LandScan USA employs a dasymetric population distribution model to depict the location of people in both space and time. The model is an extension and enhancement of the methodology used in the LandScan Global project, but also includes diurnal variations at a much finer spatial resolution. Nighttime (residential) and daytime population data are produced at 3 arc-second (~90m) resolution. The population distribution model incorporates census data at the block-level; demographic attributes including age, sex, and race; and integrates other socioeconomic data such as journey to work. Ancillary spatial data including high resolution imagery, transportation infrastructure, lidar and automated building extractions, and parcels are used to spatially refine the population distribution. Other vital information used to enhance the LandScan USA output includes cultural attractions, academic institutions, prisons, shopping malls, and commercial areas. Validation and verification is accomplished on a county-by-county basis by assessing the input data for inaccurate, missing, or outdated data.