A number of sources of error can occur that will affect the accuracy of your measurements.

Obstructions: Generally GPS signals are much better in an open field than they are in the trees, next to a building, or in a canyon. This is due to the signal being reflected off the surface of the object and bouncing around several times before it gets to your receiver. This is called multipath error and the result is an increase in the length of the signal which can degrade the accuracy of the measurement.

Atmospheric Delay: GPS signals can also bounce around in the ionosphere and troposphere, which can consequently alter the length of the signal and thus the time it takes to reach the surface. This will also change the location of the position being measured.

Selective Availability: This is the intentional scrambling of the signal to degrade the accuracy. The Department of Defense turned selective availability off in May 2000, but it can be turned back on at any time.