Area Sampling Frame
lStratify based on percent cultivated land
lSubdivide strata into primary sampling units or PSU's
–Selected PSU's divided into secondary sampling units or segments
–Segments are selected randomly & used in
–    successive years
–20 percent segment rotation/year
lEntire sample replaced every 5 years
•The Area Sampling Frame (ASF) is a stratification of each state into broad land use categories according to the percentage of cropland present.  Since 1978, satellite imagery was the major input into stratification of land based on broad land cover definitions. Previously, aerial photography mosaics were used.  Each year NASS replaces some of the state area frames because of land use changes or the need to improve acreage estimates.
•The ASF is stratified using visual interpretation of satellite imagery. This led to improved statistical precision of numerous area frame-based estimates, including coverage estimates for major probability surveys and the 1997 Census of Agriculture. In addition, beginning in 1978 and continuing today, area sampling frames have been converted from paper-based products, subject to fire and loss, to digital versions which are more accurate and better protected from loss.
•The sampling frames are constructed by defining blocks of land whose boundaries are physical features on the ground (roads, railroads, rivers, etc.). These blocks of land cover the entire state, do not overlap, and are placed in strata based on the percent of land in the block that is cultivated. The strata allow for efficient sampling of the land, as an agriculturally intensive area will be more heavily sampled than a non ag intensive area.