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GPS and Map Accuracy

Some of this is obsolete info. SA was turned off in 1998, so it no longer applies. Someday I'll update this page.

How accurate are GPS receivers? How accurate are maps? What is the accuracy correlation between the two? How do we find out?

Of course, there is Selective Availabilty (SA) in GPS receivers, that variable error that the military has built into the transmission of the signal that the receivers pick up. It is supposed to degrade the signal no more than 100 meters any given direction (100 meters radius for a total circle 200 meters across) 95% of the time. The other 5% it can be over 100 meters, but won't necessarily be so. When comparing coordinates actually punched into a unit at the site with what the map says, though, I see error far more than that at times, like 500 meters. The maps I'm often working with are less than 10 years old, so I know that the topography has not changed. I have heard of problems from people on the internet (news groups and on Web sites) complain of errors as much as 8 km. They blame the maps, not the GPS receiver.

Is the map printed wrong? Is the mathematical map in the GPS receiver wrong? Is the unit misinterpreting signals? Is one satellite transmitting a bogus signal? Is signal strength good? Was the info given the map company wrong? How much is SA affecting the actual location?

Another inaccuracy problem I just found out about is not using the same UTM grid system datum in the GPS receiver as is used on the topographical map you are looking at. More on this when I figure out more on how they differ.

Use your "Back" button or UTM Coordinate Basics or Coord Explanation