How accurate is your GPS?

[+ZombieDad]

I see a lot of logs that say coordinates are "Dead On". I also see a lot of logs that say a CO's coordinates are off by x number of feet. Whose coordinates are better? I have a Magellan Explorist GC and I almost never find "Dead On" coordinates. I assume my GPS is less accurate than others because I often have to search a wide area. I distrust my GPS so much that I rely more heavily on the tracking feature to zero in on a cache. I've read that it's common for coordinates to be off by about 20 feet, but it seems there is some level of accuracy that may be common to some GPS models. Do coordinates match up better between certain units or do I have one of lesser quality?

[+Glenn]

You can find some great information about GPS accuracy at gps.gov

 

Here is something interesting to try at least once. Turn tracks on in your GPS and let your GPS sit for a couple of hours. Then take a look at the track and see where your GPS went while it was sitting still.

[ao318]

Sometimes mine are spot on to the listed coordinates and sometimes they are over 20-30 ft off. Seems normal to me.

[+Trav'lin Two]

Find a benchmark and compare your coords with those listed for the mark.

[+Glenn]

Find a benchmark and compare your coords with those listed for the mark.

How accurate are the listed coordinates? Each page has the following disclaimer. "Coordinates may not be exact. Altitude is ADJUSTED and location is SCALED."

[+gpsblake]

Impossible to always have dead on coordinates because the error margin of the DD MM.xxx format is around 5 to 6 feet. Then throw in the potential error of the unit who placed the geocache. It's not your GPS unit in most cases. 20 feet off is completely normal, in fact, ten years ago, 50 feet was more normal of a variance.

 

On benchmarks, it's adjusted coordinates that you want to use, scaled coordinates are just taken from estimates from a printed map.

[+Ed_M]

Then take a look at the track and see where your GPS went while it was sitting still.

 

Never thought of it that way.

[+Pocileh]

By coincidence the same discussion (more or less) on the Dutch forum.

People seem to think that, due to more modern receivers, the accuracy should have improved as well. But no. Receiver accuracy basically has not changed. The receivers are better, more sensitive, indeed, so reception has improved, making it possible to use the receiver under worse conditions than "in the early days"

 

But what not has changed are things that affect the reception and quality of the GPS-signals like atmosferic conditions, Highrise buildings, narrow canyons etc.

 

But then, even in open and flat areas the atmosferic conditions can make your GPSposition wander around.

I do visit Confluencepoints sometimes. I was the first visitor of two points, both in Kasakhstan. In the middle of nowhere, see HERE and HERE .

Both located in flat area, more or lesse desert with just some low vegetation, no trees to been seen in any direction, no highrise buildings .

I tried to pinpoint the location as precisely as possible by observing my GPS for about 10 to 15 minutes and keeping it as steady as possible. Finally I was able to get a more or less dead on reading. But I needed quite a lot of pictures to get one showing the 000 reading on my GPS. In one location I gave up. Lowest I could get was 002. Even in a steady position your GPS-location will wander around. 20-30 feet is no exception.

Edited March 14, 2012 by Pocileh

[+Glenn]

By coincidence the same discussion (more or less) on the Dutch forum.

People seem to think that, due to more modern receivers, the accuracy should have improved as well. But no. Receiver accuracy basically has not changed. The receivers are better, more sensitive, indeed, so reception has improved, making it possible to use the receiver under worse conditions than "in the early days"

 

But what not has changed are things that affect the reception and quality of the GPS-signals like atmosferic conditions, Highrise buildings, narrow canyons etc...

 

Exactly. It can be difficult explaining to someone how this stuff works when they don't have a background in radio communications. Better signal strength does not necessary mean better accuracy. A way to think about is if you are talking to someone. If they start speaking louder does that make you better understand what they are saying? No. Well, only if they were speaking so softly at first that you couldn't understand them. It is the same for your GPSr. Once it gets a lock on the minimum required four satellites, in other words once it hears the satellites, the signal strength level isn't a factor that effects accuracy.

[terra_trek]

Interesting topic indeed. I am new to Geocaching and using my eTrex Venture HC. I did a set of new hides recently and had to go back to one of them because I forgot to leave a log in it. Using my coord.s and knowing exactly where it is I went right to the spot. My eTrex showed me being 10 feet away even though I was standing exactly on the hiding spot. That got me wondering about the accuracy of these things.

Another thing I have been wondering about is is it worth it trade up to a better model of GPS if accuracy doesn't get any better. Are the extra features worth the cost ?

Edited March 15, 2012 by terra_trek

[+StarBrand]

Interesting topic indeed. I am new to Geocaching and using my eTrex Venture HC. I did a set of new hides recently and had to go back to one of them because I forgot to leave a log in it. Using my coord.s and knowing exactly where it is I went right to the spot. My eTrex showed me being 10 feet away even though I was standing exactly on the hiding spot. That got me wondering about the accuracy of these things.

Another thing I have been wondering about is is it worth it trade up to a better model of GPS if accuracy doesn't get any better. Are the extra features worth the cost ?

Even my 11 year old Garmin Legend is as accurate as the eTrex 20 new one I have. In an open area anyway.

 

Yes the bells and whistles and functions and ability to hold onto the signal in tough conditions make the units better. Not much more accurate but better for Geocaching.

 

Lets just face facts, our handheld units have an average accuracy in the 15 to 20 foot range under most conditions. Add any potential error you have to the hiders error and finding a cache 10 to 20 foot off where your unit says GZ is - should not EVER be very surprising.

[+riffraff9000]

Using my eTrex 20, yesterday we found a cache and the unit read 22 feet off. The next cache, it read 2-4 feet off. So, ya never know. I mostly start looking for the cache with my "geo-sense" when the proximity alarm goes off at about 20 feet from GZ.

[+Gitchee-Gummee]

Using my eTrex 20, yesterday we found a cache and the unit read 22 feet off. The next cache, it read 2-4 feet off. So, ya never know. I mostly start looking for the cache with my "geo-sense" when the proximity alarm goes off at about 20 feet from GZ.

You also need to consider the "accuracy" of the cache placement. Either by the CO or the nearly inevitable "cache migration" effect brought about by cachers hiding it better than they found it.

[+dakboy]

Find a benchmark and compare your coords with those listed for the mark.

How accurate are the listed coordinates? Each page has the following disclaimer. "Coordinates may not be exact. Altitude is ADJUSTED and location is SCALED."

You want to find one where the location is adjusted, not scaled.

 

http://www.geocaching.com/mark/default.aspx#adjscl

[+StarBrand]

Even then....the continent is moving.

 

There was an article about a year ago about how new ultra precise measurements showed that North America is sliding off to the west at just under 2 inches a year(on average). A 12 year old cache placed with exact coordinates will now be up to 2 feet off. Benchmarks placed 75 years ago could be a lot more than that off (150 inches = 12.5 feet).

[tr_s]

Prior garmin recievers had me impressed by being able to zone in by the couple of meters... my shiny new eTrex-30, not so much.

Ten meter drift easily.

[+StarBrand]

Prior garmin recievers had me impressed by being able to zone in by the couple of meters... my shiny new eTrex-30, not so much.

Ten meter drift easily.

Finding a cache 10 meters off where your unit says GZ is should not be too much of a concern - it is near the edge of an expected error zone.

[tr_s]

Prior garmin recievers had me impressed by being able to zone in by the couple of meters... my shiny new eTrex-30, not so much.

Ten meter drift easily.

Finding a cache 10 meters off where your unit says GZ is should not be too much of a concern - it is near the edge of an expected error zone.

 

Indeed, it's not a problem and very acceptable - I don't even do much geocaching.

[+fizzymagic]

Experience really helps.

 

These days, when I approach a cache location, I automatically adjust my expectation for the accuracy of my GZ point according to the level of tree cover, nearby objects that could induce multipath errors, the "accuracy" value on my GPS, and the overall view of the sky. Out in the open, with a good accuracy reading on the GPS, and no nearby buildings or cliffs, I generally find that the GPS takes me right to the cache location, within 10 feet. In urban settings or a forest with a dense canopy, I am happy if I zero out within 20, or sometimes 30, feet.

 

The point is that GPS "accuracy" is not a single number. It is highly dependent on a number of factors, many of which you can observe and account for. If you understand the sources of the errors, it can help you adjust your expectations for accuracy when on-site.

 

I get annoyed when people in the forums say "GPS is only good to about 20 feet" and act as if being off by less than 30 feet is no big deal. Depending on the location, it might be no big deal, but in a location with a clear view of the entire sky and no objects around to induce multipath there is no excuse for coords being off by even 20 feet these days.