Is it me or my GPS

[+MysterySolvers6]

Is there a approximate standard I should expect my GPS to be off by? I would thought, considering it's GPS technology, it would be accurate but I thought last year when I was shopping for one I read a forum that mentioned devices are accurate up to X feet. Any ideas? I'm using eTrex Legend HCx.

[+DatCrazyMongoose]

Manufacturers' claims are often under perfect conditions and can be highly exaggerated. In the field, if you get below +/- 25 ft, you're doing pretty well.

[knowschad]

Manufacturers' claims are often under perfect conditions and can be highly exaggerated. In the field, if you get below +/- 25 ft, you're doing pretty well.

 

Nawww... not with today's technology. Used to be true, but not anymore. I expect, most of the time, to get down to 10 feet or less most of the time (assuming, of course, that the hider also had recent tech and took cares to make sure the coords are good and were posted w/o typos)

[+9Key]

Manufacturers' claims are often under perfect conditions and can be highly exaggerated. In the field, if you get below +/- 25 ft, you're doing pretty well.

 

Nawww... not with today's technology. Used to be true, but not anymore. I expect, most of the time, to get down to 10 feet or less most of the time (assuming, of course, that the hider also had recent tech and took cares to make sure the coords are good and were posted w/o typos)

Depends on the conditions, IMO. In a deep forest with heavy tree cover I think 25 feet accuracy is ok. Once I get within 30 or so of GZ I start looking for the cache anyway.

[+JCrew 405]

I'm using eTrex Legend HCx.

 

That is the same GPS we use. We love it. Most of the time it's within 6ft, but sometimes up to 15. Also, it really depends on tree coverage or weather. This actually has the capability of being within 6in, but for national security purposes the govt. randomizes the signal for civilian use.

 

Hope this helps!

[+flask]

accuracy will vary. it is helpful to know what kind of accuracy you're getting at any particular moment.

 

if you're getting bad accuracy, expand your search area. if you suspect that accuracy is bad at this location most of the time, expand your search area by quite a lot.

[+Prime Suspect]

I'm using eTrex Legend HCx.

 

That is the same GPS we use. We love it. Most of the time it's within 6ft, but sometimes up to 15. Also, it really depends on tree coverage or weather. This actually has the capability of being within 6in, but for national security purposes the govt. randomizes the signal for civilian use.

 

Hope this helps!

Some wrong info there. Tree cover, yes. Weather, not really. The frequency used by the GPS system was specifically chosen because it's relatively unaffected by weather. And the government turned off Selective Availability (signal randomization) back in the Clinton administration. Military GPS has better accuracy because it uses an encrypted duel-signal transmission that reduces radio transmission errors. (Yes, I know that's a gross oversimplification.)

Edited June 16, 2009 by Prime Suspect

[+Arrow42]

With my Triton I can usualy get within 5-10 feet.

 

But... getting that accurate sometimes involves approaching the same spot from a few different directions and just averaging it in your head.

[Keystone]

I am moving this thread from the Geocaching Topics forum to the GPS and Technology forum.

[+BigHef32]

With my Triton I can usualy get within 5-10 feet.

 

But... getting that accurate sometimes involves approaching the same spot from a few different directions and just averaging it in your head.

 

Same with my Blackberry using Blackstar software. I can get to within 1 ft most of the time, but 2 seconds later it might say 25 ft. Leave, come in from another direction and it's usually right back to the same spot. Anything less than 10 ft is GZ IMO and time for the senses to take over where technology got me.

[+MysterySolvers6]

thanks for the info. everyone

[+olbol]

thanks for the info. everyone

 

There are a lot of info here already, but maybe I can add something. Let me say one thing first: Your accuracy as dispayed on your GPS-device is an estimate. I think it uses the variation between successive measurements and the constellation of the satellites to estimate it. Second, to sum up some of the things that will affect your accuracy: 1: The geometry of the satellites (if every satellite used for determining your position are right above your head; the position will probably be inaccurate). 2 The number of available satellites. 3. Ionospheric activity. 4. Vegetation or other objects that dillutes your signal. Objects may either block your signal or create so called multipath which means that the signal from the satellite isn't going directly to your reciever, but has been reflected from an object. 5. The quality of your antenna. 6. Observation time. Averaging a position over time will give better accuracy. Single measurements can be off by much more that 25 ft, even in open terrain. I did an experiment were I used a Topcon 20 channel reciever observing both pseudo range and carrier phase of both GPS and GLONASS satellites (in short VERY accurate equipment) over 24 hours on a roof, and single measurements could be off by 45 ft. This illustrates that geocachers must accept a few feet +-.

[+Entropy512]

It's been summed up pretty well so far. As others have said, there has not been additional error introduced to the civilian signal since 2000. (Prior to that, error could be as much as 100 meters). The civ signal does have less resolution (precision) than the military signal, but advanced signal processing techniques have allowed the civ signal to achieve significantly better measurements than originally intended.

 

There are a number of contributing factors to error, the largest of them being:

1) Ionospheric conditions. Dual-frequency receivers can eliminate these errors without external data, but right now the second frequency is encrypted military-only. Some highend receivers can perform some tricks to allow useful information to be collected from the second frequency (because the encrypted signal is the same on both frequencies), but these are very expensive, very large, and require large data collection times from a fixed point. Ionospheric corrections can also be determined by measuring the error seen by a GPS receiver at a known location and broadcast to other receivers. This technique is known as differential GPS. Currently the most common form of DGPS is known as WAAS.

2) Multipath - objects near the receiver can add error to the signal. Trees count in this category.

 

The geometry of the visible satellite constellation determines how much an error in measurement will affect the position fix. This geometry is used to calculate a parameter called the Dilution of Precision (DOP). DOP is what many receivers use to estimate position error/accuracy, but keep in mind that while DOP is defined very clearly, reported accuracy is determined by the receiver manufacturer. So a more conservative manufacturer may report a "worse" accuracy number than another receiver, but in reality is providing the more accurate measurement. DOP also does not change from receiver to receiver, it's a sole function of satellite geometry.

 

The tendency for single readings to be off is why the new Garmin Oregons use a new waypoint averaging technique - Garmin reccommends taking multiple position samples, separated significantly in time so the satellite constellation changes. (minimum 90 minutes between samples.)