Looking up from the Valley floor from an earlier visit
This was a hike I did on a rainy day with kids, so I only went up to scouts lookout. Being a short hike, I didn't take a pack, thus I couldn't carry receivers on my pack strap. As a result, I only took the Sportrak which I carried in my hand.
As you can see from the track, the receiver wandered around a bit when first turned on prior to finally stabilizing on it's location, which is where I marked the waypoint. This is very normal with every GPS receiver I've ever used.
This track is the result of an out and back trail hike from the waypoint I saved, which appears to be pretty accurately placed on the map. Unfortunately, the track also comes to represent what I've come to expect from my sportrak in poor reception conditions, namely, very inaccurate positional data, without the receiver giving any easy to identify indication of reception problems. At no time during this hike did I ever get a lost reception indication or alarm, although the unit obviously lost reception in areas. Most of the time the receiver also indicated that I had reception from 4 to 5 satellites, although a couple of times I did notice it said I had a 2D fix even though I supposedly had 4 or more birds in view. Viewing the track data however, I find that on the return trip the receiver quit recording altitude changes for the bottom portion of the trip. As you can see, when I actually got back to the truck, the receiver thought I still had over half a mile left to go. At no time that I observed during the hike did the receiver give an EPE of greater than 60 feet once the unit stabilized initially.
It's hard to say what causes the track errors I notice on out and back trail hikes. In the area of this hike, multipath reception problems from the canyon walls are a real possibility, although my experience has generally been that multipath shows up as short term positional glitches most of the time. Based on the fact that my Sportraks behave like this on flatter ground, where tree cover is the primary reception problem, I tend to believe multipath is only part of the problem, if it's a player at all. Altitude change while in 2D mode can also cause positional errors, but generally not as great as what I get, and in this case the altitude change wasn't very large really.
Based on everything I've been able to see to date, I think the accuracy problem is primarily a result of some sort of averaging or filtering that the receiver does while moving in reception problem areas. I base this guess on the following: When moving in a generally straight direction, the receiver tends to lag my actual position by 50 feet or more in trees or terrain, often resulting in my overshooting my waypoint. The problem seems to grow if I keep moving slowly and steadily, and is much less pronounced if I stop often, and let the receiver slowly come back in line with my true position. Obvious multipath errors tend to take the whole track off course for awhile, but you will generally see it slowly drift back to a more realistic position, again indicating the momentary error influence subsequent position reporting. Finally, the more turns, altitude and other changes you encounter while moving, the more the unit seems to get confused. When carrying an etrex in the same conditions at the same time, the eTrex will often show patches of lost reception at about the same time as the Sportrak starts giving out bad data, which leads me to believe that Sportrak is also probably loosing reception, but that when it does, it does some fancy guessing which is often pretty close other than maybe a slight lag behind your real position, especially if your travel direction tends to stay steady. Unfortunately, it can also get it really wrong, which can be a problem being as how it's hard to know when it's lying. To be sure of your position with the sportrak in tough terrain, you've pretty much got to let it sit and average until it stabilizes on the same position for a minute or two. At that time I've found it tends to be pretty reliable.
Personally I'm not really concerned that the unit has reception problems sometimes, that's just the way gps works. What bothers me is that with this receiver I often can't tell it has problems, so I don't know the data it provides me is highly questionable.