Accuracy assessment estimates, identifies, and characterizes the impact that arises from all of the sources of error. This chapter reviews the history of positional and thematic accuracy assessment of maps created from remotely sensed data. From the very first days of aerial photogrammetry, positional accuracy has been assessed by comparing the coordinates of sample points on a map against the coordinates of the same points derived from a ground survey or some other independent source that has been deemed to be more accurate than the map. In 1937, the American Society of Photogrammetry established a committee to draft spatial accuracy standards for maps made from remotely sensed data. The United States (US) Bureau of the Budget published the US National Map Accuracy Standards in 1941. The establishment of the standards was a critical step in implementing consistency in positional accuracy across the US