When you record a single position with a good Personal GPS Tracking Device, the position recorded will probably be within 5 to 15 meters horizontally of the true location of the antenna. When a surveyor uses good, survey-grade GPS equipment he or she can locate a point to within a centimeter of its true horizontal position. What are the factors that allow the surveyor to be 1,000 or so times more accurate than you are? This is a complicated subject. The answer includes “very good equipment,” “measuring the actual number of waves in the carrier” (as differentiated from interpreting the codes impressed on the carrier), and “spending a lot of time” at each site.1 We can cover only the basics in a book of this scope. But you will learn how to reduce errors so that you can record a fix to within half a meter to three meters of its true location. One primary method of gaining such accuracy is called “differential correction.”
Differential Correction in Summary
In a nutshell, the differential correction process consists of setting a GPS receiver (called a base station) at a precisely known geographic point. Since the base station knows exactly where its antenna is, it can analyze and record errors in the GPS signals it receives–signals that try to tell it that it is somewhere else. That is, the base station knows the truth, so it can assess the lies being told to it by the GPS signals. These signal errors will be almost equivalent to the signal errors affecting other Portable GPS Tracker in the local area, so the accuracy of locations calculated by those other receivers may be improved, dramatically, by information supplied by the base station.
Thinking about Error
For the logging of a given point, define “error” as the distance between what your Vehicle GPS Tracking Device records as the position of the antenna and the trueposition of the antenna. It is useful to dissect the idea of “error.” We can speak of error in a horizontal plane and differentiate it from the vertical error. This is important in GPS, because the geometry of the satellites almost always dictates that no matter what we do, vertical error will almost always exceed horizontal error on or near the surface of the earth. The fact that all the satellites are necessarily above the fix being taken generally means that vertical error will be 1.5 to 2.5 as great as horizontal error. Another useful distinction is between what we might call random error and systematic error, or bias. Random errors are deviations from a “true” value that follow no predictable pattern. Systematic errors do follow a predictable pattern. An example will be illustrative. Suppose we have a machine designed to hurl tennis balls so that they land a certain distance away on a small target painted on the ground. Of course, none of the balls will hit the center of the target exactly; there will always be some error.
More information at http://www.jimilab.com/ .