# Ultimate GPS accuracy with current coordinate system



## Steve (Jan 15, 2000)

I was wondering with the perfect GPS with infinite accuracy what the is best accuracy that be acheived with the current coordinate system? What does that coordinate system go down to? Is it feet or less than that?


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## Jimbos (Nov 21, 2000)

Doesn't the government put a limit on the civilian use accuracy to about 50 feet?


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## Steve (Jan 15, 2000)

They do. My question is what is the ultimate accuracy of the existing coordinate system once you take a location fix down to minutes, seconds, etc.


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## Melon (Feb 22, 2002)

I am not sure I fully understand your question. But my Garmin Legend has stated accuracy of 11 feet.


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## kbkrause (Feb 12, 2002)

I think this answers your question. 

According to this web site Latitude and Longitude 



> For convenience, the earth has been divided into 360 degrees of latitude. Measure of latitude starts at the equator and is measured as 180 degrees north to the North Pole and 180 degrees south to the South Pole. Each degree represents 60 nautical miles on the earth's surface. The lines of longitude run north and south and are not parallel to each other. Consecutive degrees of longitude are 60 miles apart at the equator, and all come together at the North Pole and the South Pole. These are 360 degrees of longitude - 180 degrees lie east, and 180 degrees lie west of Greenwich, England, an arbitrary starting point used by all mapmakers.


Because there are 60 minutes in a degree and 60 seconds in a minute.

Latitude down to the second is 88 feet. (60 miles = 1 degree , 1 mile = 1 min, 1 mile = 5820 ft. , 5280ft/60sec = 88 ft/sec)
Longitude depends on how far away from the equator you are, at the equator 88ft/sec.

On a GPS you can use a decimal instead of a second so you can get down to much better precision.

I think this is all correct...


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## Steve (Jan 15, 2000)

That's the answer I was looking for... thanks.


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## kbkrause (Feb 12, 2002)

Steve actually a nautical mile is 6,076.11549 feet so the math changes a little bit...


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## FishTales (Feb 13, 2002)

I found this link in regards to accuracy of a GPS, I found more but this one was the first that gave it in easy to understand wording.
I also found another site that said the goverment "tweaked" some settings for the war in Iraq and got it down to 3 meters, thats about 9 feet.


http://www.kentuckyhydrografx.com/gpserror.htm


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## Michigan Hunter (Sep 24, 2001)

My Garmin Legend consistantly shows 12 to 15 feet accuracy. I also hunt Northern California (a month ago and week after next for bear). For some reason the Legend shows down to 9 feet accuracy there.


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## MSUICEMAN (Jan 9, 2002)

do not believe what your reciever thinks is its accuracy. the precision of GPS units vary with what is implemented with your receiver. with selective availability (SA), it is not very good, and varies, but with the removal of SA, the precision improves by about a factor of 10. It seems as if WAAS will not effect this either because the differential remains the same, maybe further work into the WAAS system will result in better precision, but not at its current state. EGNOS is the european version of WAAS and this hasn't resulted in better precision.

That being said, without selective availability and a good signal with no interruptions, usually the error is at or below about 21m, but this increases with bad signals which can be as much as just a really cloudy sky or tree cover or what not, up to 250m or even possibly more if extreme circumstances exist.

steve


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## MSUICEMAN (Jan 9, 2002)

i should say that these observations were done in a study by a friend who is studying and specializing in GPS systems.

steve


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## MSUICEMAN (Jan 9, 2002)

he also says that with the current DGPS system without SA, that the best accuracy available to civilians today is about 4m. though some other techniques used by the military and government can get as accurate as 10 or even 4cm! now thats accurate!

oh yeah, the accuracy of civilian GPS is set by the government and can be changed . This is intentional as at times of war, they believe it to be beneficial not to have possible threat's GPS systems accurate.

steve


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## wackoangler (Jun 1, 2003)

I use gps all the time for surveying with accuracy of 1 centimeter or less. The trick is to have more than one reciever collecting sattellite data. 

With the proper controls etc. gps can be very accurate, you can go on and on forever about this stuff, you can get a doctorate in surveying with most of your emphasis on gps. 

Don't get Gps and surveying confused with gis, the guys that do gis get the same type of accuracy "+/- 10' " as you guys do using the little hand helds. A lot of the gis people will tell you that they are getting sub centimeter accuarcy based on the coordinates that they have that go out to like 10 decimal places, but they are wrong, to get accuaracy that good, you need to have survey grade equipment and a lot of knowledge, and you have to run the data through a lot of tests. Our data guy is a phd with a lot of math background.

Sorry i'm rambling, just wanted you to know that you can get very good accuaracy, but no matter what your handheld tells you, it's +/- 10' at the best, even if it is giving you coordinates broken down to 10 decimal places.


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## Beave (Aug 26, 2002)

I know that a friend in the military says that they have real concerns about the accuracy. If terrorists were able to get ahold of the codes that surveyers and the military use they could have some very accurate and inexpensive missile guidance systems.

We're talking about photon torpedos down the exhaust hatch of the deathstar accurate here. With that kind of precision even very secure military installations are vulerable.


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## Hunt4Ever (Sep 9, 2000)

Steve,

Two things, first, switch to and learn to use the UTM coordinate system. This will allow you to use your GPS with a paper map, you won't believe what you can do. DMS (degree minutes seconds) just isn't accurate enough when your looking at a map.

Here is a link: UTM Coordinate System 

Second, in order to get good position accuracy, I mean the actual position of a single location, look for a setting or option on your GPS called "Position Averaging". What this does is takes a reading once a second and averages all the positions to create the waypoint. Set your GPS down and start the process. I'll set mine down and let it go for up to a couple of minutes, the longer the better. This takes out most of the error from SA. The only place this doesn't work well is a boat or other moving vehicle.


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## DEERSLAYER (Mar 29, 2000)

The surveyer that did my land used a GPS (a $40,000 unit!!!) and said in was accurate to within 1 inch!


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## unregistered55 (Mar 12, 2000)

My Garmin eMap is accurate to 17 feet most of the time!


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