highest accuracy ever

Most I have gotten was 15 Ft without an external ant. in the car. Thats fine with me

Virgo

6 feet with an external antenna.

6.5 feet with ext.antenna

About 13 feet, with 660 mounted on dash.

Unless I'm calling in an airstrike, I never check the accuracy.

12 ft is about as good as I have seen...8 with my Etrex...but anything around 20 feet is great..I can see that far...

10 feet on the big island of hawaii

^^^ Hehehehehe

On a good day?
Roughly 20 feet on the 350
I've seen 18 feet on the 660

No externals on either..

Is there any way to check the accuracy (or to get on some kind of satellite status page) with the Garmin C580? I have seen the "hidden pages" when you hold the battery symbol for 5 seconds, but I didn't see any satellite info in there...

Planning on send a cruise missile someplace?

launch that car from a frigate

15' w/o an external ant -- in the car.
I doubt I would ever need more...

Out of curiousity, how are you determining the accuracy of the unit?

I have a question, probably stupid, but here goes. How does the device know accurate the current reported location is? In other words, where does the information that the location is accurate to within x feet come from? And if my nuvi gets information that my current location is accurate to within say 13 feet, it must know where the actual location is, so why not just display it? Does it have something to do with the government's scrambling the signal?

The error of its position reported by the unit is an estimation of the error, not the error, so even your "best" may be much further off than you realize.

A good primer on GPS errors can be found on wiki

http://en.wikipedia.org/wiki/Gps#Error_sources

Looks like 6 to 45 feet of error in location is not uncommon.

To get much greater accuracy you need specialized receivers and clearance to use them.

For a detailed - technical answer, see here:
http://www.edu-observatory.org/gps/gps_accuracy.html

Another site's answer:
http://www.igage.com/gps_accuracy.htm

Garmin's answer:
http://www8.garmin.com/aboutGPS/

Yet another one:
http://users.erols.com/dlwilson/gps.htm

For a layman's answer (for people like me) - I was told by a wise person to just find a verified point on a map in my area, and go there. Espcecially if there's a latitude/longitude intersection anywhere near you.

The European GPS system is supposed to have enough accuracy for automobile auto-pilots. Seems like it would take one or two feet of accuracy to pull that off.
Jen

Thanks for the info. I'm learning a lot more about GPS than I thought I would!

Would need pretty darn accurate maps to pull that off. Maps that accurate?

How do you know i'm not doing exactly that?

haha, i just like my antenna.

This is why I asked the question about how they know the accuracy. When you try to determine accuracy, you have to factor in the accuracy of the GPS for sure, but even if the GPS was accurate to 1 meter, if you lay the point on a map that has 15 meter accuracy, you could still be 15 meters off. How wide is the road on screen? How does the on screen car relate to the location of the point? Turn on the GPS, don't move, and see how fast it ways you are going?

Something to keep in mind is the nuvi350 (and many others) update the display every second. At 60 mph this equates to approximately 88 feet. That means the total display error (just prior to the display update) will be 88 feet +/- the GPS error when moving at 60 mph.

RT

I believe the "accuracy" is measured as a circle. So if the unit says 15 feet, it means the "absolute" lat/lon is somewhere within a 15 radius of you. You might be right at the exact location, or it could be up to 15 feet in any direction. With the satellites 12,600 miles away, 15 feet is 1/4435200 or about .0000002%

Also, keep in mind that address locations can be off by 100's of feet, or more. There are a lot of us that key in our home address, only to find out that it's somewhere down the street

My accuracy is usually around 15 ft.

How do you measure the accuracy of your Garmin? What is the reference point?

The only way to truely check the accuracy is to position the unit over a point with known lat/lon information. This would usually be a National Geodetic Survey monument or something else that a local Land Surveyor has an accurate position on. NGS monument information can be obtained from their website.

Nuvi 350 12 feet with internal antenna in Queens NY

On what unit you have you get the figures for accurracy from the sattellite screen. Upper left corner for mine, just tap that and it shows all the sats that it is recieving and the altitude and the accuracy of it.

Actually you need accuracy to within a couple of inches. A foot or two could very easily put you in the ditch. There are systems available that can give you sub centimeter accuracy, but they use a combination of satellite and ground based transmitters to give you that level of accuracy.

I'm getting 15-20 ft which is good enough for me.

That is correct and has to be done repeatedly at different times of the day and night to be accurate.
However satellite errors also have to be taken in to consideration.

Accuracy
The degree of conformance between the estimated or measured position, time, and/or velocity of a GPS receiver and its true time, position, and/or velocity as compared with a constant standard. Radionavigation system accuracy is usually presented as a statistical measure of system error and is characterized as follows:

Predictable - The accuracy of a radionavigation system's position solution with respect to the charted solution. Both the position solution and the chart must be based upon the same geodetic datum.

Repeatable - The accuracy with which a user can return to a position whose coordinates have been measured at a previous time with the same navigation system.

Relative - The accuracy with which a user can measure position relative to that of another user of the same navigation system at the same time.

So if after hitting the home button and your in Ft. Myers Florida and home is Cleveland, Ohio and several hours later the GPSr announces "Your arriving at your destination on the left." And that destination is your driveway in Cleveland Ohio, I would have to say it doesn't get more accurate than that.

One of the more common "problems" in describing GPS accuracy is the occupation time required to achieve the claimed accuracy. Be wary you must occupy a location in order to achieve a particular accuracy. In the best case scenario, the required occupation time might be as little as one second. However, several systems that tout sub-meter accuracy are only able to achieve this after a stationary occupation of at least several minutes. Because of computer speeds and screen writes on the GPSr's they also add to inaccurate displays of accuracy.
If you need Better accuracy a Auto GPSr similar to the Nuvi is the wrong tool anyway. Either get an Hand held GPSr or what you really need is a surveyors GPSr.

So chances are what your unit is reporting is in error! But our cheap GPSr's (not in price)are generally acurate enough to find your drive way a few meters. For those that have to dead on with the coordinates try one of these http://tinyurl.com/4hwcqe

Bob

12' while mounted on the dash

@ 12 feet with 350 on dash

Maybe the Galileo system might be accurate enough, but what about the error prone maps, what will happen when the auto pilot is cruising down the road and the street isn't even there on the map? just like the 2008 - 2009 maps.

12 feet with 750 mounted on dash.

My etrex which lives on the four wheel drive handle in my jeep after about a minute never has more than 10 ft. it usually hangs right about 6 but I have seen it as low as 3 ft.
Was going to trade up but now maybe I'll hang on to the little guy.

9 ft

The best I've gotten with my eTrex Vista has been 10'... at least when I've gotten curious enough to check.

I was told on another forum (and it makes sense now that i think about it) that the unit assumes on a scale of 0 to 100 that 50 is in the middle of the block. This makes sense because the last two numbers in my address are 24 and on the map it plots my home about 1/4 the way along the street. But physically sitting in my home, it tells me the nearest address is the one ending in 48 and my house is in fact about midway along my street.

This theory also rings true on my parents address in another state.

15'

Checked today and was getting 6 FT on my 2730.

Good come back and quick on your wit!

Yeah, if Santa had a GPS with no error in it, he could just drop those presents down the chimney and keep on flying. Think of all the time he'd save....not to mention the savings on cleaning his suit.

On the other hand I'm on the end of my block and it puts my home address a house or two up the street.

I currently have a nuvi360, and it reads 13 feet often.

How do I test for accuracy?

LOL...I'm usually found right where I'm at.

... with consumer units.

However, many units will show an estimate of the accuracy, often on a "Satellite" page.

Everyone here is stating the *estimated* accuracy their unit is reporting, but only rarely do the consumer units spell out that this is reading is only an estimate.

If you post which unit you use, someone will be able to direct you on how to find the correct page on your GPSr.

On my 360 I touch the satellite bars to get to the satellite page. At the top it gives the estimated accuracy.

geocamp posted this earlier, but if you really want to test the accuracy of your unit, you need to compare the GPS coordinates to a plotted location.

The National Geodetic Survey has a list of markers with the EXACT coordinates. http://www.ngs.noaa.gov/cgi-bin/ds_desig.prl

1. under "Station Name" type in a town or landmark near you

2. select your State

3. under "Data Type Desired" select Any Horizontal Control

4. click Submit

5. you will get a list of results. highlight one or more stations, then click Get Datasheets

6. scroll to the bottom of the page and you will see Station Description. This describes in detail where the exact coordinates are located.

7. drive to that exact spot and compare your GPS coordinates to the NGS coordinates.

I retract my definitive "You can't...", but not entirely...

If your unit will continually update its displayed position, you'll also see the readout change with time, within, and sometimes beyond, the estimated accuracy.

My aviation GPSMAP 195 (VFR) does this, my Nuvi 260 does not, so taking multiple readings via tapping on the vehicle icon should show this changing position even if it is sitting on the NGS marker.

IOW you wouldn't be confirming the accuracy of your unit, but the accuracy of the estimation of the position it is reporting...

High accuracy GPS units take the average of the positions reported over minutes, hours, days to end up with a final accurate, sub centimeter position. Traveling at 35 - 75 mph, I have no need to be that accurate; A few meters more than is fine...