In the early growing season of 2012, 10 east-west field transects were established in California, spanning horizontally across the major cropland areas in California (Fig. 1). The transects ranged in distance from 20 to 54 km across the Central Valley. Data were collected for every field along each transect based on visual inspection of each field at a location on the edge of the field adjacent to an access road. Digital photos were taken for each field, and descriptive information was recorded including geographic location (latitude and longitude), initial subjective classification (e.g., crop and idle at present time), bare soil condition (e.g., present or not, tilled, beds shaped, irrigated wet soil, and flooded), weed condition (e.g., present or not, color, fractional cover, height, and type), cover crop condition (e.g., present or not, color, fractional cover, height, and type), and crop condition (e.g., present or not, type, structure, color, and condition, including emergent, growing, senescent, residue, recently harvested, fractional cover, height, and irrigation system type). Data were entered into a geographic information system database that linked geo-registered field locations and associated attributes collected during the field surveys. Two early season field surveys were conducted for each transect (April/May and May/June) and sampled all fields along each transect route. Since the original surveys contained a very small percentage of fields that did not have a clearly established crop, two late season field surveys were conducted in September and October. Due to logistical constraints, these surveys focused on a subset of fields identified as fallow by either the NASS or ACCA fallowland maps for August 2012. A general principle of ground data collection for accuracy assessment was to collect 100 samples per classification category (e.g., cultivated or fallow cropland) when dealing with a large area.41 In order to meet such sampling criteria for accuracy assessment, we sampled over 1500 fields across the 10 transects throughout the growing season of 2012. We used the early season field surveys to assess the accuracy of ACCA-derived cropland map, and late season field surveys to validate ACCA-derived fallowland map of 2012.