As California's population continues to grow, portions of the state are undergoing the process of urbanization. Although it might appear simple to determine what land is urban and what is not, different interpretations of "urban" can complicate discussions of farmland preservation, development patterns, placement of infrastructure and other issues. It is difficult to imagine how someone could define downtown San Francisco as anything but urban. But what about a Wal-Mart parking lot, or ranchette development, which to urban planners might as well be a forest but to a conservationist represents a loss of habitat? For land to be urbanized, does it merely need to be fenced off, or does it need to have a skyscraper? Although the definition of "urban" depends on the question you are interested in answering, the definition of most utility often will depend on the data available. Analysts have created several datasets that characterize urbanization (and other land uses) over broad geographical extent, including, • Census Urbanized Area (1990, 2000) • National Resources Inventory (NRI) Land Cover/Use (1982, 1987, 1992, 1997) • California Farmland Mapping and Monitoring Program (FMMP) (1984–2000) • US Geological Survey, National Land Cover Dataset (USGS) (1992). Each of these datasets takes aim at a specific definition of urbanization. The Census Bureau is interested in where people live, NRI in land removed from the rural land base, FMMP in land removed from agricultural production, and USGS in areas of constructed material. Beyond varying definitions, each dataset uses different source data and applies different methodology. The chart, which represents the Bay Area (sans San Francisco) plus Santa Cruz and Monterey counties, illustrates a large spread in values for any given year and in trends over time. For example, • The Census data shows a decrease in urbanized land between 1990 and 2000, indicating that a Census Bureau change in methodology overwhelmed changes on the ground • NRI data indicates that the region is more urban and is urbanizing at nearly twice the rate as determined by FMMP • 1992 USGS shows 40% less urbanized land than the 1990 Census. To examine differences between these datasets in spatial detail, we have mapped urbanization patterns for the City of Vacaville in Solano County. Located on the western edge of the Sacramento Valley, Vacaville is a city with steady growth in housing, retail and offices, surrounded by grazing and agricultural land. The map shows obvious disparities. For example, the Census Bureau considered the offshoot of incorporated land on the northeast end of town as urban in 1990, but not in 2000. The other datasets identified only spotty development in that area. Beyond the city limits are many pockets of development considered urban by USGS, but not by the other datasets. Conversely, the USGS data shows pockets of non-urban land within the developed area, while FMMP and Census data do not. It is also evident that transportation corridors, which are considered urban by the USGS, are not according to the Census and FMMP. Many of these discrepancies can be explained by the scales of analysis. The Census works at the census block level, which is often 2 to 3 acres in urban settings, several times greater in the suburbs, and hundreds of acres in rural areas. The FMMP is interested in plots greater than 10 acres, and USGS operates on a pixel size of only 30 meters. Other differences are due to definitions. In some places where development has occurred, there are simply not enough people for the Census to consider it urban. And while USGS considers the structures in these areas urban, the remaining spaces between the structures are often too small for FMMP to consider as valuable farmland, or too enclosed for NRI to classify as natural habitat. To those entities, that land is already urban. To highlight the ambiguity, FMMP has created classification called "other," which perhaps is overly broad to be useful, yet which often contains land in transition, or that is difficult to define. USGS has a class called "urban/recreational grasses" meant to identify green space within the built environment. Both classes have land with urban characteristics. All of this goes to show that land you might call urban, and which may be indisputably developed to some degree, may not be considered urban by someone else. Proper selection and usage of this data requires an understanding of its finer points.