First Cache found but I have an accuracy question...

[4249Nawth]

We successfully navigated to within 7 feet of the cache and found it under some leaves. As we were enjoying our find i noticed my Map 330 now showed us 30 0r more feet from the waypoint! We continued our hike and as we passed the cache point again it seems the GPS was still 35 or so feet off. Is this common? Why so accurate on the first pass? There was moderate tree cover so maybe I lost a couple of sats or lost the WAAS service? By the way, we're hooked and can't wait to find more and make some too.

[+Team StitchesOnQuilts]

I have the SporTrak Pro, which I think is similar to yours.

 

What I find is that when I slow down or stop, the averaging will get a little out of whack, and sometimes show me quite a ways from where I really am. I then do the "Magellan Hula" to reset the averaging. It almost always will then show me just where I really am.

 

I'd recommend doing a search on the Hula to find the original thread here in the forums: they explain it much better than I do. But the practical upshot is that you take the GPS in your hand, hold on firmly, and swing your hand, fast, in an arc around your body as far as it will go. Swing left, swing right, swing left, swing right.

 

Then hold the unit still for a moment and let it get a bearing.

 

Make sure to swing it vigorously. If you swing it slowly, it doesn't get enough speed to adequately reset the averaging.

 

Whatever you do, don't let go mid swing.

 

Shannah

[+Team StitchesOnQuilts]

I have the SporTrak Pro, which I think is similar to yours.

 

What I find is that when I slow down or stop, the averaging will get a little out of whack, and sometimes show me quite a ways from where I really am. I then do the "Magellan Hula" to reset the averaging. It almost always will then show me just where I really am.

 

I'd recommend doing a search on the Hula to find the original thread here in the forums: they explain it much better than I do. But the practical upshot is that you take the GPS in your hand, hold on firmly, and swing your hand, fast, in an arc around your body as far as it will go. Swing left, swing right, swing left, swing right.

 

Then hold the unit still for a moment and let it get a bearing.

 

Make sure to swing it vigorously. If you swing it slowly, it doesn't get enough speed to adequately reset the averaging.

 

Whatever you do, don't let go mid swing.

 

Shannah

[4249Nawth]

Hmmm, the "Hula"...

Ok, I'll give it a try. With all the swatting at black flies here in NH it should be very similar to what I do in the woods anyway May through June.

[+Renegade Knight]

To answer your question. No. There are a lot of factors that go into how accurage your GPS is in figuring out where you are at. I've had a day where my GPS sent me in a 50' circle with the GPS always in front of me. I've also seen the time of day make a difference and the GPS vary at seemingly random between 2 or 3 different spots that it liked as the cache location.

 

What you saw was normal.

[Kerry.]

quote:

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Originally posted by 4249Nawth:

Why so accurate on the first pass?

---

 

That's what they call Statistics (coincidence) and many things affect those statistics.

 

Cheers, Kerry.

 

I never get lost everybody keeps telling me where to go

[Kerry.]

quote:

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Originally posted by 4249Nawth:

Why so accurate on the first pass?

---

 

That's what they call Statistics (coincidence) and many things affect those statistics.

 

Cheers, Kerry.

 

I never get lost everybody keeps telling me where to go

[Dinoprophet]

I have a Garmin eTrex. According to the documentation, it is accurate to 15m (49 feet). So when it tells me, on a really good day, that it is currently accurate to 14 feet, is it *really* within 14 feet, or is it cumulative with the 49 feet (in which case, what is the accuracy really)?

[+Team Headrush]

quote:

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Originally posted by dinoprophet:

I have a Garmin eTrex. ............... is it *really* within 14 feet, or is it cumulative with the 49 feet (in which case, what is the accuracy really)?

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I'd say 14 feet was correct. You were probably receiving DGPS corrections from WAAS satelite.

 

GPS accuracy - 49 feet

DGPS accuracy (WAAS)- 10 feet

[+EraSeek]

quote:

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what is the accuracy really)?

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I'm sure Kerry can better explain. But in my experience what that 14' is, is the results of a program that someone has put in there to give you an idea of how good the unit is doing (comparatively). In general I have found that you are usually doing a bit better than what that figure tells you. When I have tested it under good conditions, I was 40% closer to the target than what the accuracy figure gave. But this was in ideal conditions. I generally see that figure as saying ''We think that the target is somewhere within this range 90% of the time".

 

Also different models and brands will use different accuracy program.

 

On the original question, normal , yes. First of all, any tree cover will cause some bounce of signal.

 

Also, you must add the error of the hider's unit's reading and your error. How do you figure up the sum of 2 unknowns. Who's unit is right and when, if ever.

 

Get close- then start thinking like the hider.

 

[This message was edited by EraSeek on August 13, 2002 at 07:25 AM.]

[Kerry.]

EPE's (or equivalents) generally appear not to report the highs and lows (that's what I've observed/recorded anyway) and has everything to do with how the EPE is programmed/derived but that information is propriety so we're really not sure what it means.

 

However it's regardless of the accuracy specifications stated in the manual.

 

The proper accuracy specification (world wide average) is less than 13m (~42 feet) 95% of the time which is signal-in-space figures and doesn't take into account of where the user is trying to operate the GPSr.

 

That 15m is generally a bit outdated now but also the important "bit" is the type of accuarcy that 15m is, CEP, RMS, 95% etc as figures alone can be a bit deceptive.

 

Cheers, Kerry.

 

I never get lost everybody keeps telling me where to go

[Kerry.]

EPE's (or equivalents) generally appear not to report the highs and lows (that's what I've observed/recorded anyway) and has everything to do with how the EPE is programmed/derived but that information is propriety so we're really not sure what it means.

 

However it's regardless of the accuracy specifications stated in the manual.

 

The proper accuracy specification (world wide average) is less than 13m (~42 feet) 95% of the time which is signal-in-space figures and doesn't take into account of where the user is trying to operate the GPSr.

 

That 15m is generally a bit outdated now but also the important "bit" is the type of accuarcy that 15m is, CEP, RMS, 95% etc as figures alone can be a bit deceptive.

 

Cheers, Kerry.

 

I never get lost everybody keeps telling me where to go

[Lyra]

I'm an amateur astronomer and subscribe to several automated e-mails that give recent astronomy news stories. One regularly gives me news on recent solar flares that generally increase the aurora borealis (Northern Lights) a few days later when the particles jettisoned from the sun reach the earth. I've noticed that my eTrex tends to lose a bit of accuracy when the magnetosphere is agitated by increased solar activity.

 

Anybody else notice this?

[+raouljan]

I have noticed this as well.

 

For those out there that are interested in sunspots, solar flares - or just want to get to know that yellow ball up there a bit more..

 

http://www.sunspotcycle.com/

[+EraSeek]

quote:

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Originally posted by Lyra:

I'm an amateur astronomer and subscribe to several automated e-mails that give recent astronomy news stories. One regularly gives me news on recent solar flares that generally increase the aurora borealis (Northern Lights) a few days later when the particles jettisoned from the sun reach the earth. I've noticed that my eTrex tends to lose a bit of accuracy when the magnetosphere is agitated by increased solar activity.

 

Anybody else notice this?

---

 

I have not noticed it personally, but YES, this is certainly the case as how much of the accuracy of you GPS depends on what is going on in the ionosphere. Distrubance in the ionosphere causes a signal delay from the satellites to your GPS, thus loss of accuracy. That is what WAAS is there to correct! It 'reads' the disturbances and creates a uniform model for correcting the delay of signal. Your GPS figures out where you are and uses the correction needed for your coordinates.