Thread: GPS accuracy vs Precision, what the gps makers don't tell us.

wow....some of you guys are dead serious...

Originally Posted by HydroGrafx

Hey Tom!

Here is a test I did a few years ago (see attached image). It is a rough test, but gives clues to precision. A word commonly used in high accuracy gps circles is "repeatability". GPS drift a key aspect of repeatability, which is simply variation of indicated position (vs. actual position).

Initially when folks see this kind of thing, they are rather disappointed. The key thing is to note the scale bar. Now imagine the average length boat overlayed on the plots. That makes things seem better and reinforce why the gps is useful. It is common to get within a boat length or slightly more of a previously saved waypoint. If one can get that close, the object of interest can usually be found quickly with the depth sounder.

Users of GPS units for agriculture were commonly interested in the "relative" accuracy of their hardware. A quality submeter gps would only drift 1 foot in a 10-15 minute period. If a farmer was spraying a field he might be getting <1 foot pass to pass accuracy as long as he didn't take longer than 10-15 minutes to make another swath. Now if he wanted to run the same recorded path again an hour (or hours) later, it is likely he might be off a few feet, even with a submeter gps.

Pat H.

Were you making passes on a fixed object when you recorded this data? What was your setup, speed, etc.?

Thanks for the info.

Tom

Originally Posted by poppop

wow....some of you guys are dead serious...

Yeah, they are drifting out of fishing into that dreaded area called WORK.

Some tech info for you electronic gurus.
These pages are currently under construction

http://www.gpsworld.com/gpsworld/art....jsp?id=388666

http://www.maps-gps-info.com/gps-accuracy.html

Of course it's all over my head, I just use my GPS 72 and GPSMap 76C to get me close and look for it with my 480.

Originally Posted by DTG

CP- sometimes we can pull up to our mats and be dead on em. But some times they are up to 15-20ft off. Sidescan mark and original mark may not be the same but close.

Yep. I have had this happen a lot.

Originally Posted by SteveJ

Some tech info for you electronic gurus.
These pages are currently under construction

GPS Accuracy: Lies, Damn Lies, and Statistics - GPS World

GPS Accuracy - How Accurate is it?

Of course it's all over my head, I just use my GPS 72 and GPSMap 76C to get me close and look for it with my 480.

I read all this stuff Steve. Can't trust any of the manufacturers to be honest. They want show raw data or performance data such as Hydrographics just put up. That is what I want to collect, not somebody bragging how good or accurate their gps is at working.

I know sub meter GPS units are dead accurate (agriculture, surveying, etc, but that is a sophisticated and expensive technology. I could get a sub meter receiver, but wow what an expense.

If I am slow trolling and try to cover a "cluster" of close proximity brush piles, I never know what to expect. One minute it (the GPS) is showing stuff (w.points)to be in front of me, then next it is behind me. Really aggravating, especially in open water. I know I could mark them with buoys, but the units specs say less than 3 meters. Bull. Well, it is always 3 meters away from where it thought it should be.

I believe most folks know what I am talking about here. There are always some that know more about this stuff than this old dumass.

Originally Posted by ShilohRed

Tom my 76c for the most part allows me to do what your talking about. I can go by a tree sticking out of the water. And it always shows the same on it. but today when going 5 MPH. It was not showing the same Lad and Lan? Would not have thought that speed would be a factor?
And wonder if the unit was just not getting the signal it does most days?

Now my 987c-SI GPS does show the same bouey in the same place all the time. and its close enough to run by at night. But I always wonder if it may be off enough to cause me to run over a marker? So I for one do not put full faith in it yet.
Will check this again next day I am out on the water. and give you the findings of the Garmin 76C
Shoot Come to think of It I can also do it in the yard on the 4 wheeler also.

How is our buddy Todd doing? If my stress test goes well Friday. I need to get with him and go Striper fishing next week. He likes the pole dancers while striper fishing LOL
Pete

Let me know. My 76 is pretty accurate. Too many trees for me to try the 4 wheeler, but that is a good idea. Lots easier than in the boat.

I tried trolling a grid with my Tracker. Mod Vs aren't good for this type work. I roamed everywhere along the grid marks. Drunk sailor.:p

Originally Posted by Cane Pole

Let me know. My 76 is pretty accurate. Too many trees for me to try the 4 wheeler, but that is a good idea. Lots easier than in the boat.

I tried trolling a grid with my Tracker. Mod Vs aren't good for this type work. I roamed everywhere along the grid marks. Drunk sailor.:p

I want one of these things.

Trimble - GPS Pathfinder ProXRT Receiver