The accuracy of your GPS unit can vary depending on whether you set the mode to make use of differential GPS (DGPS), which augments satellite information with ground-based signals; a wide area augmentation system (WAAS) developed by the Federal Aviation Administration for more accurate air navigation; or go without selective availability (SA). The following table shows how accurate you can expect your GPS unit to be in each mode:

GPS Mode Distance in Feet Distance in Meters
GPS without SA 49 15
GPS with DGPS 10–16 3–5
GPS with WAAS 10 3