GPS accuracy

[+goneautobahn]

I'm fairly new to this and am using a borrowed GPS, Garmin Legend. Every time I'm out, it averages an accuracy of 20-30 feet, and jumps around quite a bit even when standing still. This makes finding micro's especially difficult. I've seen comments where people say they walked right to it and I walked in circles. Some comments have said came back with a better GPS and walked right to it. Do different brands have better accuracy?

[+budd-rdc]

It's not just limited to different brands. Even different models by the same brand can behave differently. It would be inaccurate to say that the accuracy is different, pardon the pun.

 

The Garmin eTrex Legend responds to position changes slower than the Garmin GPSMap 60Cx, or even the older 60C. The behavior you are describing seems to occur in tree cover, or in urban areas with tall buildings and/or power lines above.

 

The Magellan I use (GPS Companion) doesn't jump around, but it's always reporting coordinates 40' to 50' behind me, and worse when I'm in deep forest cover. It can get confused if I'm not travelling in a straight line. Once I stop to allow it to "catch up", it's as accurate as other GPS's I've used.

 

Lowrance I use (iFinder GO) is very precise in urban areas but it tends to wander unpredictably in heavy tree cover. It's accurate as other GPS's I've used as long as I keep it still for a very long time, sometimes 5 minutes or more.

[ImpalaBob]

Plus or minus 30 feet is about as good as it gets. Your accuracy means nothing if it is not in sync with the cache owner. Your Garmin Legend is as capable as any other GPS unit for finding caches. Over time you develop better finding skills!!!! Sure you can pay for a newer unit that locks on Sats quicker and does not bounce as much .... but in the long run it has no better accuracy. ImpalaBob

[+Renegade Knight]

As a general rule anyone who "walks right up" to a micro find the cache in the exact spot they would have hid it themselves.

 

Some days you are hot and some days you are not.

 

The GPS unites you can buy, and the GPS units used to mark the caches location for the listing flat out are not accurate enough to put you exactly on the cache.

 

The GPS units that can do that cost thousands of dollars and would take you some time to set up.

[+JSWilson64]

I noticed the same things with my Legend - it doesn't like being under tree cover. I'd set it to Go To the cache, and get to where it said I was within 3 feet of the cache, move 2 feet, and it says I'm 40 feet away!

 

Civilian GPS receivers will all pretty much say they're accurate to 10 feet - that's the supposed limit of the civilian GPS system. Where different models differ is in their ability to maintain signal lock in different terrain and vegetation conditions. Also, weather and atmospheric conditions can play a role, as can where you are on the earth -- reception of the WAAS satellites seems to vary with location. Models with the SirfStar III chipset seem to have an easier time of it, but it's not guaranteed.

 

I was playing with a new 60 CSx this weekend on a Boy Scout outing with my son. I waited for the unit to stabilize at about ±15 feet in the open, then moved into tree cover. Accuracy went to ±18 feet under some still-leafy trees, after first jumping to about ±25 feet.

 

Etrex Legend + magnetic compass can be just as accurate as the latest and greatest - it just takes some practice. Get to the edge of the tree cover, wait for the reception to settle down, shoot a bearing with your compass, and pace it off. When you get "there" put some sort of mark down - stick a stick in the ground, draw an X with your foot, whatever. Then go back to the edge of the trees, move to a new location, shoot a new bearing, and pace it off again. (it's easier with more people)

[ossumguywill]

Plus or minus 30 feet is about as good as it gets. Your accuracy means nothing if it is not in sync with the cache owner. Your Garmin Legend is as capable as any other GPS unit for finding caches. Over time you develop better finding skills!!!! Sure you can pay for a newer unit that locks on Sats quicker and does not bounce as much .... but in the long run it has no better accuracy. ImpalaBob

Hmmmm? I usually get ~10-13 reported foot accuracy with my explorist 600. The real accuracy is often less. Also, each gps unit has been programmed diferently and at different times. The coordinate "grid" in the memory of another unit might be slightly "drifted" from my unit or any unit.

[+KelticFrog]

Users must be very skeptical of the “accuracy” claims made by GPS manufacturers. The ultimate accuracy of any GPS receiver is determined by a wide variety of variables that are difficult to measure especially in low cost, handheld units. These include measurable variables such as satellite geometry as well as several that can only be estimated like GPS unit oscillator stability; tropospheric and ionospheric effects; and the effects of satellite clock aging. The specification for C/A code GPS accuracy is on the order of +/- 13 meters. If that sounds like a lot, remember that the specification is a goal, not the actual performance. Typical GPS accuracies are usually much better than that. If your receiver is WAAS or differential capable, you can expect accuracies at least twice as good. When engineers talk about GPS accuracy, they generally talk in terms of 95% CEP (for 2 dimensional fixes) or SEP (for 3D fixes). This stands for “Circular (or Spherical) Error Probable”. Simply stated, if you can collect the position data from your receiver over a known point for an extended period of time, you will find that the difference between the measured position and the actual position falls within a circle (or sphere) 95% of the time. The diameter of the circle or sphere is the accuracy of the GPS expressed as 95% CEP. If you could plot the measured positions you might be surprised to see how far off they can be at times. If you can locate a first order USGS survey mark, you can collect GPS data over that point for a few minutes. The average difference you observe between your receiver’s position and that of the published coordinates will give you a rough idea of the static accuracy of your particular receiver. Remember that when you’re moving, filtering errors in your receiver will introduce even more inaccuracy. Probably more information than you wanted. Short answer: A WAAS enabled receiver should give you about +/- 5 meters. When you’re searching for a well hidden cache this can seem like a mile.