How accurate is a GPS receiver?

According to the government and GPS receiver manufacturers, expect your GPS unit to be accurate within 49 feet (that’s 15 meters for metric-savvy folks). If your GPS reports that you’re at a certain location, you can be reasonably sure that you’re within 49 feet of that exact set of coordinates.
GPS receivers tell you how accurate your position is. Based on the quality of
the satellite signals that the unit receives, the screen displays the estimated
accuracy in feet or meters. Accuracy depends on

• Receiver location
• Obstructions that block satellite signals

Even if you’re not a U.S. government or military GPS user, you can get more accuracy by using a GPS receiver that supports corrected location data.
Corrected information is broadcast over radio signals that come from either

• Non-GPS satellites
• Ground-based beacons

Two common sources of more accurate location data are

• Differential GPS (DGPS)
• Wide Area Augmentation System (WAAS)

Although survey-grade GPS receivers can provide accuracy of less than two centimeters, they are very specialized and expensive, require a lot of training, and aren’t very portable. Their accuracy is achieved with DGPS and postprocessing collected data to reduce location errors. The average GPS user doesn’t need this level of precision.

Clouds, rain, snow, and weather don’t reduce the strength of GPS signals enough to reduce accuracy. The only way that weather can weaken signals is when a significant amount of rain or snow accumulates on the GPS receiver antenna or on an overhead tree canopy.