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I'm wondering if anyone with a GPS knows why I had such a bad experience with one I borrowed yesterday. I'm in the market for buying one, and a friend allowed me to test his ... is was a Garmin eTrex.

I used it when hiking First Water to Battleship Mountain. The first 3.5 miles were fine, and then all of the sudden I looked down (maybe a minute later) and it said I had gone 17 miles, and that my max speed had jumped from 4mph to 47.1mph. I'm honestly not the Dash or Flash Gordon or anything. ;)

From that point on, the reliability of the data was spotty. It never developed a consistent trail from the hike, and on the way back some of the tracking back to the waypoints I created worked...and then others said weird things like 22 miles to the next waypoint that I knew was 1-3 miles away.

Was it just a defective device, or do just GPS's do that sometimes and I just had bad luck? I'm actually now looking into getting a Magellan Triton 500, but any help anyone could give me on this would be much appreciated!

mine sometimes does the same thing. I think its spotty sattelites, regarless of clear unobstructed skies. I rely on maps more than GPS. Some folks are very impressed with the performance of GPS, some arent. ;)

I have a Garmin e trex and as long as I set it up correctly it has never failed me. Usually its operator error when something goes wrong. The most important thing is that everything in its memory is cleared before you start a track. It sounds to me like it was mixing data from something prior.

Sounds pretty much like it lost the signal for a second and plotted an erroneous data point. Generally the garmins are decent little gadgets. Maybe that was an older model? or a placement issue?
GPSjoe mentioned

Any of the newer models with an "X" in their model name (60CX, 60CSX, 76CX, 76CSX and some Etrex models) have a very sensitive antenna and the very fast SirfIII chipset. They NEVER lose sattelite signals under forest cover and only very rarely in the most challenging canyons with tight vertical walls.

I use an older color Magellan Sportrak with topo maps loaded. It has a quadrifilar helix antenna. It's watertight, crisp display, ultra light yet built like a tank with no rubber buttons. I love it.

However I would only recommend a newer Garmin with expandable memory. Nonot has a Garmin mapping project he's probably going to unveil any day now. You'll be able to load free maps with the HAZ GPS Routes on them.

Whatever you do, never go by the AEG in a GPS unit. It's virtually impossible for it to get perfect triangulation without fluctuation.

joe bartels wrote:Sounds pretty much like it lost the signal for a second and plotted an erroneous data point. Generally the garmins are decent little gadgets. Maybe that was an older model? or a placement issue?
GPSjoe mentioned

Any of the newer models with an "X" in their model name (60CX, 60CSX, 76CX, 76CSX and some Etrex models) have a very sensitive antenna and the very fast SirfIII chipset. They NEVER lose sattelite signals under forest cover and only very rarely in the most challenging canyons with tight vertical walls.

I use an older color Magellan Sportrak with topo maps loaded. It has a quadrifilar helix antenna. It's watertight, crisp display, ultra light yet built like a tank with no rubber buttons. I love it.

However I would only recommend a newer Garmin with expandable memory. Nonot has a Garmin mapping project he's probably going to unveil any day now. You'll be able to load free maps with the HAZ GPS Routes on them.

Whatever you do, never go by the AEG in a GPS unit. It's virtually impossible for it to get perfect triangulation without fluctuation.

The AEG is *always* wrong; I wish they'd jut get rid of it.

The Garmin models with the H have high-accuracy antennas, the older models don't.

C is Color

x is eXpandable memory.

Even with my GPS, I still had a poor signal in LaBarge Box where it was off about 1-2 hundred yards.

joe bartels wrote:Whatever you do, never go by the AEG in a GPS unit. It's virtually impossible for it to get perfect triangulation without fluctuation.

Interesting.. I did not know this..

..but why then do we have a "fill in the blanks space" on triplogs and hike descriptions for AEG IF it is generally not
accurate?

Grasshopper wrote: ..but why then do we have a "fill in the blanks space" on triplogs and hike descriptions for AEG IF it is generally not
accurate?

You can get AEG from measuring on topo maps and/or using GoogleEarth's (admittedly not 100% perfect) elevation readout.

Or use the elevation profile that the GPS mapping software gives you and reckon...

Yes you always want to plot it out and run it through a mapping program. Personally I prefer TOPO! However the others work too as they all use the same USGS NED indexes and such.

What happens is you only need two satellites to pinpoint lat/lon. Altitude is dependent on a third signal so it can triangulate. The tiny inaccuracies add up quick over a couple hours on the trail. Most look at the signal graph and say "hey I'm locked onto eight satellites" this has to be accurate. That graph is a combination of accumulated data over several processor cycles it's not tapped into the actual satellites. Further more it puts processor cycle emphasis on the two highest streams of incoming data. This is easily tested by standing in one spot and watching it fluctuate 5-20 feet without moving. Now some will come back and say, it held steady. Try it for four hours like a hike, then take into account it's really only recording data when you're moving. Not to mention you'll have extreme terrain differences on either side of you so 5 feet off could mean a 20 foot elevation blurp. It's pretty easy to hike a trail on the side of a mountain. Try hiking 5 feet off the trail and you're in for a whole nutha hike!

I still have a lot to learn about GPS,etc.. t-hank-s for all the good info and for all the knowledgeable ones on this great site!

Mapping is an interesting science because there are always tradeoffs. GPS inaccuracy is one problem, but so is the concept of AEG. An ant, which has to crawl over every pebble, will experience more AEG along the same section than a human, who will have to step on and over rocks, compared to say, a giraffe, who would experience the least AEG of the 3. The way AEG used to be calculated was to take known points every tenth of a mile or so and determine the change, which is equivalent to someone taking steps that big. GPS now can log much finer detail than that, so the noise of the GPS, plus the finer data, can lead to people scratching their heads over the concept of AEG. For example, if taking every point from my last route, it was over 22000 ft AEG, but really most people would have considered it maybe 8000. The GPS is not inaccurate THAT much as the track was almost perfect, so that's where the extra AEG comes from.

Joe is referring to the fact if you leave your GPS on and sitting in one place, the errors will build up, eventually recording miles travelled and AEG gained, that's the effect of noise alone!

Oh, and because of the geometery, the vertical elevation of a GPS estimate has much more signifant error (read greater) than the horizontal lat/lon estimate, the GPS's have to be above you after all!

nonot wrote:Joe is referring to the fact if you leave your GPS on and sitting in one place, the errors will build up, eventually recording miles travelled and AEG gained, that's the effect of noise alone!

So, if you stop for say a 20-30 minute lunch break and plan to just sit and/or roam around the break area some during this period, is it best to just turn-off your GPS unit for this period of time, then turn it back on right before you depart on the trail again?

I had an etrex Vista start doing the same thing. Worked great for 6 months or so then started adding distance like crazy. Really sucked on a new hike when I have to cover 11 miles thats day and I thought I was at a great pace. It showed me doing 17 miles instead. I got rid of it and have stuck with maps since when needed.

you guys have me wondering if I have a defective unit. I havent yet taken the time to look at it to see what antenna/system it uses so I'll have to do that. Unless someone knows how the Geko 201 is set up. I noticed in my other GPS the elevation would change while standing still and thought it was a satellite problem. Its of little concern as maps avail all the info I need, so far.

In response to Grasshopper:
nonot would certainly be a better source of info. I haven't read up on the stuff in years.

I think some models or a setting on them keeps them from doing that so much anymore. It used to be a major problem on older models.

I don't turn mine off as I fear it'll start a new track, though I've never tried. To me it doesn't really matter as I always edit the route in TOPO! later. Rechargeable batteries last over 20 hours in my unit. The new ones must be well over a day or two by now.

In response to davis2001r6: There was a perfect example in that 2003 hike we did... after the hike Stephen looked me straight in the eyes and smiled emphatically that we'd nailed 8400 feet in elevation. I didn't want to break his heart that the peaks are only in the 7600 range and we started at 5700 with only one short second ascent after the initial full ascent. Granted we went up and down, but not two thousand feet FOUR times

In response to te-wa: I don't think you're in much need of a GPS unit period, it's just not your style. Though I think if the GPS gods gave you a more modern unit for Christmas you'd latch on. Just glad you finally got a camera

true, but I do like my GPS. I like tracking my progress, I just dont want to rely on it especially if it gives wrong info. (like AEG)

Grasshopper, I usually just leave mine on, it makes it easy when looking at the tracks to figure out where you stopped and mark it or delete that small part if you want because all the sudden a straight track turns into a small spaghetti bowl. If you are worried about your batteries you could just turn it off, but when you start it up it will take a couple minutes to recalibrate.

AZLOT69 wrote:I have a Garmin e trex and as long as I set it up correctly it has never failed me. Usually its operator error when something goes wrong. The most important thing is that everything in its memory is cleared before you start a track. It sounds to me like it was mixing data from something prior.

actually, no ... i completely wiped all data from it entirely. all the waypoints, routes, everything. so it was not that ...