Digital soils data are produced by the Natural Resources Conservation Service (NRCS) of the US Department of Agriculture from maps prepared by soil scientists. Soil scientists develop these maps based on field data, soil samples, landscape patterns and other ancillary data.
Most soil maps in the lower 48 states are compiled at a scale of 1:12,000. Digital soil maps are digitized from the compiled maps and digitized soil map boundaries are generally within 0.01 inch of the boundaries as represented on the source maps. At a scale of 1:12,000, 0.01 inch on the map is equivalent to 10 feet on the ground. If the soil map was developed at 1:24,000, 0.01 inch on the map is 40 feet on the ground.
However, mapped soil boundaries are themselves an interpretation of where one soil unit transitions into another. In areas with an abrupt change in slope, the transition from one soil to another tends to be fairly abrupt. However, in gently sloping areas, transitions can be gradual and themselves can be many meters wide. In these situations, the boundary as depicted on a map or a computer screen is itself not all that accurate; in fact the boundary represents a transition zone rather than an abrupt change.
Finally, it is always a good idea to review any metadata that exists for the soils data. Depending on the soil dataset, you may find information that pertains to the specific data with which you are working.
