Under optimal conditions, the average GPS device is accurate to within a handful of meters. However, there are many variables that affect how accurately a GPS performs in the real world.
Understanding these variables helps you increase the accuracy of your GPS, and to understand when it might not be providing with you with the most accurate information. The quality of the GPS receiver itself also plays a large part in the accuracy of the information it provides.
According to the United States government’s website on GPS, the average GPS should be accurate to within 7.8 meters with a 95% confidence rate. This statistic assumes a device that is only relying on the pure GPS signal and not using other technologies and reference points to increase accuracy. The quality and technology of the device being used and the operating conditions can cause this number to vary greatly.
The manufacturers of GPS technology have their own claims as to how accurate their devices are. Garmin, for example, says its devices are accurate to 15 meters, on average. However, companies can manufacture higher end GPS devices that use alternate signal sources to create more accurate readings.
Lockheed-Martin plans to launch a new tier of GPS satellites starting in 2013. These “Block III” satellites are going to allow new GPS devices to be much more accurate on average than those currently on the market. According to Lockheed-Martin, the newer system should be accurate to within 3 feet of the user’s position.
Various types of interference prevent GPS devices from reaching peak accuracy. Because GPS works on what is essentially a line-of-sight signal, obstructions between your device and the sky are the most common reasons that accuracy is compromised.
Being indoors, for example, almost always makes GPS less accurate and will sometimes render a GPS device nonfunctional.
However, being underneath a tree canopy or in the shadow of large buildings could also make your GPS less accurate than it would be in peak operating conditions.