One day my GPS is spot-on, the next is 90 feet off?

[+famousdavis]

Hi, I'm a newbie. When I went searching for my 2nd geocache, the GPS I bought (Garmin Venture Cx) led me to exactly where the cache was hidden. The accuracy was amazing!

 

Today, I took my two older kids on a hunt in the city to a place I'd not been before. Strange, the location it led us to was a house -- right in front of the garage door. No public land around, so even working out from that point wasn't going to turn up anything.

 

So as not to disappoint, I took my kids to the cache where I'd been successful before. I had my son use the GPS so he could find it himself, but we ran into trouble because this time, the GPS was saying that the cache was hidden 90 feet away from where I knew the cache really was hidden!

 

Why does such variation exist? What can be done about it? I know my GPS works correctly on other days, because I've searched for and found 5 other caches with not too much trouble, so usually the coordinates are pretty spot-on. But this time...90 feet off the mark seemed excessive, and now I don't know when my GPS is having a "bad" day or when it's having a "good" day!

 

Thoughts? Advice?

[+StarBrand]

Just a quick thought - did you manually enter the coordinates? Did you enter them 2 times? Best wild guess is you had an error in the entry one time.

[+stoneswivel]

Why does such variation exist? What can be done about it?

 

90' is a big discrepancy. Try StarBrand's suggestion first, but if the coords were the same...

 

The variation comes from any number of sources. The thing to remember is that your receiver is basically a radio receiver with a good clock that does a bunch of math to come up with a position. Errors in that position can come in at any stage of the game, but most people focus on the radio reception. The signals sent by the satellite navigation subsystem are surprisingly low power. They are influenced by atmospheric distortion (much more so when they are low on the horizon), blockage by things like tree cover, tall buildings, and hills, and what are known as "multipath effects". The latter occur when the signal is received twice; once on direct line-of-sight, and again after reflecting off a nearby object (such as a hillside).

 

Another important factor is satellite geometry. The math that the system uses to calculate position is analagous to triangulation on a map. If the three points you're using to triangulate are widely separated, you get a good fix. Same with GPS--you want several satellites dispersed in the viewable sky. Most receivers ("GPSr"s), including yours, have a satellite view you can check for this. There are other factors, but people's eyes tend to glaze over when I take this discussion too far, so that's enough for now.

 

What can be done about it? Several things. First, there's a field on your Garmin called "Accuracy" that is displayed on several screens by default. ["Accuracy" is a misnomer, but that''s another rant. Suffice that it's a measure of position "goodness."] Check that first. If you're reading 10-20' accuracy, that's reasonable under most circumstances. I could narrow it down more if I had an idea of your latitude. If the Accuracy is more than 20', one of the factors above is degrading the position "goodness."

 

Try "boxing" the coordinates. Come at the cache from several different angles. Remember where the GPSr puts you at each one. Keep an eye on your Accuracy each time. If they're all different locations, trust the ones in which your Accuracy was a lower number more.

 

If you have more questions, don't be afraid to ask. And welcome to the sport!

Edited January 12, 2007 by stoneswivel

[+briansnat]

There were some issues with the eTrex CX firmware reported by some people that sometimes gave them strange results. I understand the latest firmware from Garmin fixed it. Check and make sure your unit has the latest firmware.

[+RUFFLEDOSTRICH]

I've always "Garmin Etrexed-it". I've averaged coords after taking 10-20 of them after walking away at least 500 feet in EVERY direction each time as per forum advice.

 

The best coords I've gotten come after I sit that stinkin' Garmin Yellow in one place for 20 minutes (I eat a snack) and record those...cachers have found those to be much more accurate. I'm so glad I quit all that averaging effort. The averaging was SO off.

[+Ambrosia]

And sometimes, it's just the government messing with everybody.

[+famousdavis]

Thanks for all the advice.

 

The GPS coordinates were downloaded into my GPS -- I didn't fat-finger them and cause a data entry error -- and the coordinates loaded originally into my GPS are the ones that led me to exactly where the cache was stored about two weeks ago. Then, last night, I experienced the sizeable error in where the GPS said the cache was stored. Same coordinates, same GPS, different day, big difference in experience.

 

Firmware isn't an issue, either. When I opened the box after Christmas, I applied the latest firmware update from Garmin (Sept. '06).

 

I live in South Florida, so the terrain isn't going to be a problem -- we're flat as a pancake here, and there really aren't too many high-rises, either. Where this cache is located, there is a 5 or 6 story office building about 1000 feet away.

 

I don't know anything about the "Accuracy" field...haven't bumped up against such a thing. I'll look for it...perhaps that will explain things a bit.

 

I don't know anything about averaging coordinates. What I did do is move back about 50 feet in several directions (couldn't re-approach in every direction because of a hedge obstacle) and try re-approaching, but I was getting the same general error however I approached.

 

I may chalk it up to a strange anomoly and see where it takes me from here. I have had success using my eTrex Venture, but it'll be helpful if I can know beforehand how reliable the GPS is going to be in getting me to the correct destination. Maybe that Accuracy field will be the trick to that....I'll check it out.

 

Again, thanks for your speedy and thoughtful advice! ;-)

[jazzbassplayer]

I've had a similar problem. I've geocached with my daughter in northern Texas and my old garmin streetpilot 3 usually gets me within about 10 feet. Up here in Iowa it's usually 30 or more feet off. I've placed on cache and the finders are telling me the cache is out in the river instead of on the road banks. Can the north/south location affect gps readings?

[+stoneswivel]

I've had a similar problem. I've geocached with my daughter in northern Texas and my old garmin streetpilot 3 usually gets me within about 10 feet. Up here in Iowa it's usually 30 or more feet off. I've placed on cache and the finders are telling me the cache is out in the river instead of on the road banks. Can the north/south location affect gps readings?

It sure can, as can a gross longitude change. Your receiver needs to have a rough idea of where it is on the planet, so that it will know what SVs (satellite vehicles) are viewable from its location. When you move as far as TX-IA, it needs a nudge. Haven't used your machine, but I think all modern Garmins have a satellite view page that looks like a bulls-eye with icons of satellites floating in it. Search your menu from that page, and see if there's a "New Location" option. You can then set it manually, or have the electronics do it for you. If you don't have that option, I'm sure a StreetPilot wizard will be along shortly.

[+NightPilot]

It's not the north/south location, because the GPS satellites travel in orbits that make that immaterial. What can happen is that on different days different satellites are in different places - they aren't even close to geostationary - so one day you may get poor geometry, leading to poor accuracy, and on another day much better geometry, and much better accuracy. It also depends on how and where you hold the GPS, because your body will block the signals for satellites in that direction. It doesn't matter, however, whether you're at the equator or the north or south pole, or anywhere in between, as long as the receiver has time to acquire an almanac and be sure where it is.

[+Kealia]

Best I can say is that "it happens".

 

Just remember that these things do an extraordinary job of telling us where we are on the globe to within 100 feet or so.

 

Remember that once you get close (100 feet or so) the best thing to do is to start looking around and "use the Force" - don't expect the GPS to get you to within 5 feet of it. Granted, this can be harder in urban areas when searching for micros, but it's good advice (if I do say so myself).

[+PilotMan]

You gotta remember that GPS isn't a magical system that tells you exactly where you are all the time. It's a system that involves billion dollar satellites transmitting radio waves through the atmosphere to your receiver which then uses alot of complicated math to calculate your position. Anything could have caused an error, it was probably an atmospheric distortion of the signal, or it could have been a hundred other things.

[FlagFinder]

it might not be you but the placer when i first started placing caches i could never get my accuracy low on my 100 so i was placing caches with 20-30ft accuracy well now that ive stepped up im placing caches with accuracy of 7-15ft accuracy it makes a big diference.

[+Kealia]

it might not be you but the placer when i first started placing caches i could never get my accuracy low on my 100 so i was placing caches with 20-30ft accuracy well now that ive stepped up im placing caches with accuracy of 7-15ft accuracy it makes a big diference.

 

Can I ask something of you that a lot of folks are probably thinking (and haven't said yet)?

 

Please use punctuation when typing in the forums. It becomes extremely hard to read if the post is more than one sentence long.

 

Thanks!

[+NightPilot]

I agree. Failure to use proper capitalization and punctuation is just laziness, and often results in people simply refusing to read your posts.

[4x4van]

Also keep in mind that your unit may have had 40-50 foot acuracy both days (not at all out of the ordinary)...that would have allowed it to report your position(s) 90' apart at the same coordinates. It's possible that you just lucked out on the first day, and "unlucked out" on the second day.

[+krisandmel]

And sometimes, it's just the government messing with everybody.

 

Just wear the tinfoil under your hat, and everything will be fine.

Yes, just fine.

 

You're getting sleepy.

Sleeeeeepy.

 

Yes, that's right.

 

Excellent.

 

[+krisandmel]

...

I don't know anything about averaging coordinates. What I did do is move back about 50 feet in several directions (couldn't re-approach in every direction because of a hedge obstacle) and try re-approaching, but I was getting the same general error however I approached.

...

 

If you mentioned it, I missed which exact make/model GPSr you have, but many, and I think all of the eTrex, have a built in averaging function.

When you go to the main menu, then to Mark, it goes to a new screen with the waypoint symbol and name at the top, lower down the current date/time, lower yet the coords, then elevation, then at the very bottom I think there are three 'buttons', the left one if I recall correctly either says Avg or Average.

You select that and it flips to another screen showing the count, the coords and some other stuff.

 

Some people let it count up to 20 or so, others like to wait to a hundred or two, it only takes a few minutes, about one count per second I've found.

 

Oh, as it is averaging, if possible, set the GPSr on a flat surface (screen to the sky) and the higher the better.

Then get away from it for a couple minutes so your body doesn't act as a shield or reflector to the incoming signals.

If there is nothing to lay your GPSr upon, sometimes I will just hold it on the top of my head, with the cache that I'm about to hide at my feet, and stand still a little while for the averaging.

 

When the GPSr has averaged however many readings you would like, select OK or Save or whatever it says.

(Just got off a 12 hour night shift and the ol' memory isn't working like it should. Or maybe I should just blame it on the tinfoil under my hat! )

 

Just about every post I've read on this topic seems to agree that the onboard averaging capabilities seem to work out better & easier than the alternative: Walking back and forth in many directions taking several manual waypoint readings and then averaging them yourself with a calculator to come up with a new set of coords.

 

$.02, ~K

Edited January 14, 2007 by krisandmel

[+dh2000dh]

We had a discussion at a recent event about "who has Magellan, who has Garmin".

It appears you can tell what GPSr is used by the hider!

 

Then we went looking for a cache placed by a Garmin Man(Faerbers).

I was the only person there with a Magellan and I was looking about 60ft away, on the wrong side of the road!

 

There are things you can do to improve accuracy.

Bear in mind that if you show Accuracy: 30ft(either direction<READ DIAMETER: 60FT>)

and the hider had accuracy: 30ft(either direction<READ DIAMETER: 60FT>), you are looking for something within a possible diameter of up to 120 ft.

 

Wait. - the GPSr will "settle" and accuracy will often improve.

Make sure the batteries are good! Yes, this does make a difference with my Magellan!

 

TAke trees and buildings into account. Trees shield the signal, buildings will reflect a signal.

 

and then,

 

ENJOY THE HUNT.

 

David

[fivedime]

Youi mentioned at least one of your searches was at night. Maybe it is just coincidence, or bad voodoo, or something, but I have noticed lately my accuracy is much worse when I hunt at night vs. daytime. Maybe it just has to do with which satellites are over FL day vs. night?

[+HaLiJuSaPa]

Thanks for all the advice.

 

The GPS coordinates were downloaded into my GPS -- I didn't fat-finger them and cause a data entry error -- and the coordinates loaded originally into my GPS are the ones that led me to exactly where the cache was stored about two weeks ago. Then, last night, I experienced the sizeable error in where the GPS said the cache was stored. Same coordinates, same GPS, different day, big difference in experience.

 

Firmware isn't an issue, either. When I opened the box after Christmas, I applied the latest firmware update from Garmin (Sept. '06).

 

I live in South Florida, so the terrain isn't going to be a problem -- we're flat as a pancake here, and there really aren't too many high-rises, either. Where this cache is located, there is a 5 or 6 story office building about 1000 feet away.

 

I don't know anything about the "Accuracy" field...haven't bumped up against such a thing. I'll look for it...perhaps that will explain things a bit.

 

I don't know anything about averaging coordinates. What I did do is move back about 50 feet in several directions (couldn't re-approach in every direction because of a hedge obstacle) and try re-approaching, but I was getting the same general error however I approached.

 

I may chalk it up to a strange anomoly and see where it takes me from here. I have had success using my eTrex Venture, but it'll be helpful if I can know beforehand how reliable the GPS is going to be in getting me to the correct destination. Maybe that Accuracy field will be the trick to that....I'll check it out.

 

Again, thanks for your speedy and thoughtful advice! ;-)

 

One thing no one mentioned is batteries. GPSr's get real funky like you describe when the batteries start to get low. Also, in cold weather this can happen with regular batteries and is when you splurge and use Energizer Lithium instead but I don't think it would happen in South Florida (usually happens when its in the low 40s or below).

[Find-me]

You mentioned that you could not approach the cache from one of the directions because of "a huge obstacle". There is a pretty good chance that this obstacle is messing with receipt of signals from one or more satellites. This will, of course, vary as a function of the satellite's position relative to the cache and your receiver at any particular time. It might be OK one time and relatively innacurate the next.

 

Find-me

[+larryc43230]

One thing no one mentioned is batteries. GPSr's get real funky like you describe when the batteries start to get low. Also, in cold weather this can happen with regular batteries and is when you splurge and use Energizer Lithium instead but I don't think it would happen in South Florida (usually happens when its in the low 40s or below).

At least with my unit, a 60CSx, there is no "funky" behavior when either the batteries get low (not even in the seconds before the unit finally shuts down) or when the unit is cold (a friend and I used our units all day in 15-degree F weather a couple of weeks ago, and the units performed just fine).

 

Has anyone else noticed that their GPSr gets erratic when the batteries get low or when the unit gets cold? I'm beginning to wonder if this might be an urban legend, or maybe Garmin units aren't susceptible to this.

 

--Larry

[+StarBrand]

At least with my unit, a 60CSx, there is no "funky" behavior when either the batteries get low (not even in the seconds before the unit finally shuts down) or when the unit is cold (a friend and I used our units all day in 15-degree F weather a couple of weeks ago, and the units performed just fine).

 

Has anyone else noticed that their GPSr gets erratic when the batteries get low or when the unit gets cold? I'm beginning to wonder if this might be an urban legend, or maybe Garmin units aren't susceptible to this.

 

--Larry

Mine does but on rare occasion but only when using rechargeables that are VERY low. I've seen weird readings 4 or 5 times.

[+WeatherednBoston]

On my older GPS I was getting readings that were off. At the bitter end I even had readings of 1.2 miles off lol

 

I think it was because I would put it on the dash board of my x's car and when he took a sharp corner like usual...it would go crashing down to the floor. (one of the many reason's why he is now my x)

 

Have you dropped it recently?

[+briansnat]

If you mentioned it, I missed which exact make/model GPSr you have, but many, and I think all of the eTrex, have a built in averaging function.

 

The older eTrex units (yellow, Venture, Camo, Legend, Vista, Summit) do not average. I think the newer ones (Venture CX, Vista CX and Legend CX) do.

[+emb021]

After almost 200 finds, I've several times had my GPSr take me right to the cache, and other times, uh, not.

 

My experience with bad finds:

 

* mis-entered the coords. (I now almost always download them)

 

* bad coords of the hider. (there have been several caches in which the hider has had to update the cache with new coords. Sometimes they were pretty far off)

 

* problem with signals due to 'terrain'. (terrain is not just elevation. I'm in south florida. Heavy tree cover can also mess things up).

 

* combination of 2 & 3.

 

This is why, for me, its SOOO vital to have full info on the cache, including hints and previous logs. Many times information on the hide (what it is, where it might and might not be hidden, comments by other successful finders), will point you to the right location when your GPSr says its someplace else.