One of the more useful concepts in spatial analysis is density — the density of humans in a crowded city, or the density of tracks across a stretch of desert, or the density of retail stores in a shopping center. Density provides an effective link between the discrete-object and continuous-field conceptualizations, since density expresses the number of discrete objects per unit of area, and is itself a continuous field. Mathematically, the density of some kind of object is calculated by counting the number of such objects in an area, and dividing by the size of the area. This is easily done when calculating population density from census data, for example, by dividing the population of each census tract or county by its physical area. But this leads to multiple values of density, depending on the objects selected and the definition of the areas used to make the calculation. Techniques of density estimation try to avoid this problem and are discussed in detail in Section 4.3.4, Density, kernels and occupancy. The book by Silverman (1998) provides additional background reading on this issue.