I'm really impressed with the accuracy of my latest GPS receiver, the Garmin eTrex 30. Capable of receiving the Russian GLONASS satellites as well as the aviation WAAS satellites, it has been achieving accuracy of 7 or 8 feet in the few days that I've been testing it. The eTrex 30 seems to consistently be several feet more accurate than my other trail GPS, a Garmin GPSMAP 62s. The 62s can receive the WAAS satellites but not GLONASS.
So what? The guaranteed civilian GPS accuracy of 30 feet is plenty good enough for road, trail, and cross-country navigation. The most accurate maps available (at least in the U.S.), USGS 7.5-minute series topographic maps, are not as accurate as civilian GPS.
If you can't find a trail, road, or campsite when you're within 30 feet of it, your basic wilderness skills need a bit of work.
More accuracy will certainly make it easier to find a geocache, but on the other hand being able to walk right up to a geocache instead of using the written clues to find it seems to take some of the fun out of it. Of course, when you hide a new geocache, you should take advantage of all the accuracy features of your GPS receiver, including WAAS, GLONASS, and waypoint averaging, if your GPS receiver supports them.
The only other time the 30-feet accuracy standard fails to be good enough is when you're hiking and trying to measure trail distance and speed on the GPS receiver. As I've written previously, the track points recorded at walking speed are too scattered to allow the GPS to accurately measure distance and speed. The more accurate your GPS receiver is, the less scattered the track points are, and speed and distance measurements become more accurate. You can improve speed and distance measurements on most GPS receivers by setting it to record less track points, thus averaging out your route.
Far more important than accuracy is the sensitivity of the GPS receiver and the number of satellites it can receive simultaneously. GPS satellites are in 12,000 mile orbits and the signal is extremely weak by the time it reaches the Earth. Also, the radio signals are line-of-sight and are easily blocked by forest cover and buildings.
The first civilian GPS receivers were not nearly as sensitive as current units, and even worse, they multiplexed. Just one receiver channel was available and it was switched through each of the satellites in turn. These receivers rarely worked well if a significant portion of the sky was obstructed. In forest, I'd usually have to stop in a clearing to get a satellite lock.
So what really impresses me about the eTrex 30 and GPSMAP 62s are their ability to lock on to satellites and stay locked on in forest and canyons where the sky is partly obscured. Even the cheapest of the current trail GPS receivers from the major manufacturers all have high-sensitivity receivers that receive 12 or more satellites simultaneously. Since a minimum of 4 satellites are required for an accurate position fix, the newer receivers stay locked on and generate an accurate position fix even when part of the sky is hidden.
The moral of the story is- don't buy an old, used GPS receiver for serious back country navigation. And don't depend on smartphone GPS for the same reason. The GPS in phones is intended to let 911 dispatchers locate you in an emergency, and is not nearly as sensitive as dedicated trail (or street) GPS units.
One more rant- don't hike along staring at your GPS screen! GPS is a very useful tool. It makes my job as a hiking and outdoor guidebook author much easier. I can set the GPS to record a track as I hike, save waypoints at trail junctions and other landmarks, and then download the data and map the trail back at home. But it's all too easy to pay too much attention to your GPS instead of the trail and the (presumeably stunning) scenery you're hiking through. Put the GPS in your pack as a backup and follow the trail!
