Primary sample unit
A Primary sample unit or PSU is an area of land, typically square to rectangular in shape, that is approximately 40, 100, 160, or 640 acres in size. Within the PSU, sample points are assigned. Certain data elements are collected for the entire PSU, while others are collected at the PSU points. The size of the PSU is based on the shape, size, and complexity of the resources being inventoried. In 34 states, PSU’s are often 160-acre square parcels measuring 0.5 mile on each side. In the western United States, PSU’s are often 40-acre or 640-acre square areas; the 40-acre units are used in most irrigated areas, and the larger PSU’s are used in relatively homogeneous areas containing large tracts of rangeland, forest land, or barren land. In the 13 northeastern states, PSU’s are defined to be 20 seconds of latitude by 30 seconds of longitude, ranging from 97 acres in Maine to 114 acres in southern Virginia. In Louisiana and parts of northwestern Maine, PSU’s are 0.5 kilometer squares (61.8 acres).