Evaluating the accuracy of spatial data is important to determine appropriate use of these data. However, a good method has not been documented to measure locational accuracy. The Global Positioning System (GPS) reduces the difficulty of measuring the location of objects and enables non-surveyors to determine their location with relative ease. This study applied a straight-forward, repeatable, and statistically sound method of estimating the horizontal accuracy of GPS-derived location data. We concentrated on the spatial accuracy of points because points represent simple locations and not cartographic abstractions such as lines or polygons. When GPS coordinates are taken at surveyed locations, the quantity of interest is the difference from the surveyed (assumed true) coordinates. This difference in coordinates is a bivariate quantity and the probability distribution function (PDF) can be described by an ellipse with the center at and . An ellipse is an appropriate shape for a PDF; it has two dimensions but is not rectangular because the joint probability of points occurring in the corners is very small, and it is generally not circular because X and Y are not necessarily the same. There are three ellipses of interest: the standard ellipse, the confidence ellipse, and the tolerance ellipse. The standard ellipse is a descriptive tool used to visualize the ellipse's shape and orientation. It contains about 40% of the sample, is not dependent on the sample size, and cannot be used for statistical inference. The other two ellipses have identical shapes and orientation but different major and minor axes. The confidence ellipse is an estimate of accuracy; the sample mean is or is not significantly different from the survey locations at a given �. The tolerance ellipse is an estimate of precision; a given percentage of the population sampled is enclosed in the tolerance ellipse at a given �. Thirty-six locations were measured and compared to surveyed locations. The average offset was -1.13 m in the northing (Y) direction and 0.18 m in the easting (X) direction. Hotelling's one- sample test determined that H0 (no significant departure from the survey locations exists) was rejected at the 0.05 level, which indicates there was a systematic error in the sample in the south and east directions. Ninety-five percent of the population sampled (at the 0.05 level) was contained in an ellipse that was centered on 0.18, -1.13, and had a major axis of 7.49 m, and a minor axis of 5.12 m with an angle of 87.74o. Thus, if an additional point were taken, we are 95% confident that it would fall within this tolerance ellipse.