At present, accuracy of the GPS is generally accepted as being ± 20 metres in general outdoors use with hand-held GPS units.
The GPS unit must be in contact with a minimum of three satellites to make the calculations required. With three satellites, the position is referred to as a two dimensional (2D) position, and is not sufficiently accurate for most purposes, as a 2D position requires the GPS to make assumptions about some of the variables, adding error. To effectively fix a three dimensional (3D) position, the GPS must be able to see and communicate with at least four satellites. That is, it must have a clear unobstructed view of the satellites.
For greater accuracy, the satellites need to be spaced equally around the sky and not all overhead or to one side of the sky, which can occur when under trees, near mountains or in canyons or valleys. This can affect the GPS’s ability to accurately calculate position. Many of the good GPS units give an indication of the number and position of satellites being used at any given time, yielding a good indication of likely accuracy.
Another variable which affects the accuracy of the GPS is the information it is given by the user about where it is and the map being used. When setting up your GPS it is important to enter the correct time zone and map datum for the map you are using. This information should be checked prior to use and may need to be updated whenever the unit suffers a total power loss, or if changing to maps with a different datum.