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how accurate can a GPS get?


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I was on the ocean today in Maine(it was freezing cold, by the way, water = air = 50F!), first time with my Extrex, and got an accuracy of +/- 7 feet. That's the best I have ever seen personally. Is there an equipment-based accuracy threshold? I'm talking about typical handhelds, not military stuff.

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GPS accuracy is a statistic so it depends on what and how one defines as accuracy. Accuracy means different things to different people.

 

A unit which can display/output to 4/1000' has the accuracy resolution capability of around 0.09m (~4 inches) and they can/will do that (maybe about 0.5% of the time) but that's not what one would call accuracy.

 

There's no great difference (at all) in accuracy between typical handhelds and equiv military receivers.

 

Cheers, Kerry.

 

I never get lost icon_smile.gif everybody keeps telling me where to go icon_wink.gif

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I've had a GPSr show me within 6 feet of the coordinates. Unfortuantely, I was still a good 60 feet from the real cache location. The next time I was back in the area and double-checked, the same receiver had me elsewhere. Don't assume that the numbers have an absolute meaning...

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quote:
Originally posted by EScout:

...To me, the accuracy of a GPSr is how close you are to a known place on earth, when you entered those coordinates as your waypoint.


 

That's about what it is and anything that a GPS shows in any way shape or form doesn't even come close to the definition of accuracy what so ever. Some "official" definitions

 

"Positioning accuracy: The statistical difference between position measurements and a surveyed benchmark for any point within the service volume over a specified time interval"

 

"Positioning accuracy represents how well the position solution conforms to "truth". Truth is defined to be any specified user location where the position is known within acceptable error tolerances and with respect to an accepted coordinate system, such WGS84 Earth-centred, Earth-fixed coordinate system"

 

EPE simply fails to meet those requirements.

 

Cheers, Kerry.

 

I never get lost icon_smile.gif everybody keeps telling me where to go icon_wink.gif

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Originally posted by Kerry:

GPS accuracy is a statistic so it depends on what and how one defines as accuracy. Accuracy means different things to different people.

 

A unit which can display/output to 4/1000' has the accuracy resolution capability of around 0.09m (~4 inches) and they can/will do that (maybe about 0.5% of the time) but that's not what one would call accuracy.

 

quote:

There's no great difference (at all) in accuracy between typical handhelds and equiv military receivers.
quote:

 

GREAT difference is subjective, as well! Military GPSr's, accuaracy can be measured in centimeters v. meters for civies...

 

Cheers, Kerry.

 

I never get lost icon_smile.gif everybody keeps telling me where to go icon_wink.gif

 

Never say Never...Never say Always!

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quote:
Originally posted by timberlane74 & pumpkin:

 

GREAT difference is subjective, as well! Military GPSr's, accuaracy can be measured in centimeters v. meters for civies...


 

"Military GPSr's accuracy measured in centimetres", another one of the myths and misconceptions.

 

Quite frankly military PPS cm accuracy is rubbish and if you want to go into the details well that won't be a problem either.

 

Cheers, Kerry.

 

I never get lost icon_smile.gif everybody keeps telling me where to go icon_wink.gif

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Well, kerry can believe whatever he wishes, but off-the-shelf Trimble GeoExplorer and Trimble Pro XRS consistently get sub meter accuracy here. Cm accuracy is possible with good mission planning and post differential correction.

 

The rec grade models are generally 10 meter or so.

 

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"The time has come" the Walrus said "to speak of many things; of shoes and ships and sealing wax, of cabbages and Kings".

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Dave54, I'm only too well aware of accuracy capabilities and it's not a case of believing in anything.

 

As I mentioned accuracy means different things to different people and all accuracy is relative depending on how that accuracy is achieved.

 

quote:
but off-the-shelf Trimble GeoExplorer and Trimble Pro XRS consistently get sub meter accuracy here

 

That statement is not totally correct, yes a Pro XRS can achieve sub-metre accuracy BUT only in real-time and generally only from a TRS and then it's all dependent on distance.

 

And yes CM accuracy is possible but that's also time dependent, the more accuracy requires more time and again this is distance dependent.

 

Other units are capable of sub millimetre but that really besides the point in this discussion about military type accuracy.

 

What some have to realize that a PLGR in comparable mode is no more accurate than many commercial handhelds. Sure PLGR have some aumentation capabilities but so does a lot of other uinits, really need to compare apples to apples and accuracy to accuracy.

 

One thing the PLGR can't do is post process but the DAGR will and being dual freq then that will have different accuracy capabilities again.

 

In the real world any manufacturer who quotes SPS accuracy less than (better than) what the system specifications are really kidding themselves.

 

Future performance can not be guarantted based on historical performance.

 

Cheers, Kerry.

 

I never get lost icon_smile.gif everybody keeps telling me where to go icon_wink.gif

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I typically get around 1.5 feet horizontal and 5 feet vertical witha ProXRS. I do pay a lot of attention to the almanac files though and generally only work when I know I'll be seeing at least 9 satellites on a good spread.

 

What we need is a handheld unit that can download the almanac files. It'd make hunting, and more importantly hiding a whole lot more accurate.

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How accurate do we really want them to be? I mean think about it, do you really want to be lead directly to every cache? What fun would that be? I admit that I can usually spot a cache location as I am nearing ground zero, but I like a good search every now and then.

 

As for accuracy, there are so many variables that come into play both with the hider and the finder.

 

Kar

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20 feet is what I consider good.

 

Best I have had up here is 12 feet.

 

WAAS dont work up here. icon_frown.gif

 

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quote:
Originally posted by Citizensmith:

I typically get around 1.5 feet horizontal ....


 

Could you explain how you achieve that.

 

quote:
What we need is a handheld unit that can download the almanac files.

 

All GPS receivers (including handhelds) have to download an almanac, no almanac, no work. Even the most simple Garmin can download and almanac.

 

Cheers, Kerry.

 

I never get lost icon_smile.gif everybody keeps telling me where to go icon_wink.gif

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Related to this accuracy thing. I did a little test with my 76S. I created a new waypoint and used the average location. Except I let it average for about 12 hours. Neat thing about this was that the reported accuracy was around 1 foot when I finally saved the point It seems to decrease the more with the number of points taken for the average. Did this 3 times and downloaded the waypoints into ExpertGPS. This is where I discovered something new. In part of the setup you can enable sub-meter accuracy. This does not change the displayed LatLon numbers but the UTM gets carried out further. All 3 points were with 1.5'.

Just some interesting observations.

 

Spange & Crew

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People keep coming up with stories about this mystical GPSr that is capable of pinpoint accuracy down to centimetres, even millimetres. They call them commercial units or military units and so on. So.....where are these miracle devices.

 

The truth is they don't exist. There is no such thing as a unit that can pinpoint so finely. Before all the whingers get on and tell me I don't know what I'm talking about you better find some good info/facts about it because I do know my stuff. Also my brother is a mapping/surveyor and they don't use GPSrs for this sort of work because the signals are not accurate or reliable enough.

 

Also, another revealed fact that gets all the show offs backs up is that there is little or no difference between GPSr accuracy at all these days. So if you bought an ultra ultra expensive Magellan and an expected to get better accuracy than the base model Etrex, you wasted your money (A WAAS activated Unit is slightly more accurate but that's not what I'm talking about). You can buy lots of features built in but at the end of the day a 12 channel unit is a 12 channel unit and the accuracy is the same.

 

In the sky accuracy is vastly improved overall, and ANY unit will display far superior reliability than one on the ground where there are far to too many obstacles and variables too measure with more than 5 or 10 metres of true accuracy.

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Averaging these days has limitations and time is one of them. The amount of time most are prepared to average coordinates is just about useless and in fact can actually degrade the accuracy.

 

Certainly 12 hours will show some results and 24 hours even better but 5 or 10 minutes basically a waste of time.

 

I'd have to agree with the 2 dogs that there is in no way shape or form a typical standalone SPS single freq (or even dual freq for that matter civil or military) receiver capable of some of the rather outrageous accuracy some "claim" from their receivers. In technical terms it simply ain't possible.

 

Augmentation, differential and some of the other methods totally different but those that claim 1 foot from something like a ProXR isn't a standalone type receiver.

 

An example (only a few weeks old) of 2 different receivers both measuring comparable SPS accuracy.

 

The (red) one on the left 95% accuracy 2.8m, the (blue) one on the right 95% accuracy 3.1m. These values are ACTUAL measured accuracy figures and the accuracy "difference" is less than a foot (0.3m).

 

The left numbers are from a $25,000 9 channel (dual freq but not that it matters in this case) and the right from a $500 garden variety 12 channel. The difference in accuracy is bugga all and considering a few other things totally irrelevent. Mind you that receiver on the left is "capable" of milliletre (even sub mm) type accuracy if "used" in certain ways but certainly not simply by itself in Standard Position Service mode just like most recreational receivers use, same system, same signals.

 

icon_smile.gif everybody keeps telling me where to go icon_wink.gif

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Two Dogs is right about different models. They all have the same accuracy today - only the features (mapping, on-board compass, memory size, etc.) make the difference in price. The displayed accuracy is just the manufacturer's assumption of the how close you probably are to the actual spot. If they knew exactly, then they could tell you when you were actually there. The displayed accuracy is probabgly based on an algorithm of how strong the signals are and how many are being received based on what the manufacturer has seen from actual measurements of how close you should be within a circle of accuracy. But the feet displayed is an assumption. It's also a good marketing tool. (Our unit is more accurate then your unit kind of thing). For example, when I get 7 feet today, I used to get 18 feet under the same conditions a year and a half ago before I downloaded Garmin's updated program. They just changed the displayed reading but the actual accuracy never changed. Garmin and other are probably right though that there old assumptions were too conservative. Their units are actual better than that.

 

The only real way to verify accuracy is against a known position like a USGS benchmark. These are calculated down to the centimeter. There's no point in checking against a location you or others created since the initial reading is inaccurate. How are you ever going to know whether your actually there. Maybe you just duplicating the original wrong reading.

 

My virtual cache at a benchmark has been gathering accuracy data for almost two years. Over 90% of the geocaching finders have reported maximum accuracy within 3-4 meters maximum regardless of the GPS model.

 

I computed the results over a 4 month period a year and a half ago. I haven't updated it since. But downstream results follow the same accuracy.

 

The following GPS’s were reported used including my own Vista when I set up the cache. Readings were taken with 13 different GPSr’s on 12 different days between 11/25/01 and 3/24/02 at different times of the day. Weather conditions were not reported.

 

Garmin Vista (4)

Garmin etrex (2)

Garmin Summit

Eagle Expedition II

Magellan Map 330

Magellan 300

Unknown

“Cheap” Garmin

Garmin Street Pilot

 

Summary of results. Difference calculated from NGS coordinates. WGS84 coordinates in DD MM.MMM were converted to UTM using Mapsource program. Distance from benchmark calculated taking the square root of the sum of the squares of the difference between the UTM latitudes and UTM longitudes in meters and converting to feet.

 

12 readings under 12 feet

5 readings between 13-25 feet

1 reading at 40 feet (note that this was the Magellan 300 which is an older unit and reads to 2 decimal places only - about 50 feet accurate I’ve been told).

 

All were single readings except for Eagle (13 readings = 3.3 feet avg.) and one of the etrexs (7 readings = 25 feet avg.). There was one WAAS reading reported at 3.3 feet.

 

The key to accurate readings in my opinion is clear sky and not moving the GPS. I think the results speak mountains as to the accuracy of the equipment we are using.

 

PS: If anyone's interested in updating the survey results from 3/24/02, please feel free to do so and post the results here.

 

Alan

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I have concluded that entering the coordinates of the cache from the listing only duplicates the reading the author's GPS provided at the time the cache was hidden. That means, assuming his/her equipment is accurate within 3 feet and my Meridian Platinum is accurate to within 3 feet, I should expect an accuracy of nothing better than 6 feet when I reach the approximate cache point. Therefore I don't expect to step on the cache when my GPS device reads "0" - I simply set up a radius of 12 to 15 feet and begin searching, working from my "zero" point and spiraling outward. That technique generally does the trick.

 

"Today's truth remains valid only as long as it withstands the test of tomorrow's discoveries" - George Hicks

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I think my post may have been mis-interpreted. I was not claiming to have 1' accuracy only that with the averaging function the greater number of data points brought the number lower for the waypoint being averaged. Viewing the track log indicates a simlar pattern to Kerry's plots. The individual points drifted approx. 3 meters.

If the data from Kerry's plots were averaged, I am curious as to how close to the true position would that number be?

When cache hunting I accept the posted coordinates and my GPSr to be only within 40-50 feet. Yes, Sometimes they are close to the same but more often they are not.

 

Lets have fun!

Spange & Crew

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I just get out my trusty Garmin 12 and follow the arrow. 50% of the time, it points right at the cache. Often times, when the bell starts ringing, I just bend down and pick up the cache. This week I actually stepped on a cache while looking at the GPSr. And who needs accuracy, anyway. If you can get within 40-50 feet, put the darned (beloved) thing away and use your head.

 

==============="If it feels good...do it"================

 

**(the other 9 out of 10 voices in my head say: "Don't do it.")**

 

.

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quote:
Originally posted by gallahad:

I have concluded that entering the coordinates of the cache from the listing only duplicates the reading the author's GPS provided at the time the cache was hidden. That means, assuming his/her equipment is accurate within 3 feet and my Meridian Platinum is accurate to within 3 feet, I should expect an accuracy of nothing better than 6 feet when I reach the approximate cache point. Therefore I don't expect to step on the cache when my GPS device reads "0" - I simply set up a radius of 12 to 15 feet and begin searching, working from my "zero" point and spiraling outward. That technique generally does the trick.

 

"Today's truth remains valid only as long as it withstands the test of tomorrow's discoveries" - George Hicks


 

So it worked on the 1 cache you've found? icon_biggrin.gif

 

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quote:
Originally posted by Kerry:
Originally posted by Citizensmith:

I typically get around 1.5 feet horizontal ....


 

Could you explain how you achieve that.

The ProXRS is a survey grade DGPS unit. The antenna alone is bigger than my little etrex. I'd need to check the trimble web site to find out what it does that lesser units don't. One thing it does do is record a ton of extra data on every point, line, or area feature you map so you can always go back a check how accurate you where at the time. Most of the time I'm recording point features and normally allow 30 seconds of averaging. For line and area I typically give 5 seconds per point.

 

quote:
What we need is a handheld unit that can download the almanac files.

 

All GPS receivers (including handhelds) have to download an almanac, no almanac, no work. Even the most simple Garmin can download and almanac.

 

Didn't realise that, but I don't have a manual for my etrex. It is the base model with no data transfer abilities though.

 

Thanks

 

Citizensmith

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quote:
Originally posted by The 2 Dogs:

People keep coming up with stories about this mystical GPSr that is capable of pinpoint accuracy down to centimetres, even millimetres. They call them commercial units or military units and so on. So.....where are these miracle devices.


 

I've done milimeter grade surveying. We never used GPS though, Everything was done by shooting from benchmarks using total stations.

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I own an outdated Magellan 2000 XL and at times it's been only a couple feet off. I found my last cache very easily -- my unit led me right to it -- but other people who logged the cache said their GPS unit was going haywire because of the cache being in a canyon and lots of trees around. Maybe GPS accuracy hasn't improved a whole lot over the years.

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quote:
Originally posted by Spange & Crew:

.... If the data from Kerry's plots were averaged, I am curious as to how close to the true position would that number be? ....


 

Averaging, basically a 50/50 chance for better or worse but of course depends on the length of time but many here don't have the time to average with any integrity. The thing is at the time one doesn't know if the position is better or worse.

 

If your curious, the following highlight the actual affects of averaging

 

http://www.cqnet.com.au/~user/aitken/gps/gps_avg.htm

http://www.cqnet.com.au/~user/aitken/gps/avg_pii.htm

http://www.cqnet.com.au/~user/aitken/gps/gps_obs.htm

 

Cheers, Kerry.

 

I never get lost icon_smile.gif everybody keeps telling me where to go icon_wink.gif

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