poverty among African Americans in this part of New Orleans. A substantial number of census tracts show 50 percent or more at or below the poverty line in the critical area around the river and moving towards Lake Pontchartrain, the area that proved so prone to flooding with the storm and the collapse of the levy and canal system. Mapping this change or lack thereof over time can give the viewer a significant new perspective on the history of people and their economic and social situation in a community, and could have been used to help plan for the disaster that many expected to come to New Orleans sooner or later—it came sooner, before anyone had the foresight to use data like these to more effectively prepare for such a disaster.
Once the census tract map with the associated census data on poverty, displayed in an attribute table, is obtained, the procedures described elsewhere to construct thematic maps can be followed to make the 1970 map. Once this is in place, you can left-click on that layer, opening up the layer menu, and click on copy the layer. Left-click on the Layers indicator and click on Paste Layer(s); this will add a duplicate of the 1970 map. You can then double-click on the duplicate layer and construct the thematic map for poverty, 1980; repeat the process for 1990 and 2000 and you have the four maps needed for this example.
In Section 2 thus far, you have learned how to make maps a little more complicated, display different kinds of data, and to construct maps that tell us something about the situation of cities and their residents. These maps can inform planners for the future, anticipate needs and potential problems, and provide researchers with answers to questions of interest. The power of maps to show a great deal of information in a compact, convenient, and useful format has not been fully illustrated by any means in the examples so far. In the next examples we will attempt to demonstrate some of this ease of conveying complex information with multiple variables in ways that can be easily perceived by citizens, researchers, and policy makers alike.
