Environmental scientists are beginning to use the Global Positioning System (GPS) for in situ determination of the location of point and line features. The accuracy of data collected by inexpensive GPS units can be quite variable. In this study, we estimated the accuracy and precision of a simple GPS receiver and community base station system. We found that, under ideal conditions, 95 percent of the locations we derived were within 73 m of true without differential correction and within 6 m of true with correction. Taking the average of repetitive fixes at a single location increases accuracy and precision, especially if more than 50 sequential fixes are used. There is little correlation of positional accuracies obtained at different stations or between raw and differentially corrected data