4Chapter Differential Correction, DOQs, and ESRI Data
When you record a single position with a good GPS receiver, the position recorded will be probably 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.* 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 3 meters of its true location. One primary method of gaining such accuracy is called “differential correction.”