The International GPS Global Positioning System Waypoint Registry ^TM

GPS Errors, SA, EPE, and Datum

The Waypoint Registry Database is a navigation aid only and not, under any circumstances, to be used as primary navigation for any application. It is not a substitute for good navigational practice. There are many sources of errors that can occur.

400 foot box

This image is a plot of a GPS track. The GPS was not moving! The box is 400 feet tall. the GPS was a Garmin 89 with a Lowe antenna, 1.5 hour sampling time, 5 or more Satellites in view, EPE between 100 and 200 feet, date August 20, 1998, location Massachusetts (before the SA switch was turned off).

Half way through the test, the datum was changed from WGS84 to NAD27-CONUS.

The radius of the path is well within the EPE value.
This gives me confidence that the unit knows how far off it can be. Have you ever checked your EPE or DOP?

From my experience with this test, The SA, and atmospherics are a larger source of errors then changing datum. You will have to decide what level of accuracy is necessary for you. If you are out in the woods, would 200 feet be enough to find the trail you are looking for, or where you left your car? 200 feet is enough to find a turnoff of a road or find a restaurant or store or how far till the end of the trail, with plenty of confidence.

Errors can also be cumulative. If you get a Waypoint off of a map, you have map errors to deal with, if you get a Waypoint from someone else, you have to add the EPE of their unit at the time the Waypoint was collected, with the EPE of your unit at the time you are looking for it. That is assuming he/she copied it down correctly and you entered it correctly. Even if you record your own track, you have to be careful, this plot shows that I moved 350 feet in just a few minutes.

I have some more ideas for some tests I would like to run, as soon as I get a few more units to experiment with, I will do so. If you have suggestions, comments,or a GPS to lend me, please send me email.

* As of May 1, 2000 President Clinton ordered the military to turn off the SA Switch. This should cause an immediate increase in the speed and accuracy of all nonmilitary GPS units. Your EPE's should now be in the range of 50 feet instead of 200. The Military still has the option to turn the switch back on in case of need.

If you still want to know more, here is a link to a site from NOAA, About Removal of GPS Selective Availability and other errors

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The International GPS Global Positioning System Waypoint Registry TM

GPS Errors, SA, EPE, and Datum

The International GPS Global Positioning System Waypoint Registry ^TM