How accurate are gps units?

How accurate are GPS units? I marked something in my back yard and when I went back to it a couple of days later I was 20 feet off . I have a Lowrance I finder . I was thinking of using it to relocate fish cribs.

In my experience there are some things to keep in mind when using any GPS unit. 1) Anytime the US military is "engaged" the government turns on increased variance for non-military GPS units which, depending on your location, throws in a variance that could go into the kilometers range but is normally in the meters or less. This variance is really the big difference maker from system to system and day to day. Some GPS units and software have been designed to minimize this variance, but that can be pointless for off the road applications. 2) The GPS network is native metric, depending on your system you could get additional variance depending on how accurate the software converts from metric to whatever you are using. 3) Just like triangulating with radio waves and terrain association, the GPS network reports more accurate coordinates if you have 3 or more satellites locked. 4) Simply put, some map datums are better and more accurate than others depending on your application.

I would say regardless if it's a Garmin, OEM unit in your car, Lowrance/Hummingbird...in your boat or iphone/android/google maps. There only as good as YOU keep them updated. My Android phone gps is accurate within 1' - 2'. I don't think we can ask for any better than that. Last year on Green Bay my Lowrance found a pre installed coordinance to what I believe was within 3' or less. just my .02 / experience with em.

I have an old Garmin 45XL that I bought to get me home off of Lake Michigan when I got caught out in the fog. Its job was to find the lighthouse which wasn't too much of a stretch. Over the years, I've added many crib locations on it now that I mainly fish up north. I rarely have a problem finding the cribs using that GPS unit and my fish finder. I may have to drive around in a circle a few times but it puts me on it reliably. I got the Navionics app for my iPhone and it's also pretty accurate within a few feet each time.

Once as a stander on a deer drive, I left my strap-on silent swivel seat on a tree. Helped drag a deer out and drove on another drive. After dark I realized I'd left the seat in the unfamiliar woods. Had it marked on GPS, so I went for a walk in the dark. It brought me right to it. Another time coming out after bowhunting, saw the skeletal remains of an 8 pointer with the hinds about 30 yards away. Marked each on the Gps. The next day after the rain and with clouds thinning, went back and got the antlers. Same thing, brought me right there. One other time, went out of sheboygan and the fog rolled in. Used trackback and was brought right to the harbor. I'd call these successes, but at no time did I not have a compass in case.

I remember we used to use a Loran before there was GPS and those things were pretty much spot on. Then we got a GPS and it was way off. GPS then got better over time but still not as accurate as Loran. Don't think they transmit the Loran signals anymore though based on my googling.

I have an OLD explorer with firmware from 1997 for in the woods and Lowrance units with WAAS on my boat. The explorer when compared side by side with a new Garmin handheld gives the same exact waypoint coordinances. Go back the same day and they are spot on. Go back a week or year later and they are off by up to 50'. Go back the another year and it is spot on... The Lowrance units also have a smaller miss factor from the original waypoint location. It ussually gets me close and then I TM around a few feet till I find the exact spot I want. Have not gone back to an anchor on my IPilot yet to see how close that is.

They'll never be 100% dead on accurate but people expect them to be. They'll get you close which is nice because it helps narrow things down a ton but there still is the human element required to do a little bit of work. The best GPS system I use is lining up the fourth tree past the birch tree with the radio tower and the nearest corner of the house on the northwest side of the lake. Those will never move. Even when I drop a waypoint on one of the units in my boat, if it is a location I want to make 100% certain I can get back to a notepad comes out with the corresponding waypoint number and is fine tuned with landmarks.

I have 5 different GPS units. 2 handheld, 2 automotive and one marine unit. NONE of them are what I would call accurate. I marked my home from my driveway and my car GPS's never put me back to the same spots. On the water, they often show me up on the bank while I'm still floating. While I'm in the car, the lack of accuracy isn't a problem, I can see what I'm looking for, but over the water, when you can't see the crib or rock it's a lot tougher.

do you have "waas" on your gps. if so, make sure it is turned on. it's a little more accurate.

i have marked a spot and went back a couple times and it is always within 5-10 feet. even on my navionics on my iphone i marked a sunken boat and i can see it on bottom on calm days and it always puts me right on it

I've heard the GPS units for general use, non-military, are within 50'. There is a more accurate Differential Global Positioning System (DGPS) that is an improvement to the Global Positioning System that provides much improved location accuracy, from the 15-meter nominal GPS accuracy to about 10 cm in case of the best implementations. DGPS uses a network of fixed, ground-based reference stations to broadcast the difference between the positions indicated by the satellite systems and the known fixed positions of the land based antennas. So if you're looking for something and find it and punch in the coordinates on a GPS unit with DGPS you could be within 4" of the stored location. Good Luck SS