The study of two or more geographic distributions varying over the same area is a study of spatial covariation , an idea we shall meet repeatedly in this book. When the two maps look alike and the two distributions ‘fit’ one another closely, we say that the two phenomena are associated by area; that is high values for population density in one area correspond with high values for environmental quality in the same area , and vice versa. Other hypothetical cases with little covariation are also illustrated in Figure 1.10.Comparing pairs of maps in this manner often tells us a great deal about the spatial covariation of different phenomena. Distributions can also be compared by statistical methods, but a discussion of these lies outside the scope of this introductory text.