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Signal Drift

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I did a cache today under heavy tree cover (glad it was winter, would have been worse with leaves). I am using a Sportrak Pro. It seemed that that the location would move 45 degrees in any direction if I walked slow, if I walked fast it worked fine. Is this normal in heavy tree cover? Why would I have to walk at a fast pace to make it work correctly?


It was a 3 terrain, I'm glad I got out before dark or I might have spent the night! LOL

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It's hard to say what you were seeing. I often experience the same thing with my Sportraks in troublesome reception areas though. The receiver seems to do a lot of filtering, and will develope positonal errors while moving generally about a hundred feet or so. During this time it will often pick up turns, speed changes etc and stay fairly accurate with them, albeit ofset by a fixed amount. When you stop, or slow down to where you are almost stopped, the unit will slowly catch up with where you really are, and you'll see the slow drift you describe. Generally moving forward, and recovering from the direction in which the position was offset. Most of the time the drift is only 50 to a hundred feet, but I've seen it over half a mile when I've been trudging along slowly and steadily for a long period of time. It can take anywhere from a couple minutes to 20 or so until it catches up. It also makes for some pretty bizzare tracks.


For what it's worth





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We had a rather strange experience the other weekend. Clear, wide-open sky, 15-foot accuracy, and the GPS pointed about twenty feet off the trail. It didn't look like a likely spot -- there was a steep bank and some dense vegetation, in a park that prohibits bushwhacking, on a cache whose description states that you don't have to leave the trail. I walked past, turned, walked past again, and it still indicated the same spot. Very dubious, we started looking for a side trail that would perhaps take us around the other side of the cache spot.


And then, abruptly, the GPS arrow flip-flopped and the unit said that the coordinates were actually 60 feet in the opposite direction -- which turned out to be right on. We had a lock on every satellite in sight for the whole time, never worse than 15-foot accuracy. I've never seen an abrupt 80-foot leap like that, much less in those conditions. I have seen coordinates that seemed to drift slowly around a forty-foot circle under heavy tree cover, but this was just weird. I'm glad we didn't blindly follow it for the first five minutes at the cache area.

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(Play Twilight Zone music here)


My bros (Spiderhike) Garmin E-Trex did that to. We both entered the same coords (I have a Magellan GPS 310) and we went, his said the cache was 31 miles away and mine said 2.53 miles away. Mine was right. He was a little upset but it has never happened again since that one incident. I might add that we were walking around in sage brush, nothing over 2 feet tall, clear sky, perfect conditions.


Happy. Hunting. burnout.gif

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Originally posted by gefd28:

Why would I have to walk at a fast pace to make it work correctly?


Dont't know about your sportrak, but this is what the

FAQ at magellan says. And I assume this is without any kind of signal disruption. I think they always give the best sennario.


Navigation Screens


Q: Why does my compass not point in the right direction?


A: The Meridian computes direction by comparing your present position to the last position taken, it requires that you be moving to compute your compass heading. To eliminate error, you must be moving at a minimum of 2.0 mph. The exception is the Meridian Platinum, which has an electronic compass that will compute the compass heading below 2.0 mph. This includes computing your compass heading while you are standing still.


I have flouted the wild, I have followed its lure, fearless. familar, alone; yet the wild must win,

and a day will come when I shall be overthrown. By: Robert Service

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