I spend a lot of time out on the trails in Jasper National Park with a GPS these days, so I want to know how accurate my unit is. My Garmin Vista HCx displays an accuracy number (EPE, or Estimated Position Error), typically ranging from +-3m to +-10m. One would think that this means that I am within 4-10m of my true location. However, this is not the case. An EPE of 4m actually means that there is a 50% chance that you are within 4m, but also a 50% chance that you are further off! However, there is a 95% chance that you are within 10m, and a 98.9% chance that you are within 10.2m. See the bottom of this article on GPS accuracy for more information.
Worse, one cannot easily compare the EPE among different GPSs, even units made by Garmin. This is because the EPE is a number that Garmin arrives at through a proprietary formula. Different manufacturers arrive at their EPE differently, and even among models made by the same company, the calculations may vary. So EPE is merely a qualitative measure of accuracy.
Interestingly, older Garmin units, like the 12 series, used to display DOP (Dilution of Position). This measurement is calculated according to a standard formula, so it can be used to compare accuracy of a location among different units. Yet it was dropped from more recent models, presumably because it is just a number (a DOP of 3 is better than a 4), so it doesn’t mean much to the average user.
Once you have recorded a trail, there is no way to compare its relative accuracy to other GPS recordings of the same trail because none of the accuracy information is stored with the waypoint or track data. For more details on all this, read this forum thread on GPS accuracy.
What does this mean for the average user?
1. After you turn on your GPS, allow it to record its position in one spot for a while before taking a waypoint. Accuracy tends to increase once a GPS has been on for a few minutes.
2. Keep an eye on the accuracy numbers. If they go up to for example 15m, then your track may be off by as much as 50 m. Normally this is not a big deal, but it could mean that a trail running close to a creek may appear to be on the other side of the creek. Keep a close eye on the map, and know how to read it. I always carry paper maps as backup.
3. If you use your GPS to record trails, get one of the more advanced units, like the GPSMAP 62s. They have more sensitive antennas, which work well even within buildings or under tree cover. My Vista HCx also performs well in the forest.
But most importantly, get out there, and have fun!