Geographic Information Systems (GIS) are a valuable tool for many types of environmental analysis. Errors within GIS data, however, can limit the usefulness of GIS technology. To control and document the error introduced during data entry, the authors have instituted a series of Quality Control (QC) procedures for use within the U.S. Environmental Protection Agency's Direct/Delayed Response Project (DDRP). Central to their approach has been the use of formalized log sheets detailing the steps necessary for validating and digitizing mapped data. Additionally, they have evaluated the accuracy and precision of the data at specific stages of the data entry process. At critical stages, these formalized evaluations were independently repeated. The authors believe that the establishment of QC procedures within the DDRP has led to very low error rates associated with GIS data entry. Their procedures and techniques described within the paper have other applications within the GIS community. (Copyright (c) 1989 American Society for Photogrammetry and Remote Sensing.)