How Accurate is a GPS Receiver's Calculations.

I have often wondered about the accuracy of my Global Positioning System (GPS) receiver. 

Outdoor Quest/Blake Miller image

The accuracy inherent in my receiver has generally been "good enough for me." That said, I was interested in finding technical information to build on my field experience.

I did a bit of google surfing and found a fine article from Outside Magazine writer Erin Beresini titled Your GPS Is Lying to You About Distance,  Outside Magazine, Dec. 7, 2015.  She distills a complex scientific paper by researchers into understandable terms.

The bottom line for backcountry hiker is that GPS receivers overestimate distance.  There are three reasons for this:

1. "The first is positioning error, or the fact that there’s a difference between where you actually are and where your GPS thinks you are at any given point in time.
2. The second error is the variance of the GPS measurement. Even if you don’t move, the samples your GPS takes won’t each be in the same location. In other words, your samples will form a cloudlike cluster of points around your actual location. The smaller that cluster and the closer it is to your actual location..,
3.  The third is the autocorrelation of GPS measurements. If each measurement is off from your actual location by approximately the same amount, they’re said to be highly autocorrelated, "  Outside Magazine, Dec. 7, 2015. 

There are many other factors that impact the accuracy of a GPS receiver.  These include atmospherics, solar flares, heavy treed canopy, terrain masking and freeway overpasses.   ]In discussions with serious back country hikers ( Search and Rescue members) heavy weather can impact accuracy.

To improve GPS receiver accuracy consider enabling the Wide Area Augmentation System (WAAS).  Newer units can also take advantage of the Russian satellite system known as GLONASS. It is similar to our GPS system. Enabling WAAS and GLONASS combined offers a significant increase in available satellites for navigation process and give the receiver the time needed to establish solid positioning information.

Hard numbers are only vague estimates.  For example, a basic recreation receiver should be accurate to +/- 15 meters.  With WAAS enabled accuracy could become as good as +/- 3 meters.

A GPS receiver is great to have but don't leave the map and compass at home.

Outdoor/Blake Miller image

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