Issue with the Magellan SporTrak

[+mrmnjewel]

Have noticed an issue with our base-level Magellan SporTrak. We have done all the proper rigamarole as far as initializing it originally and letting it sit out to get an "almanac" of satellites and such. What we have noticed as of late is that we can be standing perfectly still under no tree cover to speak of, and the coordinates will slowly change...just very slightly...and, also, when hunting a couple of caches recently, the coordinates took us to one spot originally and then took us to a different spot a few feet away a few minutes later. I know it's not an exact science, but, in your opinion(s), is our gpsr having problems, or are we?

 

The kingdom of heaven is like treasure hidden in a field. When a man found it, he hid it again, and then in his joy went and sold all he had and bought that field.---Matthew 13:44

 

Matt & Julia

 

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I wish outer space guys would conquer the Earth and make people their pets, because I'd like to have one of those little beds with my name on it.

- Jack Handey (aka Jack Handy)

[+briansnat]

It's you, not your GPS. You're expecting pinpoint accuracy and consumer grade GPS's don't offer this. There are many variables that will affect your accuracy. Trees, cloud cover, buildings, rocks and even your body position and the direction you are facing.

 

What you describe is perfectly normal.

 

"Give a man a fish, he'll eat for a day. Teach a man to fish, he'll sit in a boat and drink beer all day" - Dave Barry

 

[This message was edited by BrianSnat on August 03, 2003 at 04:28 AM.]

[DocJ]

Welcome to the sport. I have a sportrak pro, and I'm pretty sure that some small fluctuation in coordinates even while perfectly still is normal, as a few feet accuracy is the best one can expect even with WAAS working. That should be just fine for finding a cache, and in fact, I would love if I could get that kind of accuracy all of the time. As for the GPSr leading you to different spots while zeroing in on a cache, try this: When you get to the general cache location, try backing off maybe about 50 ft of so and approaching the cache from different directions until you're sure that each approach is leading you toward the same point. Sometimes loss of a perfect satellite lock due to tree cover..etc.. can cause some slightly screwy readings. Using a few separate approaches really helps home in on the target. Hope that helps.

[+O'Clan]

I agree with these guys... My magellan does the same and will jump around occasionally.

[+Mark 42]

Can we hear from the Garmin users?

 

Is this common?

 

BTW, how do y'all like the Magellan Sportrak pro?

I may be buyiong one since they're on sale for $129.

 

Why do so few GPS receivers have a jack for an external antenna?

 

On the drifting readouts:

 

You may want to look up a good description of the difference between precision and accuracy.

(I'll give it a shot).

 

Essentially, you can have a precision of .001 feet and still be of by 100 yards.

 

You can have an accuracy of .001 ft, even though you machine reads in units of 10's of feet.

 

Precision is how many decimal places a measuring device has on the readout.

 

Accuracy is how far off of the perfect measurement it can be.

 

A scale that reads .001 Oz. is high precision, but it can be off by half a pound if it's out of calibration. So the precision would be .001 Oz and the accuracy would be only +/- 8 OZ.

 

My theory for the fluctuation:

The error on a gps will allow what is displayed values to fluctuate a bit because the readout's precision is greater than the unit's accuracy.

 

Hukt un fonix werkt fur mee

 

[This message was edited by Mark 42 on August 03, 2003 at 12:25 PM.]

[+briansnat]

It happens with Garmin, Magellan, Lowrance, Cobra and any other brand of consumer grade GPS. I have a Garmin Vista and have seen this.

 

One additional "problem" with the Magellans...well it's not really a problem, but part of the design is the "slingshot effect" of the device's auto averaging feature. This may cause you to overshoot the cache. Magellan users need to take this into consideration when cache hunting.

 

"Give a man a fish, he'll eat for a day. Teach a man to fish, he'll sit in a boat and drink beer all day" - Dave Barry

[darkfrog]

It is probably "averaging" your position when you're just standing still... This means that although the position reading is changing, it's probably becoming more accurate. You should be letting it average for a few minutes before you mark a waypoint for something important important like home or a cache that you are placing. Some people go so far as to average the coords of a cache over a period of several days just to make sure its not off.

 

IT Ninja for Hire|High-Tech Adventures

[+Searching_ut]

What you're seeing is very normal. When you look at the complexity involved in computing your position, it's actually somewhat amazing the units do as good as they do.

 

The filtering and waypoint averaging on your sportrak actually tend to keep the displayed position more stable than what you'll often see on something like say an eTrex, but not necessarily more accurate. Which method of providing data to the GPS user is best pretty much depends on the preferences of the user. Luckily, there are a lot of good choices.

 

By the way, get out and cache in a canyon and you might find returing to the exact same spot a few minutes later gives you a different position by distance measured in tenths of a mile instead of a few feet.

[+Desert_Warrior]

Download VisualGPS. Then plug in your GPS and watch the display. Let it run over a time, and come back to see what it did. You will be surprised at the amount of movement

 

One day I will try it with WAAS on and again with WAAS off.

 

Mike. Desert_Warrior (aka KD9KC).

El Paso, Texas.

 

Citizens of this land may own guns. Not to threaten their neighbors, but to ensure themselves of liberty and freedom.

 

They are not assault weapons anymore... they are HOMELAND DEFENSE WEAPONS!

[+TotemLake]

Briansnat has it right although I have never experienced the "slingshot" effect while hiking. What the Magellans are famous for is averaging when you are standing still and this will show position change.

 

Another factor for this movement that you noted is known as the bumblebee dance as the satellites you are communicating with are not stationary. That is, their geometric position with each other and you changes rather quickly and the calculations will vary based on some very complicated algorithms and timing.

 

Cheers!

TL

[cappd]

My question is close enough that I'll add it to this thread. I have an older Magellan 315. It works great....seeming to always guide us to within just a few feet of a cache location. Today however, I was with a friend who had a spanking new SportTrak. After getting both units up and running I noticed that they had differing coordinates displayed. I left both units side by side in an open field for about 30 minutes or so. Although the coordinates did get a little closer to the same numbers they were still off by enough that my kids would have given up looking for a cache and complained all the way home! I'm guessing they were off by about 80-100 feet from each other. Why?

[+Cornix]

quote:

---

Originally posted by cappd:

After getting both units up and running I noticed that they had differing coordinates displayed.

---

 

Perhaps the SporTrak is using a different map datum. The default setting for the secondary coordinate system / map datum on my SporTrak has been UTM / NAD27. I've changed this to WGS84.

 

Cornix

[+geospotter]

For what it's worth, let me add some purely anecdotal observations.

 

A friend and I were doing a multi-cache the other day. He has an etrex, I have a Meridian. At each location his unit seemed to bring him to the GOTO much more quickly than mine. His would note that "it's about 20 feet ahead" while mine would note "it's about 90 feet ahead". If I stood still for a while it would eventually agree with his and we always ended up at the same spot, but the Meridian seemed to lag behind.

 

I don't know if it takes fewer readings per minute/second or what, but its lag time seems more pronounced. Don't get me wrong, my MAG315 and my Meridian have never let me down, but...

[+Renegade Knight]

quote:

---

Originally posted by geospotter:

For what it's worth, let me add some purely anecdotal observations.

 

A friend and I were doing a multi-cache the other day. He has an etrex, I have a Meridian. At each location his unit seemed to bring him to the GOTO much more quickly than mine. His would note that "it's about 20 feet ahead" while mine would note "it's about 90 feet ahead". If I stood still for a while it would eventually agree with his and we always ended up at the same spot, but the Meridian seemed to lag behind.

 

I don't know if it takes fewer readings per minute/second or what, but its lag time seems more pronounced. Don't get me wrong, my MAG315 and my Meridian have never let me down, but...

---

That's the slingshot effect. It has to do with the way the Magellan uses it's information to direct you to the waypoint. They always get there but often the magellan user has to turn around and come back.

 

This has proven handy at least once in our caching adventure. The Magellan said "it's on top of that cliff" while my Garmin was telling me "Sorry boss I lost my signal but it really looks like it's down here".

 

The magellan was right.

 

As for the topic...That is normal variation you can expect to see in a GPS. You will get used to the nuances of using htem. One thing I always did (which is why I never understood why people needed a compass) was to sight ahead to where the GPS is pointing. It's sort of a game to see if the GPS will point right at the cache. As you get used to this you will have the spot you need to be looking for the cache in mind long before you get there. When I stop walking I'm normally within 5' of ground zero per what the GPS says because I saw where it was telling me to go.

[+TotemLake]

quote:

---

Originally posted by Renegade Knight:

quote:

---

Originally posted by geospotter:

For what it's worth, let me add some purely anecdotal observations.

 

A friend and I were doing a multi-cache the other day. He has an etrex, I have a Meridian. At each location his unit seemed to bring him to the GOTO much more quickly than mine. His would note that "it's about 20 feet ahead" while mine would note "it's about 90 feet ahead". If I stood still for a while it would eventually agree with his and we always ended up at the same spot, but the Meridian seemed to lag behind.

 

I don't know if it takes fewer readings per minute/second or what, but its lag time seems more pronounced. Don't get me wrong, my MAG315 and my Meridian have never let me down, but...

---

That's the slingshot effect. It has to do with the way the Magellan uses it's information to direct you to the waypoint. They always get there but often the magellan user has to turn around and come back.

 

This has proven handy at least once in our caching adventure. The Magellan said "it's on top of that cliff" while my Garmin was telling me "Sorry boss I lost my signal but it really looks like it's down here".

 

The magellan was right.

 

As for the topic...That is normal variation you can expect to see in a GPS. You will get used to the nuances of using htem. One thing I always did (which is why I never understood why people needed a compass) was to sight ahead to where the GPS is pointing. It's sort of a game to see if the GPS will point right at the cache. As you get used to this you will have the spot you need to be looking for the cache in mind long before you get there. When I stop walking I'm normally within 5' of ground zero per what the GPS says because I saw where it was telling me to go.

---

 

Aaaah.... I see. Perhaps because I approach the final 200 feet a little slower as I start investigating the area, my MeriPlat does catchup.

 

As for using the compass... I've noted with my GPSr, that sometimes it just can't seem to settle down on whether it is pointing north or not, that is, the map orientation bounces back and forth under some very heavy canopy as it loses track of a satellite but picks up another. Using my Suunto compass, I was able to determine North pretty quickly, but to settle down the GPS, I set everything to reference Magnetic rather than True. Within a couple moments, my MeriPlat settled back down and pointed in the correct direction.

 

It was for me, an interesting exercise because I was in completely unfamiliar territory and without a tip sheet to boot. I had to use other means to augment my navigation and search for the cache using my brief history as a guide to successfully ferret out the cache. Not that there was a faint fear of getting lost as the trails in this particular park were well marked, but it gave me an opportunity to learn how to get out of trouble when the gadget goes whacky.

 

Cheers!

TL

[blocko1000]

I have wondered about this also. I figured it was normal. You may be standing still, but what about the satelites? Could they be moving about a bit up there?

[+TotemLake]

quote:

---

posted August 05, 2003 11:34 AM

I have wondered about this also. I figured it was normal. You may be standing still, but what about the satelites? Could they be moving about a bit up there?

 

---

 

They all are except for the WAAS satellites. Those are geosynchronous.

 

But I think it was the constant loss of one satellite and gain of another, and so on with each satellite the GPSr locked onto that was causing my problem. And... I was walking, but it was a behavior that started after I had been walking for about 20 minutes. I noticed this behavior even more a few days back when it took 15 minutes to get a good communication lock.

 

Btw... reading over what I typed in my previous post... I was never a guide. I meant using my brief searching history as a guide to me... (just for clarification).

 

Cheers!

TL

[+Mark 42]

It'd be nice if there were a single button to switch to "Simulation" mode... I hate when I'm programming the GPS and right in the middle of inputting a waypoint I get that alarm and taken to a different screen!

 

It'd be nice if I could quickly go to simulation as soon as I turn the unit on!

 

BTW, is there a way to get rid of the stupid disclaimer screen at startup?

I hate having to hit enter repeatedly (twice?) at startup each time!

 

I wish manufacturers would realize that the ability to disable features is desirable.

 

Can I turn of WAAS & averaging on my SporTrak Pro?

 

I hope that someday we will be able to put away

our fears and prejudices and just laugh at people.

[+TotemLake]

I normally don't reply this way, but look for my answers preceded by >>> inside your quoted text.

 

quote:

---

Originally posted by Mark 42:

It'd be nice if there were a single button to switch to "Simulation" mode... I hate when I'm programming the GPS and right in the middle of inputting a waypoint I get that alarm and taken to a different screen!

 

It'd be nice if I could quickly go to simulation as soon as I turn the unit on!

 

>>> You are kind of stuck with what comes on first as this is an assumption that everybody wants. You are proving to be the exception to the rule. As for the alarms, turn them off. I'm sure they are in your setup someplace.

 

BTW, is there a way to get rid of the stupid disclaimer screen at startup?

I hate having to hit enter repeatedly (twice?) at startup each time!

 

>>>No. But there is probably a way to modify it. It might be the same way as the Meridian series, but your best bet is to go to this Yahoo Group to find out how best to work with your particular model. You will need to become a member to take advantage of it.

 

I wish manufacturers would realize that the ability to disable features is desirable.

 

>>>Provide them feedback on your desires. How else will they know?

 

Can I turn of WAAS & averaging on my SporTrak Pro?

 

>>>Yes, but again, I suggest you go to the Yahoo Group I provided a pointer to in my previous answer. It is a hidden menu support and when you use it, you will probably void your warranty.

 

_ I hope that someday we will be able to put away

our fears and prejudices and just laugh at people. _

---

 

Hope that helps!

 

Cheers!

TL

[+Mark 42]

Thanks.

I'll go join that yahoo group (what's one more yahoo to a guy like me... )

 

I'll probably do more reading in the manual.

 

I have a 1-1/2 hour commute each way, so I have time to read the manual before I go ask more dumb questions in another forum.

 

I hope that someday we will be able to put away

our fears and prejudices and just laugh at people.

[+TotemLake]

I haven't joined, but I was thinking they might have an FAQ there that might be able to answer a few of your questions.

 

Cheers!

TL

[+MattLarson29]

I have noted the same thing with my SportTrak Pro. It seems to wander for a little bit when I stop. I try to give it a couple minutes when I get close, and that seems to help.

 

Matt