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Gps Seems To Be Off


jocatch
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I am new to this GEO Caching and I am a little fustrated! My first cache, the one at the tower at Bowdin Park in NY was easy to find because my GPS was right on the money. The next cache, The Inn at the Falls, I couldn't find without the help of the hints and when I found it I noticed my GPS was about 30' off. Today I tried to find My Hometown (Hopewell JCT, NY) and couldn't find it. Looking at the reviews of others for that cache, I notice it seems to be near a bench. Where my GPS brought me, there was not a bench in sight.

 

I have the Magellan Sport Track Color with the latest flash memory upgrade, set to WGS84 and set to DEG/MIN.MMM.

 

I noticed that as I circled around where the unit said it should be, one time it put me in one spot and if I walked around a little, it would me about 15' away and if I walked some more, it put me back to the first spot. There are lots of trees around. Any suggestions on how to test and check the accuracy of the unit? How accurate can the GSP be? I heard they should be with 10'.

 

I assume these caches are on the ground covered up with branches, rocks and the like.

 

JC :)

Edited by jocatch
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You can expect variation of up to 30' or so in normal use without a ton of tree cover. If you have a WAAS-capable GPS and are receiving signals from the WAAS satellite, it may claim better accuracy, but the accuracy situation can change moment to moment depending on what satellites you're receiving, where they are in relation to one another, etc. Plus, of course, the person who hid the cache also had a certain margin of error, so don't be surprised to find caches that appear to be 30-40 feet off where your GPSr claims they are. Once you're within fifty feet or so, if you slow down and just follow the GPSr around you'll end up doing a bee dance. At that point it's time to stop watching the needle and start looking around and thinking like a geocache. (Sometimes there are freak incidents where your coordinates will suddenly leap 100' away -- those are rare, but do glance back at the GPSr occasionally. Just don't follow it slavishly.)

 

Your bog-standard ammo box in the woods is typically on the ground, covered in sticks/rocks/leaves, or tucked under a branch, but not all of them are. (Memorably, one was dangling over our heads, tied to a tree branch. It never hurts to look up.) Read logs and look for hints of how large or what kind of container the cache is. Micro caches are often hidden in more subtle ways -- magnetized to the bottom of a bench is extremely common. You'll soon develop a sense of where to look -- don't be reluctant to pick easy caches that have been found recently and have lots of spoilery hints while you're new.

 

I don't know what to tell you about your missing bench, though! Did the spot otherwise look right, or might you have transposed a digit in entering the coordinates? How was the GPSr's claimed accuracy at the time? Have people had trouble finding the cache recently?

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I went to a new cache yesterday where the original coordinates were off. A previous finder posted new numbers in his log, which helped me get to the right spot.

 

Usually, if my GPS gets me within 30 feet of the cache location, I just start looking, up, down, all around . . . but when GZ is in the middle of the second lane of traffic from the curb, as it was yesterday . . . well, that's a problem. :)

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You cannot adjust the accuracy of a GPSr. They don't measure anything. They are computers analyzing data received from the satellites and displaying the results. They either work or they don't. Their accuracy is dependent on the quality of the signals they receive, and the quality of the algorithms they contain.

 

You do need to be sure the Datum is set to WGS84, but since you were right on one cache it probably is.

 

Any suggestions on how to test and check the accuracy of the unit?

 

You can get a feel for the accuracy they are capable of by hunting high accuracy benchmarks. This is good practice for beginners. Just remember the benchmarks are more accurate than the coordinates posted by another random geocacher, so don't expect to get as close to caches as you will to benchmarks.

 

http://forums.Groundspeak.com/GC/index.php...dpost&p=1549550

Edited by Thot
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A GPS receiver doesn't have any accuracy to adjust.

 

An uncertainty circle with radius of thrity feet is entirely reasonable in most circumstances. Likewise, the cache owner may have had a similar degree of uncertainty in collecting the coordinates.

 

Bottom line: GPS is unlikely to put you on top of the cache. You'll have to search for it.

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You cannot adjust the accuracy of a GPSr.  They don't measure anything.

Sure they do. They measure time intervals, and if the internal clock is miscalibrated, the coordinates will be off. And since temperature affects the accuracy of the clock, there's an internal temperature sensor that's used to adjust the clock data on the fly. So if the temperature sensor is not functioning correctly, you can also get bad coordinates.

 

GPSs do measure, and the can go out of calibration. But it's very rare, and when it does happen, it needs to go back to the manufacturer.

 

But in the case of the OP, the readings he is getting are perfectly normal.

Edited by Prime Suspect
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You cannot adjust the accuracy of a GPSr.  They don't measure anything.

Sure they do. They measure time intervals, and if the internal clock is miscalibrated, the coordinates will be off. And since temperature affects the accuracy of the clock, there's an internal temperature sensor that's used to adjust the clock data on the fly. So if the temperature sensor is not functioning correctly, you can also get bad coordinates.

 

GPSs do measure, and the can go out of calibration. But it's very rare, and when it does happen, it needs to go back to the manufacturer.

I don't think any of that's correct. As I understand the nature of the way GPSrs work, they have accurate time without regard to the accuracy of their internal oscillator (clock). After four satellites are obtained they calculate the time exceedingly accurately using the satellites themselves.

 

For these gadgets to work they require the time accuracy of atomic clocks. This would make them cost prohibitive for ordinary people to own, and no other clock is accurate enough. This exceptional accuracy is why GPSs are used for high precision time applications that have nothing to do with navigation.

 

Of course the internal oscillator could fail. In that case the unit would quit working, like I said in my first post. The idea that the internal oscillator can be "miscalibrated" or drift, causing it to produce incorrect results is a non sequitur. To repeat what I said earlier, GPSrs are computers analyzing data received from the satellites and displaying the results. They either work or they don't. Their accuracy is dependent on the quality of the signals they receive, and the quality of the algorithms they contain.

Edited by Thot
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I have owned 2 Garmin E-maps, and one E-trex in the past. My newest GPS is Garmin GPS V. I got it cheap, because it was outdated (2003). The signal fluctuates terribly, sometimes it will say I'm 100 feet away, and then jump to .10 of a mile briefly. My other units never did this. One day I was walking up to a cache and was 40 ft away and then the unit suddenly changed and said 6.2 miles. @#$%^&!!! I waited a few minutes, but it never changed back. I shut it off, and turned it back on and then it worked fine. I dont understand why it would do this. Sometimes I notice signals from only 2 satellites and the accuracy will say 30 feet. ??? I know that you need at least 3 satellites to get a location marked, and 4 for altitude, so how can this be? Also, the directional arrow seems to have a time delay of 5 seconds. Does anyone know if this is a design fault, or just a crappy unit(which may need repair)??

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I don't think any of that's correct. As I understand the nature of the way GPSrs work, they have accurate time without regard to the accuracy of their internal oscillator (clock). After four satellites are obtained they calculate the time exceedingly accurately using the satellites themselves.

The original poster was correct. The quartz ocillator in your receiver does indeed flywheel off GPS time. If you had a bad crystal in your receiver, you would notice a large fluctuation in your calculated position.

 

Peace,

TeamRJJO

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If you had a bad crystal in your receiver, you would notice a large fluctuation in your calculated position.

Can you clarify what you mean by "a bad crystal?" If you mean one that has failed or is defective that's one thing. If you mean one that is out of calibration or has drifted as a result of something like temperature effects that's another. In either case I don’t think of a crystal oscillator as “measuring” anything.

Edited by Thot
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I don't think any of that's correct.  As I understand the nature of the way GPSrs work, they have accurate time without regard to the accuracy of their internal oscillator (clock).  After four satellites are obtained they calculate the time exceedingly accurately using the satellites themselves. 

 

For these gadgets to work they require the time accuracy of atomic clocks.  This would make them cost prohibitive for ordinary people to own, and no other clock is accurate enough.  This exceptional accuracy is why GPSs are used for high precision time applications that have nothing to do with navigation. 

 

Of course the internal oscillator could fail.  In that case the unit would quit working, like I said in my first post.  The idea that the internal oscillator can be "miscalibrated" or drift, causing it to produce incorrect results is a non sequitur.  To repeat what I said earlier,  GPSrs are computers analyzing data received from the satellites and displaying the results. They either work or they don't. Their accuracy is dependent on the quality of the signals they receive, and the quality of the algorithms they contain.

The following is from a Garmin engineer (emphasis mine):

 

As we know from GPS 101, A GPS receiver must be precisely synchronized with the satellites to operate. We must know the exact time the signal left the satellite, and the time the signal arrived at the receiver, to calculate the time difference and corresponding range to the satellite. By knowing the precise location of the satellite when the signal left, and the range (pseudorange), we know our exact distance from that satellite (at that point in time). By performing this calculation on multiple satellites, we can triangulate and calculate our position.

 

Obviously, timimg is everything. We must have a precise timing source for this to work. We could install a rubidium or cesium beam oscillator in our GPS receivers, but this would be a little pricey, use a lot more battery power, and the unit would be a little bulky. Instead, we use a relatively cheap oscillator, and a lot of software finesse. Oscillator compensation data is stored in the unit as a table based upon temperature. When the unit locks on, it calculates the unit's oscillator error and enters a correction factor into this table based upon current internal temperature. In this fashion the unit is CONSTANTLY "learning" and fine tuning itself. We burn these units in when new to calculate and store calibration constants across the entire rated temperature range of the product.

 

So, as I said, the accuracy of a unit is tied to internal clock, and the internal temperature sensor.

Edited by Prime Suspect
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We burn these units in when new to calculate and store calibration constants across the entire rated temperature range of the product.

 

So, as I said, the accuracy of a unit is tied to internal clock, and the internal temperature sensor.

Unfortunately, what you just quoted shows that your original claim, that a GPS unit can get "out of calibration" without failing entirely, was incorrect.

 

If the temperature compensation tables get screwed up, your GPS is broken, not "out of calibration."

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We burn these units in when new to calculate and store calibration constants across the entire rated temperature range of the product.

 

So, as I said, the accuracy of a unit is tied to internal clock, and the internal temperature sensor.

Unfortunately, what you just quoted shows that your original claim, that a GPS unit can get "out of calibration" without failing entirely, was incorrect.

 

If the temperature compensation tables get screwed up, your GPS is broken, not "out of calibration."

Broken, yes. But also still able to generate a navigation solution in some cases. I think that's what the original post was asking.

 

Peace,

TeamRJJO

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Ok...I scanned the responses and didn't see anything about this so I will add my two cents. I have a Sportrak color and YES you can calibrate the GPSr. I don't know of any other Magellan that can be (or has to be) calibrated, but this one has to be recalibrated every time you change batteries. Press your Menu button and go to Setup...towards the bottom of the list is Calibrate. The system will walk you through the steps. This must be done every time you change batteries and your GPSr should prompt you to do so.

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Broken, yes.  But also still able to generate a navigation solution in some cases.  I think that's what the original post was asking.

But, we are now in a digression from the OP. My post that spawned this "does it measure anything" digression started over my reply to the OP's question that's the Title for this thread where he asks:

Gps Seems To Be Off, How to adjust accuracy?

I replied to this question saying:

You cannot adjust the accuracy of a GPSr. They don't measure anything. They are computers analyzing data received from the satellites and displaying the results.
Edited by Thot
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Ok...I scanned the responses and didn't see anything about this so I will add my two cents. I have a Sportrak color and YES you can calibrate the GPSr. I don't know of any other Magellan that can be (or has to be) calibrated, but this one has to be recalibrated every time you change batteries. Press your Menu button and go to Setup...towards the bottom of the list is Calibrate. The system will walk you through the steps. This must be done every time you change batteries and your GPSr should prompt you to do so.

But that calibration is only for the built-in compass, right?

 

JC

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I have a SporTrak Pro as one of my GPs's. I have noticed that it has difficulty keeping a lock amongst tall buildings. Other than that it has always been extremely accurate. I rarely have been more than 20" from the cache and more often less then 10. That being said you have to take into account whether the cache hiders coordinates are correct as well.

 

I found 1 cache that was more than 1/2 mile from the hiders coordinates. If it hadn't been for a previous finder posting corrected coordinates I never would have found it.

 

:)

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We burn these units in when new to calculate and store calibration constants across the entire rated temperature range of the product.

 

So, as I said, the accuracy of a unit is tied to internal clock, and the internal temperature sensor.

Unfortunately, what you just quoted shows that your original claim, that a GPS unit can get "out of calibration" without failing entirely, was incorrect.

 

If the temperature compensation tables get screwed up, your GPS is broken, not "out of calibration."

Your GPS (well, Garmins at least, I don't know about Mags) can function without the temperature tables. Just not as well. If this weren't true, they wouldn't have made it possible to wipe the table with the right combination of button presses. Once wiped, the unit won't perform as well until it rebuilds a new table on its own. Whether the degraded performance is considered "broken" is a matter of semantics.

Edited by Prime Suspect
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Sure they do. They measure time intervals, and if the internal clock is miscalibrated, the coordinates will be off. And since temperature affects the accuracy of the clock, there's an internal temperature sensor that's used to adjust the clock data on the fly. So if the temperature sensor is not functioning correctly, you can also get bad coordinates.

At the risk of belaboring the point others have made. This is 100% wrong. The clock in a GPS receiver is the same thing as in the $10 Timex on your wrist, with possibly a little temperature compensation circuitry added. It never has to be calibrated externally and can't be “miscalibrated” because it is calibrated continuously while receiving satellites by syncing with the signals being sent by those satellites.

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Ok, I'll let the brainiacs argue over the deep internal workings of the GPSr, but to help the OP, I'll ask your method of searching.

 

If you are turning on your GPSr right in the cache area (or searching for one that is only a short walk) the receiver may not have had sufficient time to download the almanac data. This can cause significant errors in your reported position.

 

Try turning it on and leaving it where it has a good view of the sky for 15 or 20 minutes, then try the cache again.

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Ok, I'll let the brainiacs argue over the deep internal workings of the GPSr, but to help the OP, I'll ask your method of searching.

 

If you are turning on your GPSr right in the cache area (or searching for one that is only a short walk) the receiver may not have had sufficient time to download the almanac data. This can cause significant errors in your reported position.

 

Try turning it on and leaving it where it has a good view of the sky for 15 or 20 minutes, then try the cache again.

I sat on my living room couch and turned on the unit and watched to see how my position changed with time. When the unit first locked in to the satelites, it showed my position as 42 36.054, 74 50.870. As I watched over the next 10 mins, it went to 42 36.050, 74 50.868 and more or less stayed there. My new question is how many feet is .001 degree?

 

JC

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Ok, I'll let the brainiacs argue over the deep internal workings of the GPSr, but to help the OP, I'll ask your method of searching. 

 

If you are turning on your GPSr right in the cache area (or searching for one that is only a short walk) the receiver may not have had sufficient time to download the almanac data.  This can cause significant errors in your reported position.

 

Try turning it on and leaving it where it has a good view of the sky for 15 or 20 minutes, then try the cache again.

I sat on my living room couch and turned on the unit and watched to see how my position changed with time. When the unit first locked in to the satelites, it showed my position as 42 36.054, 74 50.870. As I watched over the next 10 mins, it went to 42 36.050, 74 50.868 and more or less stayed there. My new question is how many feet is .001 degree?

 

JC

If I figure right, it is about 350'

 

JC

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I sat on my living room couch and turned on the unit and watched to see how my position changed with time. When the unit first locked in to the satelites, it showed my position as 42 36.054, 74 50.870. As I watched over the next 10 mins, it went to 42 36.050, 74 50.868 and more or less stayed there. My new question is how many feet is .001 degree?

 

It looks more like you want to know how many feet are in 0.001 minute, not 0.001 degree.

 

It's different for latitude and longitude. For latitude (the N coord), it's always 6.074 feet. For longitude (the W coord), it depends on the latitude where you live. (For me, it's about 4.55 feet.) See this section of Markwell's FAQ for a description.

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.001 minutes is about 6 feet in latitude, and less than that in longitude, depending upon how far north you are (degrees are big at the equator and very small near the poles. Your measurements changed about (24^2+9^2)^.5=26 feet or so.

 

Edit: to account for your latitude of 42° 35.054.

Edited by Sputnik 57
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I sat on my living room couch and turned on the unit and watched to see how my position changed with time. When the unit first locked in to the satelites, it showed my position as 42 36.054, 74 50.870. As I watched over the next 10 mins, it went to 42 36.050, 74 50.868 and more or less stayed there. My new question is how many feet is .001 degree?

 

It looks more like you want to know how many feet are in 0.001 minute, not 0.001 degree.

 

It's different for latitude and longitude. For latitude (the N coord), it's always 6.074 feet. For longitude (the W coord), it depends on the latitude where you live. (For me, it's about 4.55 feet.) See this section of Markwell's FAQ for a description.

Yes, you are right. My mistake. I just remembered those are mins, not degrees. Duh!

 

After work I plan to calibrate the compass and leave on for a while and then try to find that cache again and see what happens. If nothing else it will tell me something about how consistant the unit is because I remember exactly where it placed me last time.

 

Thanks everyone for your help.

 

JC

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I sat on my living room couch and turned on the unit and watched to see how my position changed with time. When the unit first locked in to the satelites, it showed my position as 42 36.054, 74 50.870. As I watched over the next 10 mins, it went to 42 36.050, 74 50.868 and more or less stayed there. My new question is how many feet is .001 degree?

 

It looks more like you want to know how many feet are in 0.001 minute, not 0.001 degree.

 

It's different for latitude and longitude. For latitude (the N coord), it's always 6.074 feet. For longitude (the W coord), it depends on the latitude where you live. (For me, it's about 4.55 feet.) See this section of Markwell's FAQ for a description.

Yes, you are right. My mistake. I just remembered those are mins, not degrees. Duh!

 

After work I plan to calibrate the compass and leave on for a while and then try to find that cache again and see what happens. If nothing else it will tell me something about how consistant the unit is because I remember exactly where it placed me last time.

 

Thanks everyone for your help.

 

JC

:huh:

Well, I give up on this one (HomeTown in Hopewell Jct, NY). I went and calibrated and recalibrated the compass, left on for 25 mins before using, etc and I was still getting bad results at the site.

 

I went there again and as I was walking along the path I got to the point where it last time said the cache was but the distance now indicated about 200' feet more to go which I said "Great, because the pictures of the cache posted by others were taken at a park bench" and the bench was about 200' from this position. However, as I got within 100' of the bench, the unit jump around and said it was 100' behind me and it brought me back to the place I was a few days ago. As I walked around it placed me at various spots within a 30' radius of the original spot. I didn't realized how inacurate these things are. It must be the heavy tree cover but I had good signals on at least 4 satellites. But I couldn't get a consistant fix to the cache. With all the trees and all the bushes and hidding places there were, I looked a little and just gave up and went home. Maybe the last person hid it REALLY good or maybe it is gone. Who knows. Oh, well. Maybe the next one will be easier.

 

JC

Edited by jocatch
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Ever thought when compass is in a spin, that its telling you are right on top of cache location as GPS interprets it... Kind of like a magnet compass will spin when its over the north or south magnetic pole...

 

Maybe you should set GPS down at the 200 foot mark and ignore it from then on and walk in to location and search.... You may be relying on GPS to much and not using your instincts enough....

 

But then again I have had same type of problem, when I have been within about 20 feet or so of cache location..the spin that is, not 200ft...

 

Dale

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Ever thought when compass is in a spin, that its telling you are right on top of cache location as GPS interprets it... Kind of like a magnet compass will spin when its over the north or south magnetic pole...

I think he's talking about his real compass built into his GPSr, and you are talking about the pseudo compass that points at the cache.

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I sat on my living room couch and turned on the unit and watched to see how my position changed with time. . . .

Unless your couch is against a window I'm surprised you got satallite lock at all inside your living room.

My couch is about 12 feet into my livingroom, & away from the nearest picture window. From that couch, my iFinder Pro will go from 0-to-satellite-lock in about 40 seconds, sometimes less :huh:

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Hmm . . .  Kinda makes you wonder what Line Of Sight means.  Garmin says:
The signals travel by line of sight, meaning they will pass through clouds, glass and plastic but will not go through most solid objects . . .

No, kind of makes you wonder about the quality of newer construction...

"Line of sight" just means that since the determination of position is based on knowing the precise signal travel times, it is dependent on the signals going in a straight line from the satellite to your receiver rather than bending around obstacles or reflecting off them (the latter is called multi-path and results in substantial position errors). But many materials are opaque to visible light and still pass microwave frequencies with little attenuation.

 

I wouldn't draw any conclusions on construction quality based on absorption of microwaves. Any thin metallic foil layer (such as used on some types of insulation) will effectively block the signals, but many sturdy materials will allow microwaves to pass through with only minor absorption. The heavy spanish tiles on our roof still let me get signals quite easily while inside.

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