An area frame is a collection of well-defined land units that is used to draw survey samples. Common land units composing an area frame include states, provinces, counties, zip code areas, or blocks. An area frame could be a list, map, aerial photograph, satellite image, or any other collection of land units. Area frames play an important part in area probability samples, multi-stage samples, cluster samples, and multiple frame samples. They are often used when a list of ultimate sampling units does not exist, other frames have coverage problems, a geographically clustered sample is desired, or a geographic area is the ultimate sampling unit.