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GPSr accuracy


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I have an up to date Garmin 64st and have noticed that it GZs anywhere from 25 to 50' feet north of the cache location consistently.   I turned off Glonas to see if that made a difference but it didn't seem to make much if any difference. I'm going to get out my  old 60csx and see if there is a noticeable difference between the two, the 60 was always rock solid and brought me to with 10 feet of a cache even under a dense rainforest canopy.

I got to thinking that I've read that there has been significant movement in the earth's poles -- I wonder if that is playing a part in the accuracy issue?  Anyone else experience consistently poor accuracy?

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Is your GPS set to WGS84 datum?

As for the magnetic inclination, that only has an effect on the compass not on the GPS coordinate calculation (as you noticed comparing with an other GPS). It will not point to true North (N90°) but to a point in Northern Canada not to far from Greenland. We noticed this when walking almost "North" according to the compass but actually walking almost East on the map.

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1 hour ago, Difranco said:

I'm going to get out my  old 60csx and see if there is a noticeable difference between the two, the 60 was always rock solid and brought me to with 10 feet of a cache even under a dense rainforest canopy.

 

Comparing the two is a really great idea. Do that several times on several locations.

 

The problem with accuracy of the GPS device versus cache coordinates is how do you know that those are "exact" (they are not, never!). If the cache owner's coordinates are 7 metres off and your GPS device shows an accuracy of 3 metres (what ever the shown accuracy means for reality!) the distance between the cache and your "point 0" may be up to 10 metres or even more as the shown accuracy is nonsense in my eyes.

 

Surely there is a difference between random deviations (normal!) and systematic ones (shouldn't be). And if your new device is always North of the coordinates of the etrex it's a systematic one. In that case I would contact the support of GARMIN a both devices are GARMIN devices. If you have a list of coordinates (please more than 2 or 3) it shows a systematic problem and they might help. It might be a setting problem or your new device has other issues - but I am sure they know some things to try out. At least they should know best what may lead to such problems.

 

Perhaps you'll even find out that it's not a North problem but a genuine distance problem - and I fear that's a normal phenomenon.

 

Good luck!

Jochen

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I can only anecdotally provide something that worked for me in the few situations where what is described has happened to me.  All I know is that my two primary units (60 CsX and Montana) would occasionally vary as described and that my recalibration, for whatever reason, cleared up the problem enough to render the discrepancy to a point that was negligible.  I can't explain why it worked, only that it did work.  That doesn't mean it would work for this situation or anyone else experiencing something similar.  While it seems illogical, it worked for me.  

 

The only other thing I've done that seems to have worked when I felt one of my GPS devices wasn't really locking in and believed my accuracy was suffering (when I didn't have access to both units to compare accuracy) is what is suggested when you travel a long distance without the unit on.  I turn the unit on and leave it in a secure spot with a clear view of the sky for a longer period of time (15-30 minutes) so it can acclimate.  Again, this is something I've only had to do a couple times and for whatever reason, it managed to clear up the issue enough that it wasn't a problem moving forward from that point.

 

I should also point out that both of my units didn't/don't include the ability to acquire GLONASS.  I don't expect that would make much difference but it's not quite the same comparison to the OP, whose unit can acquire both sets of signals.

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11 hours ago, Difranco said:

I have an up to date Garmin 64st and have noticed that it GZs anywhere from 25 to 50' feet north of the cache location consistently.   I turned off Glonas to see if that made a difference but it didn't seem to make much if any difference. I'm going to get out my  old 60csx and see if there is a noticeable difference between the two, the 60 was always rock solid and brought me to with 10 feet of a cache even under a dense rainforest canopy.

3 hours ago, Difranco said:

I'll try a re-calibrate.   

 

That is odd.     :)      Civilian GPS is still only "accurate" to around ten feet. That may change eventually, but not for now.

"Always" brings you to ten feet is phenomenal. 

You see, that ten foot "accuracy"  in finding a container in the woods is for both you and the CO.  It's rare when both of you have a perfect day.  

Add in that your GPS unit may have a +/- location  accuracy of (let's say) 20 feet, and you could be a good distance off.

Are the caches you're finding now newer ?   

We haven't seen new folks using phones as concerned with "accuracy" as those years earlier using a GPSr.

 - They believe their top-o-the-line phone is the most accurate simply because it's the newest model out.

Some don't understand , or figure it's not needed to "average" coordinates.

 

Cool.  It'd be fun to see what you come up with.  :)

Edited by cerberus1
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4 hours ago, Difranco said:

I'm going to get my 60csx out compare about dozen caches to see if there is any difference --  I'll leave the 64st with the GLONAS off to make sure the comparison is apples to apples.    I'll  report back in a week.

 

I'll be interested to see the results.  As Cerberus mentions, a +/- of 10 ft. is considered normal so something that puts you at 20 feet is still within a +/- 10 of your coordinates and the posted coordinates.  Occasionally I get poorer reception and find myself with a +/- of 20 feet, which gives me a possible +/- of 30 feet and I'm generally OK with that.  Much over that and it's either bad coordinates on the CO's part, a bad signal on my part, or a combination of the two.  However, if were a continually repeated error among a wide variety of COs, then I would suspect something might be off.  I do remember finding Magellan placed caches on my Garmin consistently about 20 feet off from where I ended up finding the cache within my first couple years of caching.  Not so much now.

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I used to cache with a couple people (both north of 25K finds) that used to carry two GPSr of different brands because of how they calculate position so GZ was varied between the two and some COs were using TomToms and for some reason there was noticeable variance.   The caches I've mostly been going after are older caches, so I'm wondering if there is a difference between GPSrs of different generations.

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The problem with two units, is if they disagree which one is the more accurate?  Three is a better choice, if two agree (somewhat) that's a good indication.  It they form a triangle, try the middle.

 

I can't test the 60 vs. 64, my 60 died when I crushed the screen as it cushioned my hip from a rock.  But I do have a 62, 64 and 66 sitting outside right now to see how close the lat/lon match.

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Results of my quick test (~30 minutes):

 

Degrees removed:  62 - 26.404    11.770

                                   64 - 26.405    11.772

                                   66 - 26.4043  11.7763

 

They were sitting side by side on a table with good view of the sky.

 

ETA - All were using just GPS.

  The 62 & 64 started out agreeing form the start (about the same numbers), but the 66 first showed 26.4352 11.7780

 

Edited by The Jester
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2 hours ago, The Jester said:

Results of my quick test (~30 minutes):

 

Degrees removed:  62 - 26.404    11.770

                                   64 - 26.405    11.772

                                   66 - 26.4043  11.7763

 

They were sitting side by side on a table with good view of the sky.

 

ETA - All were using just GPS.

  The 62 & 64 started out agreeing form the start (about the same numbers), but the 66 first showed 26.4352 11.7780

 


 

 

2 hours ago, The Jester said:

The problem with two units, is if they disagree which one is the more accurate?  Three is a better choice, if two agree (somewhat) that's a good indication.  It they form a triangle, try the middle.

 

I can't test the 60 vs. 64, my 60 died when I crushed the screen as it cushioned my hip from a rock.  But I do have a 62, 64 and 66 sitting outside right now to see how close the lat/lon match.


The third unit is the CO coordinates of whatever cache I am seeking.   But now, I'm going to add a 3rd GPSr  60Csx, 64st, and we'll get out my wife's eTrex (i forget the model).  All units have the latest firmware.

The methodology will be side by side at GZ then take an averaged reading for a mark.   So in total I'd have four measurements.   All units will be calibrated at the GZ prior to a measurement being taken.

 

Edited by Difranco
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