Sportrak Accuracy Question And Funny Story

[+Karma Hunter]

My wife borrowed a GPS and went geocaching a few times last summer and really enjoyed it, so I decidied to get her a GPS of her own for her birthday last week. I did a little research and got a Magellan Sportrak Map, primarily because we live in a heavily wooded area and people seem to think that the Magellans work better under tree cover.

 

The story: I decided to be clever and hide her other birthday presents out in the woods so she could go on a treasure hunt. Unfortunately, I'd never used a GPS before, and I assumed that the accuracy was within a few feet. To make sure she was challenged, I hid the presents REALLY well (completely buried under leaves in a very steep area with many acres of identical ground cover). I also made no effort to pay attention to exactly where I was hiding them, because I thought the GPS would take care of that for me. To make a long story short, one of the presents is still lost in the woods, despite several hours of hunting on two different days. At this point we don't know whether an animal wandered off with her present, or whether it's still out there waiting to be found.

 

The question: When I buried the present I marked it as a waypoint on the GPS. But now when we go hunting for it the location it takes us to varies by 35 feet or so. As a test, I marked a waypoint on our porch, and subsequent tests at the same location have shown variations of 15 to 60 feet. Walking and standing still provide different readings, but they are equally inaccurate. Skies have varied from cloudy to clear, and there are no leaves on the trees this time of year. So my question is, are variations of up to 60 feet for readings taken over time (several days) by the same unit in the exact same location a normal occurance? The display shows at least 6 sattellites locked, and WAAS is usually either engaged or averaging. Turning WAAS off (thanks to another thread on this forum for showing how to do this!) gives about the same results. I'd hoped for more accuracy/consistency when buying a GPS. Is our experience normal, or should we consider a different unit?

[Lobo Astuto]

Did you checked the EPE before hiding it?

If Estimated Predictable Error (EPE for short) was high, you should expect that kind of variation.

I advise you on using WAAS, the sportrak is good, but a little help never hurted.

Good luck!

[+Karma Hunter]

Unfortunately I was unfamiliar with the EPE feature when hiding presents, so the accuracy at that time is unknown. I've kicked myself many times for not learning more before heading off into the woods. EPE now is usually about 15 feet, even when the displayed distance to the "reference" point on my porch is off by more than that.

[+RobRee]

what is the gift made of?

metal?

did you bury it?

ask around for friends with metal detectors. also check for locals online with metal detectors. could be a fun hunt for them.

what town city are you in? could direct you to a source.

robbie

[+robert]

Unfortunately I was unfamiliar with the EPE feature when hiding presents, so the accuracy at that time is unknown. I've kicked myself many times for not learning more before heading off into the woods. EPE now is usually about 15 feet, even when the displayed distance to the "reference" point on my porch is off by more than that.

Remember the error projection is there as a guide. If it says EPE of 15 ft and 17 ft from your porch, you can kinda subtract those numbers to get 2. Would that be more accurate for you? This isn't an exact science, just something I've found on a lot of cache hunts.

 

Remember, this is a $150 piece of electronic gadgetry receiving signals from satellites in space so it's not going to be perfect, but for the money, it's pretty darn good.

[+Karma Hunter]

Thanks for the reply. No metal in the package. It's fleece gloves in green (her favorite color), wrapped in plastic bags to protect from the rain we were supposed to get that night. Not terribly expensive, but something she would have been excited about. At this point we've more or less given up on finding the package, but we're curious about the overall mystery. The bags were shopping bags, so it's possible one had carried groceries. Maybe it could have smelled like food enough to attract an animal? We'll probably never know. It has been frustrating (but funny) though. We can't figure out if the problem is us, the GPS, or something else entirely.

[Lobo Astuto]

Don't rely only on gpsr, next time try getting another waypoints, like a singulaar tree, a rock, something like that.

You can try digging a 20 ft hole, after all, the error could be bigger!

[+embra]

One thing that can potentially magnify the error is that you have to rely on two readings: one when you placed the gift, and the other when you attempted to retrieve it. So you would need to add the two errors together.

 

Sometimes they might cancel each other out, but a worst case scenario would be something like this: your first reading could be 30 feet east of the coordinates you mark. When you come back the next day, perhaps your gps places you 30 feet west of the true coord position. So, yeah, it is quite possible to be 60 feet or more off.

 

One thing that can help for the next birthday is to let your wife's SporTrack go into averaging mode. Many GPSs have this feature (I'm pretty sure the SporTrack does this) where if you let it set for a while it averages out the readings it takes over a period of time--averaging out the error and improving the accuracy of the reading. On the position screen it should state that it's averaging and how long it's been doing that. After 20 or 30 minutes I think you reach a point of diminishing returns on the time investment.

 

Thanks for the story--you'll probably get many more years of use out of the story than she would with the gloves.

[+RobRee]

you'll probably get many more years of use out of the story than she would with the gloves.

 

that is sooo true

robbie

[EagleI]

Well, at least you didn't bury the Sportrak!