How accurate is my GPS?

How accurate is GPS? The answer is, of course, it depends.

The accuracy of your GPS depends on a number of factors:

• satellite signal. This is a complicated combination of things including internal satellite clocks and control. But there's not much you can do about this.
• satellite geometry. Your GPS determines its position using the "best four" from a larger constellation of satellites. Best accuracy occurs when three of the satellites are near the horizon spread 120 degrees apart around you with the fourth satellite directly overhead. The satellites move and your GPS may decide on a different "best four" from time to time, so accuracy changes. There is not much you can do about this either.
• blocking and multipath. The things around you (e.g., leaves, rocks, buildings, your body) can momentarily block the signals from one or more satellites or cause the signals to bounce, introducing errors. Everything else being equal, you get the best accuracy out in the open when you are standing still.
• GPS receiver. Receiver accuracy depends somewhat on the speed and sophistication of the hardware and software in your GPS, and the quality of the antenna. A dedicated handheld GPS receiver has a better GPS antenna than a smartphone.

So after all that, the position that your GPS indicates could be right on. Or it could be out by 5 or 10 m, maybe more. That means if you're using your GPS to navigate to a known location, it might get you to within 5 or 10 m (maybe farther away). But if that known spot was also determined by GPS under different error conditions, its error might be 5 or 10 m in the other direction. So your GPS might only bring you as close as 10 or 20 m. Or closer. Or farther. And it might be different an hour or day later.

Many of the points and trails in the projects here on THEN are the result of GPS fixes. Therefore...

• if you're looking for a specific point, your GPS will not necessarily lead you directly to it... but close!
• if you're following a trail on one of the downloadable GPS maps, your trajectory will not necessarily coincide exactly with that on the map... but it won't be far off. Try zooming out a bit.

TimmiT History Exploration Notebook