A fly-through represents the changing trends in a physical landscape. A spatial trend can also be evident when examining a series of related variables of population characteristics. For example, the percentage of population in age groups (5-13, 19-24, 45-54 years of age) will usually show a clear regionalization in a city, particularly in North America, with the older populations closer to the center and younger populations nearer the periphery. In this animation for the city of Omaha, Nebraska, the difference in the population by age group can be seen as a wave moving from one side of the map to the other. The city is bounded by the Missouri river to the east. Newer parts of the city with younger populations are developing on the western side. Variables such as income and housing valuation depict similar spatial trends.