What does Shape_area mean? I noticed that it is not the sum of Aland and Awater. What are the units of these three?
The units of ALAND and AWATER are square meters. E.g., tl_2015_us_county.dbf has a record for Cuming County with ALAND = 1477895811 and AWATER = 10447360. From Wikipedia the area is 575 mi^2. If you convert 575 mi^2 to sq. meters you get same result as ALAND + AWATER