The present paper analyzes the influence of the forest canopy on the accuracy of the measurements performed by global positioning systems (GPS) receivers. The accuracy of a large set of observations is analyzed in the present research. These observations were taken with a GPS receiver at intervals of one second during a total time of an hour in twelve different points placed in forest areas characterized by a set of forest stand variables (tree density, volume of wood, Hart-Becking index, etc.). The influence on the accuracy of the measurements of other variables related to the GPS signal, such as the Position Dilution of Precision (PDOP), the signal-to-noise ratio and the number of satellites, was also studied. The analysis of the influence of the different variables on the accuracy of the measurements was performed by using genetic algorithms. The results obtained show that the variables with the highest influence on the accuracy of the GPS measurements are those related to the forest canopy, that is, the forest stand variables. The influence of these variables is almost equally important without significant statistical differences. As was expected, those observations recorded in areas covered by an important forest canopy have larger errors than those obtained in areas with less canopy cover. Finally, conclusions of this study are exposed.