They usually consist of farmland, light suburban development or small towns and are classified as having an average of 100 people per square mile. In most instances, this is the type of land development that occurs as cities expand. Finally, non-urban areas are regions such as central Montana and western Maine, where only ten people or less live per square mile. Based upon satellite measurements of city lights, this image is a map of the urban population density of North America. Red yellow and green are urban areas, and blue is peri-urban.
Most major cities are in level areas along an ocean bay, large lake, or navigable river.
They then lay out their results on a map of the U.S., classifying each soil in terms of limiting factors. One limiting factor may be that the soil is too damp and needs to be drained before it can be farmed. Another limiting factor may be that the soil is too acidic and would have to be limed. Generally, a soil with three or fewer of the less severe limiting factors is considered to be prime soil because very little has to be done with it in order to plant a crop. When they tallied up the amount and type of soils covered by urbanization, they found that the soils with just one or two limiting factors were being urbanized the most.